UV-B Radiation E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 726

Veröffentlichungsjahr: 2017

Beschreibung

Ultraviolet-B (UV-B) is electromagnetic radiation coming from the sun, with a medium wavelength which is mostly absorbed by the ozone layer. The biological effects of UV-B are greater than simple heating effects, and many practical applications of UV-B radiation derive from its interactions with organic molecules. It is considered particularly harmful to the environment and living things, but what have scientific studies actually shown?

UV-B Radiation: From Environmental Stressor to Regulator of Plant Growth presents a comprehensive overview of the origins, current state, and future horizons of scientific research on ultraviolet-B radiation and its perception in plants. Chapters explore all facets of UV-B research, including the basics of how UV-B's shorter wavelength radiation from the sun reaches the Earth's surface, along with its impact on the environment's biotic components and on human biological systems. Chapters also address the dramatic shift in UV-B research in recent years, reflecting emerging technologies, showing how historic research which focused exclusively on the harmful environmental effects of UV-B radiation has now given way to studies on potential benefits to humans. Topics include:

• UV-B and its climatology
• UV-B and terrestrial ecosystems
• Plant responses to UV-B stress
• UB- B avoidance mechanisms
• UV-B and production of secondary metabolites
• Discovery of UVR8

Timely and important, UV-B Radiation: From Environmental Stressor to Regulator of Plant Growth is an invaluable resource for environmentalists, researchers and students who are into the state-of-the-art research being done on exposure to UV-B radiation.

