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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
This book is a comprehensive guide to strigolactones’ role in plant biology, growth, and sustainable agriculture.
Strigolactones, a fascinating and rapidly evolving class of plant hormones, have garnered significant attention in plant biology over the past decade. Initially discovered for stimulating the germination of parasitic plants, strigolactones are now recognized as key regulators of numerous plant processes, including growth, development, and response to environmental stresses. Their multifaceted nature and wide-ranging impact on plant physiology make strigolactones a critical study area for researchers aiming to enhance crop yield, resilience, and overall agricultural productivity.
This edited volume provides a comprehensive overview of the current state of knowledge on strigolactones, exploring their biosynthesis, signaling mechanisms, and practical applications in agriculture. The book collects contributions from leading experts in the field, offering a diverse and in-depth perspective on the various roles that strigolactones play in plant biology.
The chapters in this volume cover a broad spectrum of topics, from the molecular and genetic basis of strigolactone biosynthesis to their interactions with other phytohormones and environmental factors. The book examines the regulatory functions of strigolactones in plant architecture, including shoot branching, root development, and leaf senescence, as well as their involvement in stress responses such as drought, salinity, and pathogen attack. Also highlighted are recent advancements in understanding strigolactone signaling pathways and the potential for genetic engineering to manipulate these hormones for crop improvement.
Audience
Plant biologists, agronomists, horticulturists, and agriculture industry professionals studying plant development to address agricultural challenges.
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Veröffentlichungsjahr: 2025
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Table of Contents
Cover
Table of Contents
Series Page
Title Page
Copyright Page
Preface
1 Strigolactones: Journey from Rhizospheric Chemoattractants to Plant Growth Regulators
1.1 Introduction
1.2 Brief History of Strigolactones
1.3 What is the Origin of the Name Strigolactone?
1.4 Diverse Strigolactone Functions: From Single-Celled Alga to Terrestrial Plants
1.5 Strigolactones in Ferns and Mosses: Rhizosphere Signals or Phytohormones?
1.6 Agricultural Loss and Root Parasitic Plants
1.7 Strigolactones and Agriculture: Why Do They Hold the Key?
References
2 Nature, Structural Diversity, Biosynthetic Pathway, and Strigolactone Transport in Plants
2.1 Introduction
2.2 Chemical Nature of Strigolactones
2.3 Naming Protocol for Strigolactones
2.4 Natural Diversity of Strigolactones
2.5 Biosynthetic Pathway of SLs
2.6 Strigolactone Transport
2.7 Conclusion
References
3 Unveiling the Strigolactone Signaling Pathway: From Receptors to Responses
3.1 Introduction
3.2 The Strigolactone Perception
3.3 Strigolactone Signal Transduction
3.4 Transcriptional Regulation
3.5 Environmental Factors Modulate Strigolactone Signaling
3.6 Applications and Future Perspectives
3.7 Challenges and Unanswered Questions in Strigolactone Research
3.8 Conclusion
References
4 Diverse Roles of Strigolactones in Plant Growth and Development: Shaping Above- and Below-Ground Architecture
4.1 Introduction
4.2 The Contribution of Strigolactones to Shoot Development
4.3 Role of Strigolactones in Regulating Shoot Secondary Growth
4.4 Root Development in Plants: Potential Role of Strigolactones
4.5 Role in Leaf Senescence
4.6 Conclusion
References
5 Regulation of Phosphorus Nutrition in Tomato Plants: Unveiled Roles of Strigolactones
5.1 Introduction
5.2 Strigolactone Chemistry and Biosynthesis
5.3 The SL Signaling Pathway
5.4 Function of Strigolactones in Tomato Plants: Emphasis on Phosphorus Nutrition
5.5 Conclusions
References
6 Strigolactones’ Role in Heat and Saline Stress Tolerance in Horticultural and Field Crops
6.1 What are Strigolactones
6.2 Production, Role, and Understanding of Strigolactones in Alleviating Heat Stress in Horticultural and Field Crops
6.3 Production, Role, and Understanding of Strigolactones to Alleviate Salinity Stress in Horticultural and Field Crops
6.4 Conclusions and Future Perspectives of Strigolactones’ Use for Alleviating Stress in Plants
References
7 Role of Strigolactones in Heavy Metal Tolerance: A Case Study on Cadmium
7.1 Introduction
7.2 Cadmium Toxicity on Plants
7.3 Strigolactone-Mediated Cadmium Tolerance
7.4 Future Perspectives and Challenges of SLs for Agricultural Practices and Environmental Sustainability
7.5 Conclusion
References
8 Strigolactone Interplay with Other Phytohormones Under Stressed and Normal Conditions
8.1 Introduction
8.2 Role of SLs in Abiotic Stress Tolerance
8.3 Interplay of SLs with Other Phytohormones
8.4 Future Directions and Implications for Agriculture
8.5 Conclusion
References
9 Strigolactone Analogs: Synthesis, Structural Features and Biological Activity
9.1 Introduction
9.2 Synthesis of Natural SLS
9.3 Structures of Strigolactones
9.4 Biological Activity of Strigolactones
9.5 Conclusion
References
10 Karrikin-Related Effects on Plant Development, Stress Tolerance, and Beyond
10.1 Introduction
10.2 Direct KAR-Associated Effects on Plants
10.3 Indirect KAR-Associated Effects on Plants
10.4 Smoke Water as a Promising Agent for Agricultural Applications
10.5 Conclusions and Perspectives
Acknowledgments
References
11 Strigolactones: Key Phytohormones in Plant–Microbe Interactions and Development
11.1 Introduction
11.2 Identification of SLs as Signaling Molecules in AM Symbiosis
11.3 SL Perception by AM Fungi
11.4 Influence of SLs on AM Fungi at the Cellular and Molecular Levels
11.5 SLs and AM Fungi Mediate Root Development
11.6 Impact of AM Fungi on Nutrient Acquisition (Particularly Phosphate) and Plant Growth
11.7 AM Symbiosis: Serving as a Biofertilizer and Biocontrol Agent
11.8 Interactions of SL with Non-AM Fungi
11.9 Strigolactones and Root Nodule Symbiosis
11.10 Effect of SLs on Nodule Number (Quantity)
11.11 Impact of SLs on Rhizobia
11.12 Future Directions and Conclusion
References
12 Strigolactones and Control of Parasitic Weeds
12.1 Introduction
12.2 What are SLs?
12.3 Phenomena of Host–Parasitic Plant Interaction
12.4 SL–Carotenoid/Biosynthetic Pathway for Stimulation
12.5 Karrikins
12.6 Exploring Various Strategies for Controlling Parasitic Weed Infestations
12.7 Impact of SL Application on Future Scientific Research and Agriculture
12.8 SL Functions
12.9 Conclusion
References
13 Strigolactones and Plant Defense: Protection Against Pests and Pathogens
13.1 Introduction
13.2 Biotic Stresses in Plants
13.3 Biotic Stress-Mediated Defense Mechanism in Plants
13.4 Strigolactone-Mediated Defense Against Pathogens
13.5 Strigolactone-Mediated Defense Against Pests
13.6 Strigolactone-Mediated Plant Defense Mechanism
13.7 Conclusion and Future Perspective
Acknowledgments
References
Index
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Some known parasitic plants belonging to the Orobanchaceae family an...
Chapter 2
Table 2.1 Summary of different proteins and genes involved in the biosynthesis...
Chapter 3
Table 3.1 Some instances of SL crosstalk with other phytohormones in literatur...
Chapter 5
Table 5.1 Main root trait differences between tomato wild-type and strigolacto...
Chapter 6
Table 6.1 Endogenous generation of SLs under abiotic stresses in plants.
Table 6.2 Exogenous application of SLs under abiotic stresses in plants.
Chapter 7
Table 7.1 Impact of exogenous GR24 (SL analog) treatment on plants under Cd st...
Chapter 12
Table 12.1 Impacts of SLs on various plant species’ functionalities.
Chapter 13
Table 13.1 Major classes of plant-infecting bacterial and fungal pathogens and...
Table 13.2 Major vector-borne viral diseases affecting crops with their sympto...
List of Illustrations
Chapter 1
Figure 1.1 Milestone discoveries in strigolactones.
Figure 1.2 Germination of parasitic seeds belonging to Broomrapes and enhanced...
Chapter 2
Figure 2.1 Chemical structures of some representative strigol-type strigolacto...
Figure 2.2 Chemical structures of some representative orobanchol-type strigola...
Figure 2.3 Chemical structures of some representative non-canonical strigolact...
Figure 2.4 Conversion of carlactone into avenaol.
Figure 2.5 Biosynthetic pathway of SLs.
Figure 2.6 Transport of SLs. (a) In petunia, it was proposed that asymmetrical...
Chapter 3
Figure 3.1 Some impacts that SLs have on plant physiology (image produced usin...
Figure 3.2 (a) The central ring depicts the ABC-D ring scaffold base structure...
Figure 3.3 A simplified flowchart of the current understanding of the SL signa...
Chapter 4
Figure 4.1 Strigolactones interplay with auxins and cytokinins in the control ...
Figure 4.2 Regulation of above- and below-ground architecture in plants by str...
Chapter 5
Figure 5.1 Canonical and non-canonical natural SLs.
Figure 5.2 Schematic model of the main players involved in the regulation of P...
Figure 5.3 Conceptual summary of how strigolactones (SLs) are involved in P-de...
Chapter 6
Figure 6.1 Strigolactones’ general chemical structure (source: Wikimedia Commo...
Figure 6.2 Role, and understanding of strigolactones in alleviating heat stres...
Figure 6.3 Role of strigolactones in salt stress conditions in horticultural a...
Chapter 7
Figure 7.1 Overview of how Cd toxicity affects different plant functions.
Chapter 8
Figure 8.1 The proposed scheme illustrates how SLs help plants adapt to abioti...
Figure 8.2 Strigolactone interactions with different phytohormones.
Chapter 9
Figure 9.1 Two classes of SLs.
Figure 9.2 Chemical structures of natural strigolactones and synthetic analogu...
Chapter 10
Figure 10.1 Milestone discoveries about karrikins (KARs) (see details in the t...
Figure 10.2 Signal transduction of karrikins in plants. (a) Chemical structure...
Chapter 11
Figure 11.1 Impact of strigolactones on AM fungal colonization for mediating t...
Figure 11.2 Wild type pea plant showing normal nodule numbers (a), SL deficien...
Figure 11.3 Wild type
Lotus japonicus
shows normal number of root nodules (a),...
Chapter 12
Figure 12.1 General structure of SLs.
Figure 12.2 Parasitic plant:
Striga
species.
Figure 12.3 Life cycle of root parasitic plants and interaction of SLs.
Figure 12.4 Effects of SL analogs on the plant’s growth and signaling pathway ...
Figure 12.5 Potential application benefits of SLs in agriculture.
Figure 12.6 Future research directions and application of SL for crop producti...
Chapter 13
Figure 13.1 Role of SLs in plant growth and development. SLs regulate shoot br...
Figure 13.2 SL-mediated biotic stress response in plants. SL mediates biotic s...
Figure 13.3 SL-mediated stomatal closure in response to biotic stress in plant...
Guide
Cover Page
Table of Contents
Series Page
Title Page
Copyright Page
Preface
Begin Reading
Index
WILEY END USER LICENSE AGREEMENT
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Scrivener Publishing100 Cummings Center, Suite 541JBeverly, MA 01915-6106
Publishers at ScrivenerMartin Scrivener ([email protected])Phillip Carmical ([email protected])
Strigolactones
Emerging Plant Hormones
Edited by
Tariq Aftab
Department of Botany, Aligarh Muslim University, India
and
Kaiser Iqbal Wani
Department of Botany, Aligarh Muslim University, India
This edition first published 2025 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA© 2025 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.