Leseprobe

Table of Contents

Cover

Title Page

List of Contributors

Preface

1 An Introduction to UV‐B Research in Plant Science

1.1 The Historical Background

1.2 Biologically Effective Irradiance

1.3 UV‐B‐induced Effects in Plants

1.4 Conclusion and Future Perspectives

Acknowledgements

References

2 Stimulation of Various Phenolics in Plants Under Ambient UV‐B Radiation

2.1 Introduction

2.2 UV‐B Radiation

2.3 Phenolics

2.4 UV‐B Radiation Stimulates Phenolic Induction

2.5 UV‐B‐Induced Photomorphological Responses

2.6 Photosynthesis Under UV‐B Radiation

2.7 UV‐B Radiation Induces Phenolics Accumulation in Fruits

2.8 Conclusion and Future Perspectives

References

3 UV‐B Radiation: A Reassessment of its Impact on Plants and Crops

3.1 Introduction

3.2 Plant Production

3.3 Plant Protection Against UV‐B

References

4 Interaction of UV‐B with the Terrestrial Ecosystem

4.1 Introduction

4.2 Growth and Development

4.3 Secondary Metabolites

4.4 Susceptibility to Herbivorous Insects

4.5 Plant Sexual Reproduction

4.6 Genomic Level

4.7 Conclusion

References

5 A Review on Responses of Plants to UV‐B Radiation Related Stress

5.1 Introduction

5.2 Morphological and Yield Response to UV‐B

5.3 Targets of UV‐B in the Carbon Fixation Cycle

5.4 Photoreceptors and Signalling Pathway in Response to UV‐B Radiation

5.5 Acclimatization and Protection in Response to UV‐B

5.6 Oxidative Stress and Antioxidant System in Response to UV‐B

5.7 DNA Damage and Repair Mechanism

5.8 Exclusion of UV Components: Experimental Approach to Study the Effect on Plants

5.9 Conclusion and Future Perspectives

Acknowledgement

References

6 Oxidative Stress and Antioxidative Defence System in Plants in Response to UV‐B Stress

6.1 Introduction

6.2 Plant Protection Against UV Radiation

6.3 UV‐B and ROS

6.4 UV‐B and Antioxidant Enzymes

6.5 UV‐B and Antioxidant

6.6 UV‐B and Signalling

6.7 Conclusion and Future Perspectives

References

7 Major influence on phytochrome and photosynthetic machinery under UV‐B exposure

7.1 Introduction

7.2 Photomorphogenesis in Higher Plants

7.3 Effect of UV‐B Exposure on Photosynthetic Machinery

7.4 Conclusion and Future Perspectives

References

8 UV‐B Radiation‐Induced Damage of Photosynthetic Apparatus of Green Leaves: Protective Strategies

vis‐a‐vis

Visible and/or UV‐A Light

8.1 Introduction

8.2 UV‐B Effects on the Photosynthetic Apparatus of Leaves

8.3 UV‐A Effects on Photosynthetic Apparatus of Leaves (Damage and Promotion)

8.4 UV‐A‐Mediated Modulation of UV‐B‐Induced Damage

8.5 PAR‐Mediated Balancing of UV‐B‐Induced Damage

8.6 Photosynthetic Adaptation and Acclimation to UV‐B Radiation

8.7 Corroboration with Sensible Approach

8.8 Conclusion

Acknowledgements

References

9 Ultraviolet Radiation Targets in the Cellular System: Current Status and Future Directions

9.1 Introduction

9.2 Absorption Characteristics of Biomolecules

9.3 Action Spectrum

9.4 Targets of UV‐B

9.5 The Photosynthetic Machinery

9.6 Cell Division and Expansion

9.7 Conclusion and Future Perspectives

Acknowledgements

References

10 Silicon: A Potential Element to Combat Adverse Impact of UV‐B in plants

10.1 Introduction

10.2 The role of Silicon Against UV‐B Exposure on Morphology of Plants

10.3 The defensive role of silicon against UV‐B exposure on physiological and biochemical traits of plants

10.4 Silicon repairs anatomical structures of plants damaged by UV‐B exposures

10.5 UV‐B‐induced oxidative stress and silicon supplementation in plants

10.6 Silicon supplementation and the status of antioxidant enzymes in plants exposed to UV‐B

10.7 Silicon and level of phenolic compounds under UV‐B stress

10.8 Conclusion and future Perspectives

References

11 Sun‐Screening Biomolecules in Microalgae: Role in UV‐Photoprotection

11.1 Introduction

11.2 Global Climate Change and UV Radiation

11.3 Effects of UV Radiation on Microalgae

11.4 UV‐induced Defence Mechanisms

11.5 Sun‐Screening Biomolecules as Key UV Photoprotectants

11.6 UV‐Induced Biosynthesis

11.7 Photoprotective Function

11.8 Conclusion

Acknowledgements

References

12 Plant Response: UV‐B Avoidance Mechanisms

12.1 Introduction

12.2 Ultraviolet Radiation: Common Source, Classification and Factors

12.3 UV‐B and Human Health

12.4 UV‐B and Plant Responses

12.5 UV‐B Avoidance and Defence Mechanism

12.6 UV‐B and its Significance

12.7 Conclusion and Future Perspectives

Acknowledgments

References

13 Impact of UV‐B Exposure on Phytochrome and Photosynthetic Machinery: From Cyanobacteria to Plants

13.1 Introduction

13.2 Effect of UV‐B Irradiation on Photosynthetic Machinery of Cyanobacteria

13.3 Effect of UV‐B Irradiation on Photosynthetic Machinery of Algae

13.4 Effect of UV‐B Irradiation on Photosynthetic Machinery of Higher Plants

13.5 Conclusion and future perspectives

Acknowledgements

References

14 Discovery of UVR8: New Insight in UV‐B Research

14.1 Introduction

14.2 Photoperception in Plants

14.3 Discovery of UVR8: UV‐B Photoreceptor

14.4 UVR8 Structure

14.5 Physiological Roles of UVR8

14.6 Conclusion and Future Perspectives

References

15 UVR8 Signalling, Mechanism and Integration with other Pathways

15.1 Introduction

15.2 UVR8‐Arbitrated Signalling

15.3 Molecular Mechanism of Photoreceptor‐Mediated Signalling

15.4 UVR8 Involvements in Different Pathways

15.5 Conclusion and Future Perspectives

Acknowledgements

References

Index

End User License Agreement

List of Tables

Chapter 01

Table 1.1 Regions of the electromagnetic spectrum together with colours, modified from Iqbal (1983) and Eichler

et al

. (1993).