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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-394-30279-6
Front cover image courtesy of Adobe FireflyCover design by Russell Richardson
Preface
Strigolactones, a fascinating and rapidly evolving class of plant hormones, have garnered significant attention in plant biology over the past decade. Initially discovered for stimulating the germination of parasitic plants, strigolactones are now recognized as key regulators of numerous plant processes, including growth, development, and response to environmental stresses. Their multifaceted nature and wide-ranging impact on plant physiology make strigolactones a critical study area for researchers aiming to enhance crop yield, resilience, and overall agricultural productivity.
This edited volume provides a comprehensive overview of the current state of knowledge on strigolactones, exploring their biosynthesis, signaling mechanisms, and practical applications in agriculture. The book collects contributions from leading experts in the field, offering a diverse and in-depth perspective on the various roles that strigolactones play in plant biology.
The chapters in this volume cover a broad spectrum of topics, from the molecular and genetic basis of strigolactone biosynthesis to their interactions with other phytohormones and environmental factors. We delve into the regulatory functions of strigolactones in plant architecture, including shoot branching, root development, and leaf senescence, as well as their involvement in stress responses such as drought, salinity, and pathogen attack. Furthermore, the book highlights recent advancements in our understanding of strigolactones, signaling pathways and the potential for genetic engineering to manipulate these hormones for crop improvement.
This book is a valuable resource for researchers, students, and professionals in plant sciences, agronomy, and related fields. By consolidating the latest research and offering practical insights, it should inspire new investigations and applications that harness the power of strigolactones to address some of the most pressing challenges in agriculture today.
The editors hope that this book will catalyze further research and innovation in the field of plant hormone biology, ultimately contributing to the development of more resilient and productive agricultural systems. We are grateful to the contributing authors for their dedication and expertise, which made possible this comprehensive exploration. We extend our thanks to the reviewers who have provided invaluable feedback and guidance throughout the preparation of this volume, and to Martin Scrivener and Scrivener Publishing for their support and publication.
Tariq Aftab
Kaiser Iqbal Wani
Aligarh Muslim University, India
October 2024
1Strigolactones: Journey from Rhizospheric Chemoattractants to Plant Growth Regulators
Kaiser Iqbal Wani, M. Naeem and Tariq Aftab*
Department of Botany, Faculty of Life Sciences, Aligarh Muslim University, Aligarh, India
Abstract
Strigolactones are a broad class of plant hormones that have progressed from being identified solely as chemoattractants in the rhizosphere to becoming important regulators of plant growth and development. Originally identified by their ability to promote the germination of seeds from parasitic plants such as Striga, Orobanche, and Phelipanche, strigolactones are now widely recognized as a novel class of plant hormones that play an important role in plant growth regulation. They play an important role in developing symbiotic associations with microbes such as rhizobia and arbuscular mycorrhizal fungi (AMF), which facilitate nutrient absorption, especially under nutrient-scarce conditions. Their main influence on root development, shoot branching, and overall plant architecture has been demonstrated by numerous studies conducted in the last two decades. This novel group of hormones aids plants in integrating environmental cues for the regulation of many development processes by influencing the transport and signaling of other hormones, especially auxins. Their involvement in a wide range of growth and developmental processes in plants, under both normal and stressed conditions, highlights their potential for agricultural applications, including enhancing crop resilience and productivity in challenging conditions.
Keywords: Strigolactone, Striga, Orobanche, rhizospheric chemoattractant, agriculture
1.1 Introduction
In biology, hormones are a class of signaling molecules secreted by specific glands in organisms and transported to target organs via the circulatory system to regulate physiology and behavior. Traditionally associated with glands in animals, these molecules impart an action that is beyond simple chemical secretions either locally diffused to nearby cells or acting on the cells themselves that secrete them. Once these hormones reach their target cells, they bind with specific receptor proteins, and signal transduction pathways induce a cascade of cellular changes. Plants also have their version of such essential molecules; even though they lack dedicated hormone-secreting glands like animals, they are produced by the plant itself in trace amounts, the compounds being referred to as plant hormones. Plants are similarly dependent on the use of a varied array of natural factors that include, among others, water, oxygen, light, and a range of minerals and nutrients. The contribution of these exogenous factors to the growth of plants is considered essential. Alongside these exogenous factors, however, are intrinsic factors that help in their development. There exist factors intrinsic to the plants that are more specifically called plant hormones or phytohormones. They exercise their regulatory powers at very low concentrations either locally, in the cells adjacent to their origin, or upon distant tissues or organs. Darwin and Francis (1880) proposed the term plant hormones from the phenomenon of phototropism, which is the bending of a plant in the direction of light. Thiamann gave the term “phytohormones” to an organic substance of natural occurrence in plants.
Plant hormones play a crucial role in controlling different aspects of plant development and growth and reactions to environmental stimuli during different stages of the life cycle. The production of these signaling molecules occurs in specific tissues of plants and are then transported to other parts to exert their functions. The complex interactions between different hormones provide a framework necessary to coordinate plant physiological processes for optimal development in a changing environment. Of the many plant hormones, five are commonly regarded as “the big five”: auxins, gibberellins, cytokinins, ethylene, and abscisic acid (ABA). These hormones regulate a wide range of functions, frequently influencing several events at the same time. Auxins promote growth [1], cytokinins regulate cell division, which aids in the formation of new plant organs [2], gibberellins contribute to stem elongation [3], ethylene orchestrates processes such as ripening [4], and ABA regulates moisture levels by mediating stomatal behavior [5]. The intricate interplay of hormones constitutes a complex mechanism, many of which are still unidentified.
During the last 15 years or so, several classes of new chemical compounds that have hormone-like activity have been independently discovered. The name for this new class of phytohormones is strigolactones. In this intricate domain relating to plant–soil interactions, strigolactones are complex compounds with far-reaching consequences for plants and the environment surrounding them, particularly in rhizospheres. Initially identified as rhizospheric chemoattractants [6], these small chemical molecules came out to be versatile signaling molecules governing a wide array of activities integrally related to plant growth, development, and environmental adaptation [7]. This chapter briefly reviews the history of these multi-faceted novel phytohormones, from their detection in root exudates to key plant growth regulators.
1.2 Brief History of Strigolactones
This is the sixth decade since the first pivotal publication elucidating the isolation and characterization of strigol as an effective seed germination stimulant for Striga lutea[8]. Since then, a number of key discoveries have been made concerning strigolactones, as indicated in Figure 1.1 in the form of a timeline. The work by Cook et al. was seminal not only in aiding the discovery of a group of compounds known as strigolactones but also in establishing their activity in stimulating the germination of seeds belonging to parasitic plants such as Striga. More recently, their identification as key players in mediating the plant–arbuscular mycorrhizal fungi symbiosis [9] and their activity as shoot-derived endogenous plant hormones have driven new research opportunities [10, 11]. This structurally related group of compounds collectively called strigolactones occupies a prominent position in the scientific world today for their multifaceted involvement in rhizosphere signaling processes and their potential in parasitic weed management. This newfound understanding of their regulatory functions as plant hormones unraveled many intricacies of plant physiological responses.
Figure 1.1 Milestone discoveries in strigolactones.
Among various applications, strigolactones are most commonly used to control parasitic weeds. Parasitic weeds from the Striga, Phelipanche, and Orobanche genera pose significant agricultural threats in many regions around the world, necessitating immediate action [12]. The field has made significant progress, culminating in the organization of the inaugural international strigolactone congress in Wageningen in March 2015.
The first strigolactone, (+)-strigol, was discovered in 1966 and extracted from cotton root exudates as already discussed at the beginning. However, it was not until nearly two decades later that its structure was explained [13–15]. The ability of strigol to induce seed germination of the Striga and Orobanche species is significant. Initially, strigol was the only naturally occurring strigolactone known. After 1990, novel strigolactones were isolated from various root exudates, including sorgolactone from sorghum [16], orobanchol from red clover [17], and solanacol from tobacco [18].
Strigolactones are found in trace amounts in root exudates, making it challenging to determine their structures and assign stereochemistry [19]. All strigolactones have a fundamental structure consisting of an annulated system of three rings known as the ABC scaffold, which is connected to a butenolide ring via an enol ether bridge. Two families of strigolactones have been identified: one with (+)-strigol-like stereochemistry at the BC junction and the other with (-)-orobanchol-like BC stereochemistry. Across all natural strigolactones, the D-ring configuration at the C-2′ position is consistently R.
As the interest in strigolactones is growing day by day, new information about their biological properties is emerging. It has been discovered that strigolactones function as branching factors for arbuscular mycorrhizal (AM) fungi [9, 20] and as inhibitors of bud outgrowth and shoot branching, with the latter involving an interplay with auxins [10, 11]. Furthermore, they have a significant impact on plant architecture (both above- and below-ground), playing a key role in its regulation. These signaling molecules are now recognized as a new class of plant hormones with a promising future.
1.3 What is the Origin of the Name Strigolactone?
Strigolactones were first reported in root exudates because of their ability to induce the germination of seeds from the parasitic plant Striga. S. hermonthica, also known as purple or giant witchweed, is a notable species in this plant family. The name “Striga” originated from the observation of these witchweeds by subsistence farmers in Africa. These plants would germinate unexpectedly, destroying the crops. The term “witchweed” was derived from this idea because farmers thought that these plants germinated mysteriously similar to how a witch would appear all of a sudden without an introduction. The scientific term Striga originates from ancient Rome, where it was referred to as some sort of mythological witch due to her vicious nature, specifically her malice toward others, especially that of children. Striga species are part of the broomrape family (Orobanchaceae), and the majority of them grow as parasites on other plants.
The lactone component of strigolactones indicates its chemical structure. Lactones are cyclic esters formed by the condensation of a carboxylic acid and an alcohol group within a single molecule. Strigolactones, in particular, contain two lactone rings. The chemical variations to the core structure and differences in stereochemical configurations cause variation within the strigolactone family [21]—for example, strigol and orobanchol are frequently cited examples of A and B rings undergoing oxidation, resulting in distinct stereochemical differences between the B and C rings.
1.4 Diverse Strigolactone Functions: From Single-Celled Alga to Terrestrial Plants
Strigolactones have been reported in simple single-celled algae and primitive terrestrial plants like liverworts and mosses [22]. Their early function probably included intercellular signaling and control of growth and differentiation in primitive plants—for example, strigolactones are produced in liverworts, mosses, and the alga Chara coralline; they induce the outgrowth of rhizoids there. In filamentous mosses, like Physcomitrella patens, they regulate the branching of protonema and the growth of filaments in neighboring colonies [23]. As a result, strigolactones help neighboring organisms coordinate their growth and competition—a principle that also applies to higher plants, regulating the growth of roots and shoots. As terrestrial colonization progressed millions of years ago, fungal symbioses emerged. Certain liverworts formed symbiotic relationships with mycorrhizal fungi, though the role of strigolactones in this interaction is unknown and requires further investigation. With the evolution of vascular plants came complex patterns of shoot branching and the possibility of long-distance strigolactone transport. The use of strigolactones by witchweeds, which they extract from host plants, is the most recent evolutionary adaptation in strigolactone history.
1.5 Strigolactones in Ferns and Mosses: Rhizosphere Signals or Phytohormones?
Genetic and physiological studies have reported that strigolactones act as plant hormones in angiosperms [24, 25]. Moreover, in most plant species, strigolactones are secreted by roots into the rhizosphere, hence acting as a signal that attracts AM fungi, making nutrients most available for uptake [26, 27]. This phenomenon is observed in a variety of plant groups, including bryophytes, indicating a long-standing evolutionary relationship [28]. This raises the question of whether the original function of strigolactones was primarily hormonal regulation or as a rhizospheric signal.