Chapter 02

Table 2.1 An overview of recent publications (since 2000) reporting the accumulation of phenolics induced by UV‐B radiation in different plant species and organs (leaf, callus, fruit). Estimated biologically effective UV doses (UV‐B

BE

, kJ m

–2

day

–1

), PAR (μmol m

–2

s

–1

) and experiment duration are presented where available. The experiments were performed in the temperature range 20 ± 5 °C, unless otherwise indicated.

Table 2.2 Changes in the content of phenolic compounds (determined as aglycones) in

Ramonda serbica

leaves exposed to moderate UV‐B

BE

for five days.

Chapter 09

Table 9.1 Showing the effects of UV radiation on photosystem II with the help of different variables of OJIP transient.

Chapter 10

Table 10.1 UV‐B induced impacts on plants.

Table 10.2 Impact of silicon application in plants exposed to UV‐B radiation.

Chapter 11

Table 11.1 Occurrence of some common UV‐absorbing MAAs in different microalgae.

Chapter 12

Table 12.1 List of treatment and genes affected by UV radiation (Courtesy of Morales

et. al.

, 2014).

Chapter 14

Table 14.1 Physiological responses mediated by UVR8.

List of Illustrations

Chapter 01

Figure 1.1 Absorption spectra of protein and DNA at equal concentrations

Chapter 02

Figure 2.1 Global erythemal UV index in 2015 (http://www.temis.nl/uvradiation/world_uvi.html).

Figure 2.2 Chemical structures of the main sub‐classes of phenolic compounds.a) Common hydroxybenzoic and hydroxycinnamic acids, and umbeliferon.

p

‐HBA,

p

‐hydroxybenzoic acid; PrcA, protocatechuic acid, VA, vanillic acid; GA, gallic acid; SyA, syringic acid;

p

‐CA,

p

‐coumaric acid; CA, caffeic acid; FA, ferulic acid; SA, sinapic acid.b) Representatives of flavonoids with labelled A, B and C rings in the eriodictyol structure.c) Representatives of stilbenes, lignans and presentation of a part of lignin polymer from poplar.

Figure 2.3 HPLC chromatograms recorded at 340 nm, showing sunlight‐induced accumulation of flavonoids in bamboo (a) and linden (b) leaf hydrolyzed methanol extracts.1, homoorientin; 2, luteolin; 3, tricin; 4, quercetin; 5, kaempferol.Grey—sunlight; black—shade.

Figure 2.4 Schematic overview of the link between photosynthesis, sugar content and phenolic induction under ecologically relevant UV‐B doses, in green and white leaf sectors of variegated (a)

P. coleoides

and (b)

P. zonale

. Arrow directions indicate increased or reduced concentration of specific metabolite. Dotted arrows represent transport between source and sink leaf tissue. (c) Photographs of representative leaves of

P. coleoides

(left) and

P. zonale

(right) plants after exposure to: UV‐B

BE

: 7.0 kJ m

–2

day

–1

combined with 48.8 mol m

–2

day

–1

of PAR (HL). Tre, trehalose; Glc, glucose; Fru, fructose; Suc, sucrose; Gal, galactose; HBAs, hydroxybenzoic acids; ECat, epicatechin;

A

, CO

2

assimilation rate; ETC, linear electron transport chain. For detailed results, see Vidović

et al

. (2015b, 2015c).