Non-seed plants lacking essential components of strigolactone signaling, such as D14 and SMXL7-type proteins, may need an alternative mechanism for perception and response [29]—for example, Marchantia polymorpha has lost the ability to produce strigolactones due to the absence of CCD8 and MAX1 enzymes [30]. If strigolactones primarily served as developmental or hormonal signals in this species, the loss of synthesis ability would likely lead to evolutionary selection pressure against it. Consequently, M. polymorpha has lost its capability to attract AM fungi, unlike other Marchantia species that still retain this ability.
What about the role of strigolactones in mosses? As already known, P. patens produces carlactone, which suggests the presence of strigolactone-like compounds [22, 31]. Furthermore, normal development in P. patens is dependent on carlactone biosynthesis. But what if P. patens is the exception? Here strigolactones appear to regulate plant growth in a way similar to quorum sensing, which could be viewed as a limited developmental response to what is essentially a signal in the rhizosphere. Given that P. patens has also lost the capability for AM symbiotic association, unlike its immediate relatives, the use of strigolactones as a developmental signal in this species may be an independent evolutionary adaptation unique to it or its immediate relatives.
1.6 Agricultural Loss and Root Parasitic Plants
Root parasitic plants of Orobanchaceae family are one of the most important agricultural problems in Southern Europe, Africa, and Asia, where there are a number of obligate parasites of this family that require the availability of a host for development and reproduction. They include Broomrapes, genera Orobanche, Phelipanche, and witchweeds, genus Striga [32–34]. Notably, witchweeds are a real danger to African food security [35]. Indeed this weed infests close to 50 million hectares of agricultural land across the continent, especially to the detriment of 300 million farmers who stand to lose over $7 billion USD worth of economies annually [36]. Parasitic plants can infect a wide range of plant species, but staple crops such as cereals and legumes are especially vulnerable. They germinate due to strigolactones exuded by the roots of host crop plants (Figure 1.2) and cause severe agricultural loss. The seed production of a single Striga plant may vary from 10,000 to over 200,000 seeds, and these seeds could remain dormant in the soil for up to 20 years [37]. The control of their propagation is a daunting task because the weight of these seeds is very small, about 7 μg. They are easily dispersed by wind, water, animals, and the human population as well as agricultural equipment [38, 39]. Furthermore, cases of infection of crops previously considered to be non-host species by Striga have been reported [40].
Figure 1.2 Germination of parasitic seeds belonging to Broomrapes and enhanced AM colonization by strigolactones exuded from host plant roots in the rhizosphere.
For poor farmers without access to efficient control techniques, the threat posed by parasitic plants is particularly serious, while pesticides, ethylene, methyl bromide, and stringent quarantine regulations were necessary in the United States to successfully eradicate infestations of Striga asiatica [32, 41]. These solutions are frequently unavailable to or prohibitively expensive for African farmers. As an alternative, they turn to less successful techniques like hand weeding, intercropping, and more fertilizer [42]. Farmers are occasionally compelled to give up their land or crops because there are no sustainable alternatives [35]. Therefore, there is a pressing need for novel, approachable strategies to eradicate these parasitic weeds.
Some of the known parasitic plants belonging to the Orobanchaceae family, along with their host plants, are given below in Table 1.1.
Table 1.1 Some known parasitic plants belonging to the Orobanchaceae family and their host plants.
Parasitic plant
Host plant
Type of parasitism
Type of haustoria
Striga
spp. (witchweeds)
Maize, rice, sorghum, millet, sugarcane
Obligate hemiparasiteInfects the roots
Terminal and lateral haustoria
Orobanche
spp. (broomrape)
Tomato, potato, pea, tobacco, sunflower
Obligate holoparasiteInfects the roots
Terminal and lateral haustoria
Triphysaria versicolor
Maize,
Arabidopsis
, rice, tomato
Facultative hemiparasiteInfects the roots
Lateral haustoria
Phtheirospermum
Arabidopsis, Medicago
Facultative hemiparasiteInfects the roots
Lateral haustoria
1.7 Strigolactones and Agriculture: Why Do They Hold the Key?
The Green Revolution introduced us to shorter, high-yield cereal plants that thrive in intensive farming with fertilizers and pesticides. These shorter varieties require less energy for stem growth and allocate more toward producing seeds by reducing their sensitivity to gibberellins or their production [43, 44]. In a future with finite global phosphate reserves and most nitrogen fertilizers requiring vast amounts of fossil fuel, it will become more and more important to breed crop varieties for their ability to scavenge nutrients more efficiently. It could also become necessary to foster symbiotic associations between plants and soil microbes to maximize plant growth. Variation in plant strigolactones opens up opportunities for breeding crop varieties with better nutrient efficiency and/or symbiotic properties [45, 46]. The structure of roots and shoots is controlled by the exuded strigolactones, offering an opportunity for plant breeding improvement. Concurrent efforts can be made to minimize witchweed parasitism, particularly in Africa, where it causes maximum losses to subsistence farmers. This could be achieved through the use of either plant breeding techniques or strigolactone analogs that trigger self-destruction—the so-called suicidal germination—of witchweed seeds before crop planting. In that respect, strigolactone research gives significant promise to finding solutions in connection with the important issues of crop improvement and agricultural management.
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Note
*
Corresponding author
:
2Nature, Structural Diversity, Biosynthetic Pathway, and Strigolactone Transport in Plants
Ishrat Mehmood1*, Kaiser Iqbal Wani2 and Tariq Aftab2
1Department of Biophysics, Panjab University, Chandigarh, India
2Department of Botany, Aligarh Muslim University, Aligarh, India
Abstract
Strigolactones (SLs) are a class of phytohormones with diverse roles in plants which range from regulation of plant growth and development and their interactions with the environment. Their original roles, associated with seed germination of root parasitic plants of broomrapes, were extended later to include the regulation of shoot branching inhibitions, root development, and plant–microbe interactions. The SLs hold great importance in the agricultural sector, particularly in combatting root parasitic weeds via suicidal germination and other means. Recent research has revealed their involvement in inter-plant signaling, adding a layer to their ecological significance. This chapter is about the chemical nature, structural diversity, biosynthetic pathway, and naming protocol of SLs. Canonical SLs comprise a tricyclic lactone ring system coupled to a fourth ring by an enolether bridge. However, the non-canonical SL variants exhibit structural deviations, influencing their biological activities. The naming protocol of different SLs ensures clarity and precision, facilitating communication in scientific conversations. The biosynthetic pathway involves enzymatic steps, including carotenoid cleavage, cyclization, and modification, mediated by enzymes like CCD7, CCD8, MAX1, and D27. Investigations for understanding their nature, diversity, and biosynthesis offer valuable insights and hold the key for agricultural and ecological applications, emphasizing their potential to address global challenges.
Keywords: Strigolactones, biosynthesis, nature, transport, canonical, non-canonical, Striga
2.1 Introduction
Strigolactones (SLs) represent a class of phytohormones that have emerged to take central roles in the complex web of signaling mechanisms controlling various aspects of plant growth, development, and environmental interactions. Derived from carotenoids, they are secreted into the rhizosphere, where they interact with root parasitic plants and symbiotic fungi. These small molecules were primarily identified for their role in stimulating the germination of root parasitic plants but have extended to include many physiological processes not only within host plants but also extending into non-host plants [1]. Their key role which gives them immense significance is their role in the regulation of plant architecture [2]. They participate in shoot branching and root development, senescence, and overall plant morphology. These signaling compounds negatively regulate axillary bud outgrowth, thus inhibiting the formation of lateral branches for the adaptability of the plant to environmental changes [3].
Strigolactones are linked to plant–microbe interactions, particularly in the context of symbiosis and parasitism. In the case of symbiotic associations, these compounds act as signaling molecules in the rhizosphere that attract arbuscular mycorrhizal fungi (AMF) to establish a mutualistic association with plant roots. They act as rhizosphere signaling molecules, attracting AMF hyphae toward the roots and stimulating the branching of fungal hyphae in the soil [4]. On the other side, they facilitate parasitic relationship between certain plants and root parasitic weeds. They arbitrarily facilitate the invasion of host plant roots by inducing parasitic relationships between certain plants and root parasite weeds, leading to nutrient and water deficits.
The significance of SLs extends even further. Recent studies [5] suggest that these compounds mediate immediate inter-plant signaling, acting as allelochemicals that convey information about the local environment. This form of chemical communication allows plants to respond collectively to threats, such as herbivore attacks or impending stress conditions, by adjusting their growth and defence strategies. Also, the historical context of SL discovery is interesting to know.
The story unfolds against a backdrop of scientific curiosity, agricultural challenges, and the pursuit of knowledge that has reshaped our understanding of plant biology. The journey of SLs began in the mid-20th century when researchers were grappling with the enigmatic phenomenon of unwanted seed germination of root parasitic plants [6]. Scientists sought to unravel the mystery of how these parasitic plant species, such as Strigaand Orobanche, detect the presence of host plants and initiate the germination process. The role play between parasitic plants and their hosts posed significant challenges for agriculture, leading researchers to seek answers. In the early 1960s, scientists made a pivotal breakthrough when Italian researchers led by Pietro C. Rubiales identified a compound in root exudates of host plants that stimulated the germination of seeds of parasitic plants which then parasitize the roots of host plants. This compound, later named strigol, marked the first glimpse into the world of SLs. The term “strigolactone” was coined years later when additional related compounds were discovered, broadening the scope of these signaling molecules [7, 8].
The discovery of SLs was not merely an academic pursuit; it held profound implications for agriculture. Root parasitic weeds, witchweeds, or Striga species, were known to cause devastating losses in crop yields, particularly in Africa, where they posed a serious threat to staple crops like sorghum and millet [9]. Understanding the signaling mechanisms behind their germination provided a potential avenue to develop strategies to control these parasitic plants and mitigate agricultural losses. As research on SLs progressed, scientists uncovered their role not only in parasitic plant interactions but also in various other aspects of plant biology.
The 21st century has witnessed an explosion of research on SLs, revealing their involvement in shoot branching, root development, plant–microbe interactions, responses to abiotic stress, and even inter-plant communication. In 2008, the identification of the first SL receptor, DWARF14 (D14), by a collaborative effort led by researchers from the United States and Japan, marked a significant milestone [10, 11]. This discovery provided insights into the molecular mechanisms through which SLs exert their effects on plant growth and development. What began as a quest to understand the mechanisms behind parasitic plant germination evolved into a broader exploration of plant signaling, architecture, and ecological interactions.
2.2 Chemical Nature of Strigolactones
Strigolactones (SLs) constitute a class of sesquiterpene lactones initially identified for their role in promoting seed germination of parasitic weeds, including Striga and Orobanche species. These compounds feature a tricyclic lactone structure (ABC tricyclic ring) linked to a fourth ring (the D ring or α, β-unsaturated furan ring) through an enol-ether bridge. SLs are synthesized from carotenoids, which are themselves made from terpene or isoprene building blocks. Structurally, SLs consist of four rings (A, B, C, and D), with the D-ring being the key characteristic feature. This related group of compounds is defined as substances with a butenolide (D-ring) structure [12, 13].
The fundamental structure of SLs consists of three interconnected rings, designated as rings A, B, and C. A six-membered ring containing an enol, ether, or lactone moiety is known as Ring A, while Ring B is a fivemembered ring typically bearing a ketone functional group. Ring C, the third ring, is also five-membered, connected to ring B by an oxygen atom. Their tricyclic lactone ring structure serves as the molecular basis for a variety of biological activities. A varied tail that extends from ring A is used to identify various SL compounds. Because of this tail’s high sensitivity to modifications, several SL variations with unique biological roles have been identified. The tail consists of a polyenyl side chain, characterized by a series of double bonds. The length and arrangement of these double bonds vary among different SLs, contributing to the specificity of their interactions with receptors and downstream signaling cascades [14].