Chapter 05

Figure 5.1 Plants’ response to ambient UV‐B radiation at cellular and molecular level (schematic presentation), emphasising the effect of ambient UV‐B radiation on major cell organelles like Chloroplast (A), Nucleus (B), and Mitochondria (C).(

A

) Chloroplast: UV‐B radiation causes reduction in photosynthesis due to loss of thylakoid membrane integrity, chlorophyll pigments and downregulation of genes associated with photosynthesis. UV‐B targets several components of Z scheme in chloroplast (text in red). Abbreviations: Mn‐ manganese complex containing four Mn atoms, bound to Photosystem II (PSII) reaction centre; Tyr‐ tyrosine in PSII; O

2

‐ oxygen; H

+

‐ protons; P680 (Primary electron donor)‐ PSII reaction centre in chlorophyll (Chl), on receiving a photon of light, P680 gets excited to P680*; Phe‐ pheophytin molecule, primary electron acceptor of PSII; Q

A‐

plastoquinone molecule; Q

B

‐ loosely bound plastoquinone molecule to PSII; FeS‐ Rieske iron sulphur protein; Cyt.f‐ Cytochrome f; Cytb6(L & H) for Cytochrome b6 (low & high Energy); PC‐ copper protein plastocyanin; P700‐ primary electron donor of PSI excited to P700* by absorbing energy; Ao‐ chlorophyll molecule and primary electron acceptor of PSI; A1‐ phylloquinone

Figure 5.1

?>(continued) (Vitamin K) molecule; FX, FA, and FB‐ three separate iron sulphur centres; FD‐ ferredoxin; FNR‐ Ferredoxin‐NADP‐oxido‐Reductase(FNR); NAD

+

‐Nicotinamide adenine dinucleotide; NADPH is the reduced form of NADP

+

.(

B

) Nucleus: Plants sense UV‐B through UVR8 photoreceptor that activates a UVR8‐dependent photo‐morphogenesis, signalling leads to interaction with the E3 ubiquitin ligase COP1 and stabilization of the bZIP transcription factor HY5 that transmits the UV‐B signal resulting in changes in gene expression, which further leads to encoding of proteins helps in UV protection by (1) increasing level of DNA repair enzymes (e.g. photolyases) can act on CPDs and 6‐4 PPs lesions (2) Increased anti‐oxidative proteins (antioxidants) can act as ROS scavengers and (3) Increased level of UV‐absorbing sunscreens gives acclimation response Abbreviations: HY5‐Elongated hypocotyl 5; UVR8‐ UV resistant locus 8; COP1‐constitutive photomorphogenic 1; ROS‐ Reactive oxygen species.(

C

) Mitochondria: In the presence of UV‐B, mitochondrial DNA (mtDNA) becomes photoactivated, and UV‐induced polymerase (DNA repair enzyme CPD photolyase) helps in repairing of DNA; also, UV radiation causes release of ROS from the electron transport chain, resulting in membrane lipid and protein oxidation. Abbreviations: ROS‐Reactive oxygen species; Cyclobutane pyrimidine dimer (CPD)].

Chapter 06

Figure 6.1 Targets of ROS generated by UV‐B stress.

Figure 6.2 UV‐B‐induced ROS generation followed by Asada‐Halliwell pathway of oxyradicals scavenging and involvement of various antioxidant enzymes.

Chapter 07

Figure 7.1 UV‐B‐mediated dose‐dependent response of higher plants.

Figure 7.2 Photoreceptors‐mediated signalling in higher plants.

Figure 7.3 Schematic representation for signalling of low fluence rate of UV‐B through UVR8. COP1, Constitutively Photomorphogenic 1; HY5, Elongated Hypocotyl 5; UV‐B, ultraviolet‐B radiation; UVR8, UV Resistance Locus 8 (Jenkins, 2009).

Figure 7.4 Different centres of electron transport chain susceptible to UV‐B damage

Chapter 08

Figure 8.1 A schematic representation showing different sites of damage in the PSA of green leaves, as induced by UV‐B radiation. The scheme depicts the arrangement of major protein complexes of PSII, PS I, cyt b6/f complex, oxygen‐evolving complex and components of electron transport chain within the thylakoid membrane. The sites of damage are indicated by the thick arrows.