Ring A
Ring A is a six-membered ring and is characterized by the presence of either lactone or an enol ether moiety. Whereas the lactone configuration has a cyclic ester group, the enol ether configuration has a double bond between carbon atoms, forming an oxygen–carbon double bond. The choice between these two configurations influences the chemical properties of ring A, which, in turn, contributes to SLs’ overall adaptability [12].
Ring B
Ring B is a five-membered ring that has a ketone functional group that is connected to ring A. The biological activity of the SLs is dependent on the ketone group, which also influences how SLs interact with receptors and downstream signaling pathways. The presence of the ketone group in ring B sets SLs apart from other classes of plant hormones and imparts specific chemical properties to this ring, contributing to the overall structure and functionality of the tricyclic lactone ring system [15].
Ring C
Ring C is another five-membered ring connected to ring B by an oxygen atom that completes the tricyclic lactone ring system. One important structural feature that contributes to the stability and conformational flexibility of strigolactones is the linkage between rings B and C. As a connecting element, the oxygen atom influences the molecule’s overall three-dimensional structure and facilitates its linkage with other molecules in biological systems [16].
In addition to being SLs’ structural hallmark, the tricyclic lactone ring system also influences their functional characteristics. The distinct configuration of atoms in this ring system gives SLs certain chemical and physical characteristics that influence their solubility, stability, and reactivity. In addition, the tricyclic lactone ring system plays a vital role in the detection and binding of SLs to their receptors, which starts downstream signaling cascades that control various physiological processes in plants [17].
2.3 Naming Protocol for Strigolactones
Stereoisomers have several stereogenic (chiral) centers; strigol, for instance, contains three of these centers, and there are eight possible stereoisomers. The International Union of Pure and Applied Chemistry (IUPAC) recommends the so-called CIP system, named after the proposers Cahn, Ingold, and Prelog, for the notation of the configuration at a stereogenic center [18]. The sensation of chirality is indicated by the stereochemical descriptors R and S in the CIP system. Strict abstract requirements must be adhered to assign the appropriate description to a particular chiral center. It is frequently challenging to imagine a structure that corresponds with such R and S descriptors since the R, S nomenclature is based on abstract principles. These days, the ChemDraw application makes it easy to produce these descriptions. The majority of scientists want an “at a glance” classification of the perfect stereochemistry. To do this, a notation known as (+)-3aR,5S,8bS,2′R)-strigol is utilized, which describes the stereochemical interaction with a parent reference structure. The prefixes ent, which stands for enantiomer, and epi, which is an acronym for epimer, denote the opposite configuration at a particular atom, indicating the association with (+)-3aR,5S,8bS,2′R)-strigol. Given that (+)-strigol is the first completely studied and verified structure, using it as the reference chemical makes sense. Due to the three stereogenic (chiral) centers in strigol, eight possible stereoisomers exist. Eight stereoisomers of 5-deoxy-4-hydroxy-strigol, also known as orobanchol, are known to exist. These may be named unequivocally using the R and S descriptors or by prefixing the word orobanchol with the relevant information to indicate the relative stereochemistry concerning (+)-strigol [19, 20].
For the ent/epi approach, it is required to select a parent molecule or reference substance. The naming of SLs was straightforward before the structural rectification of orobanchol: the stereochemistry of all other SLs was tied to (+)-strigol, which was the logical parent molecule. However, there were two choices available which either adhere to the naming convention that used (+)-strigol as the parent compound or utilize the newly discovered structure for natural orobanchol as the parent compound for the orobanchol family [21, 22]. Because both approaches are being used, things might get complicated. This approach has few advantages and might be difficult for people who are not familiar with stereochemistry. Using both (+)-strigol and (-)-orobanchol as standards, Scaffidi and colleagues proposed an alternate name and notation in the structural correlation of GR24 stereoisomers. As a result, several stereoisomers were given two names: for example, ent-2′-epi-5-deoxystrigol is also known as 4-deoxyorobanchol [23]. This is a complex way for people who are not as knowledgeable with stereochemical difficulties, and it offers little additional benefit.
2.4 Natural Diversity of Strigolactones
Naturally occurring SLs in plants are carotenoid-derived hormones or their precursors that control a vast array of growth and physio-biochemical processes in plants [24, 25]. Based on the diversity in chemical structures, they can be classified into two major groups:
Strigol and its related compounds containing ABC-ring system connected to the methyl butenolide D-ring through an enol-ether bridge are called
canonical SLs
(
Figures 2.1
and
2.2
).
Certain germination stimulant compounds which lack the ABC ring system but possess the D-ring have been characterized, and such compounds are called
non-canonical SLs
(
Figure 2.3
).
Through purification bioassays, approximately 25 canonical SLs have been identified and isolated from root exudates thus far [24, 26]. Except for 5-deoxystrigol, which was first described as the hyphal branching stimulant for AM fungi [4], the majority were identified as germination stimulants of root parasitic weeds. Carlactone (CL) [27] is the most basic non-canonical SL and acts as a common precursor for the biosynthesis of SL. The root exudates of a number of plants have been shown to contain carlactonoic acid (CLA), an oxidized metabolite of CL [28, 29]. Non-canonical SLs participate in chemical communications in the rhizosphere along with canonical SLs.
Figure 2.1 Chemical structures of some representative strigol-type strigolactones.
Figure 2.2 Chemical structures of some representative orobanchol-type strigolactones. Note: While drawing the molecular structures, stereochemistry is shown by plain lines connecting carbon atoms in the plane of the page. The wedges project out of the plane of the page, with dashes projected into the plane of the page.
Figure 2.3 Chemical structures of some representative non-canonical strigolactones.