Scheme 8.1 A model schematizing the defence mechanisms exhibited through visible/UV‐A light by green leaves to counter UV‐B‐induced damages. [For color representation in this figure legend, please refer to the online version of this book.] Inactivation of PSA occurs due to damage to DNA, PS II (OEC, reaction centre II proteins, Q

A

, Q

B

electron acceptors, electron transport between PSII) and PSI and thylakoid disorganization. The red arrow indicates the damaging path of PSA. Green leaves respond to UV‐B assault in two distinctly different modes:i. Educing defence responses, comprising of: (a) adaptation (through promotion of anthocyanin and flavonoid synthesis that filter out UV‐B radiation and activation of gene expression for D1 & D2 reaction centre protein); (b) repair and reactivation (activating the enzyme photolyase to repair the damaged DNA and diminishing the degree of DNA damage); (c) enzymatic (activity of super oxide dismutase, catalase ascorbate peroxidase, glutathione reductase, monodehydroascorbate reductase, dehydroascorbate reductase, guaiacol peroxidase, and glutathione‐S‐transferase) and non‐enzymatic (ascorbic acid, reduced ascorbate, reduced glutathione, α‐tocopherol, β‐carotene, carotenoids and xanthophyll cycle) defence;ii. Photomorphogenic responses involving phytochrome and/or blue/UV‐A photoreceptor, which act at the level of development and protection, and overlap with the mechanism of adaptational responses. The green arrow indicates the mechanism involved in protecting different sites of damage.

Chapter 09

Figure 9.1 Absorption spectra of DNA and a protein at equal concentrations

Figure 9.2 Diagram showing the regulation of cyclobutane‐pyrimidine dimer (CPD) photoreactivation. Transcription of genes encoding photolyases is minimal in the dark, but is induced by UV‐A and PAR, possibly involving a UV‐A/B photoreceptor.

Figure 9.3 Mechanisms of oxidation of Trp by direct UV absorption and

1

O

2

‐mediated pathways. Key to structures (where the

a

‐amino acid moiety is represented by R CH(CO

2

H)NH

2

):

1

, indolyl radical;

2

, C‐3 peroxyl radical;

3

, C‐3 hydroperoxide;

4

, dioxetane intermediate;

5

, C‐3 alcohol;

6

, 3α‐hydroperoxypyrroloindole;

7

, 3α‐hydroxypyrroloindole;

8

,

N

‐formylkynurenine;

9

, kynurenine;

10

, 3α‐dihydroxypyrroloindole.

Figure 9.4 Oxidation pathways for Tyr via direct UV absorption or

1

O

2

production. Key to structures (where the a‐amino acid moiety is represented by R CH(CO

2

H)NH

2

):

1

, tyrosyl radical;

2

, a C–O linked isomer of a dityrosine crosslink;

3

, Tyr endoperoxide;

4

, HOHICA (3α‐hydroxy‐6‐oxo‐2,3,3α,6,7,7α‐hexahydro‐1

H

‐indol‐2‐carboxylic acid).

Figure 9.5 Direct UV absorption by Phe leads to formation of the triplet state and subsequent radical formation, or photo‐ionization, followed by hydration, to form Tyr isomers.

Figure 9.6 Reaction pathways for

1

O

2

‐mediated oxidation of His residues (where the α‐amino acid moiety is represented by R CH(CO

2

H)NH

2

).

Figure 9.7 Schematic presentation of photosystem II (PSII), indicating photosensitizers proposed to be involved in its UV‐B‐mediated inactivation. P

680

is the primary electron donor of PSII. Z is redox active tyrosine located on the D1 and D2 proteins, respectively; Z normally serves as the electron donor to P680. Electrons originate from water, the splitting of which is catalysed by a cluster of four manganese atoms. Extrinsic proteins are involved in stabilizing this reaction. On the acceptor site, a pheophytin (Pheo) serves as the primary electron acceptor. The plastoquinones, Q

A

and Q

B

, are the secondary electron acceptors.