2.4.1 Canonical Strigolactones
Canonical strigolactones (SLs) are classified into two categories based on the stereochemistry of the C-ring: strigol-type (illustrated in Figure 2.1) and orobanchol-type (illustrated in Figure 2.2) SLs, with β- and α-oriented C-ring stereochemistry, respectively [25]. Typically, plants predominantly produce either strigol-type or orobanchol-type SLs; however, certain species, such as tobacco, synthesize both types [30]. Gymnosperms like Gingko biloba, Pinus thunbergii, Cryptomeria japonica, and the lycophyte Selaginella moellendorfi have been reported to produce orobanchol-type SLs [26, 29]. While many angiosperms are also orobanchol-type SL producers, some, including cotton and strawberry, exclusively produce strigol-type SLs. Within the Poaceae family, sorghum is a strigol-type SL producer, whereas rice produces orobanchol-type SLs [11]. It has recently been demonstrated that some cultivars of sorghum resistant to Striga produce orobanchol, a powerful branching stimulant for AM fungus but a feeble germination stimulant for Striga[31], although these cultivars are resilient to Striga infestation but still act as suitable hosts for AM fungi. The chemical structures of some of the common canonical strigolactones are shown in Figure 2.1 (strigol type) and Figure 2.2 (orobanchol type).
2.4.2 Non-Canonical Strigolactones
Despite reports that strigol is exuded by maize, a primary host of Striga, Jamil and others [32] used LC–MS/MS analysis to detect non-canonical SLs, including strigol, in the maize root exudates. Two new germination stimulants, SL1 and SL2, were reported instead. Likewise, no known canonical SL that potently stimulates the germination of broomrapes has been found in the root exudates of Avena strigosa[33]. A great deal of work has been done to identify and elucidate the germination stimulants generated by black oats and maize. Two non-canonical SLs, zealactone [34] and avenaol [33], have been extracted from their respective root exudates.
The stereochemical structure of avenaol, which has been reported in A. strigosa root exudates, has been elucidated by total synthesis. It has shown strong activity on Phelipanche ramosa. It is produced from precursor CL via hydroxylation, elimination, and cyclization (Figure 2.4). The non-canonical SLs, although lacking the A, B, or C rings, nevertheless have the enol-ether-D ring moiety, which is obligatory for SLs to function biologically. Several non-canonical SLs have been identified as being produced by maize; among them, SL2 is zealactone [35], while SL1 is a combination of at least two isomers. Canonical SLs are not known to be produced by sunflower plants. Heliolactone, a non-canonical SL, has been extracted from sunflower root exudates, another Orobanche host. However, non-canonical SLs rapidly break down during purification and storage and are thus less stable than their canonical counterparts.
The first non-canonical SL to be reported is carlactone, an SL biosynthetic precursor. The amount of CL in plant tissues appears to be fairly high compared with canonical SLs and is unaffected by phosphate limitation, which indicates that the response to phosphate deficiency is regulated in steps after CL formation [36]. The exudates from the roots of several plants have been shown to contain CLA, an oxidized metabolite of CL [37]. Based on these findings, CLA likely plays a role in rhizosphere chemical communications with neighboring plants or microbes.
Figure 2.4 Conversion of carlactone into avenaol.
The simplest canonical SL, 5DS, and a non-canonical SL, lotuslactone, are both exuded by Lotus japonicus, a model legume plant [38]. Except for acetoxyl derivatives, which have two extra carbons, such as orobanchyl acetate, and sorgolactone, a C18 SL, most canonical SLs are C19 compounds. Non-canonical SLs, on the other hand, are C20 compounds because they have an extra carbon atom (excluding CL and CLA). Methyl carlactonoate (MeCLA), its isomers, and their hydroxyl derivatives appear to be the source of these non-canonical SLs as this extra carbon most likely originates from the ester methyl group [28]. Plants of the Solanaceae family, including potato, tomato, aubergine, and tobacco, release many isomers of presumptive didehydro-orobanchol in addition to the canonical and non-canonical SLs [16, 28, 39]. The structures of these isomers are yet unknown and need to be elucidated.
2.5 Biosynthetic Pathway of SLs
Strigolactones are biosynthesized via the carotenoid pathway, as demonstrated by studies using carotenoid biosynthetic inhibitors and carotenoid-deficient mutants. These observations were made before SLs were identified as a novel class of plant hormones influencing shoot branching. Afterward, a group of recessive mutants with increased shoot branching allowed researchers to gain a much better understanding of SL biosynthesis. These mutants, which include pea ramosus (rms), rice dwarf/high-tillering dwarf (d/htd), petunia decreased apical dominance (dad), and Arabidopsis more axillary growth (max), were intended to find a novel shoot branching regulator substitute for conventional hormones like auxin and cytokinin. The Carotenoid Cleavage Dioxygenase 7 (CCD7) and CCD8 mutants were identified as SL-deficient. These included pea rms1 and rms5, rice d17 and d10, and Arabidopsis max3 and max4 mutants [10, 11]. Then, it was discovered that rice D27 and Arabidopsis AtD27, two new iron-binding proteins, were involved in the production of SL. In 2012, Alder et al. [40] used recombinant D27, CCD7, and CCD8 proteins from rice, peas, and Arabidopsis to conduct an excellent in vitro biochemical investigation. They demonstrated how these three enzymes sequentially convert all-trans-β-carotene via 9-cis-β-carotene and 9-cis-β-apo-10′-carotenal to CL [27] (Figure 2.5). A thorough evaluation of the catalytic properties of CCD7, CCD8, and D27 enzymes was recently conducted [40]. Eventually, an LC–MS/MS study confirmed that CL was the endogenous molecule in rice and Arabidopsis[36]. Furthermore, feeding studies using stable isotope-labeled CL showed that CL is transformed into orobanchol and 4-deoxyorobanchol (4DO), two endogenous classical SLs in rice, indicating that CL represents an intermediary of SL production in vivo