Figure 9.8 Radar plot of JIP test parameters from 45° fixed angle leaves. F

O

is the minimal and F

M

is the maximal fluorescence. F100 μs, F300 μs, F

J

and F

I

is the fluorescence measured after 100 μs, 300 μs, 2 ms and 30 ms. Area is the area under induction curve. Derived parameters are: quantum yields for trapping TRo/ABS, dissipation DIo/ABS, electron transport ETo/ABS and reduction offend acceptors REo/ABS. ABS is the absorption energy flux; CS is the excited cross‐section of the leaf sample; TR is the excitation energy flux trapped by the RC and utilized for the reduction of Q

A

to Q

A

; DIo is the dissipation energy flux at the level of the antenna chlorophylls; ETo is the flux of electrons from QA

−

into the intersystem electron transport chain; RC is the PSII reaction centre; RC/CS is the concentration of RC per excited CS of leaf sample; PIabs and PICSm are the performance index on absorption basis and sample per cross section

Figure 9.9 Showing the overall effect of UV‐B on the cellular system.

Chapter 10

Figure 10.1 Stress responses in plant organelles against ambient UV‐B radiations (modified after Nawker

et al

., 2013; de Andrade

et al

., 2015; Michaeli and Fromm, 2015). The figure describes that, after exposure to UV‐B radiation, major alterations occur in the organelles of the plant cell. As indicated in the diagram, the photoreceptor for UV‐B radiation is called as UVBR8, which is generally found in dimeric form. When UVBR8 enters into the nucleus, its monomeric form interacts with COP1 (Constitutive Photomorphogenic 1) and forms a complex of UVBR8‐COP1. This complex blocks the expression of UV‐B induced genes. However, HY5 (Elongated Hypocotyl 5) blocks the expression of flavonoid synthase and chalcone synthase pathways. A cascade of mitogen‐activated protein kinases (MAPK) is also activated from exposure to UV‐B stress and ultimately leads to PCD. In the chloroplast, an excessive amount of electrons is released and hence elevates the amount of ROS. Cytochrome C is released from the mitochondrial transmembranes, activating the activity of caspase and finally causing DNA laddering.? denotes that the photoreceptor for UV‐B is unknown, its function in the UVR8 stress pathway is unclear and the roles of AtDAD1, AtDAD2 and AtBI are still in doubt.

Figure 10.2 Model illustrating the overall damaging effects posed by UV‐B (enhanced or ambient) on higher plants at multiple level.

Chapter 11

Figure 11.1 A generalized diagrammatic presentation of ozone depletion caused by anthropogenically released chlorofluorocarbons

Figure 11.2 A simplified view for effects of UV radiation and probable defence mechanisms adopted by microalgae

Figure 11.3 Effects of UV radiation on relative electron transport rate (A) and phycobiliproteins phycocyanin (PC) and phycoerythrin (PE) in the cyanobacteria

Anabaena variabilis

PCC7937 and

Lyngbya

sp. A09DM, respectively

Figure 11.4 Chemical structure of some common mycosporine‐like amino acids reported in microalgae.

Figure 11.5 Occurrence of scytonemin in the extracellular polysaccharide sheath of

Lyngbya

sp. (

A

) shows the UV‐absorption maximum at 386 nm (for details, see Rastogi and Incharoensakdi, 2014a)

Figure 11.6 UV‐induced biosynthesis of shinorine (A) and scytonemin (B) in the cyanobacteria

Gloeocapsa

sp. CU2556 and

Lyngbya

sp. CU2555, respectively

Chapter 12

Figure 12.1 Plant photoreceptors and their absorption in the solar spectrum. Regions of maximal absorption are indicated by solid lines, while sensitivity towards other wavelengths is indicated with dashed lines

Figure 12.2 Epidermal and anatomical characteristics of first fully expanded leaves of

Vigna unguiculata

(L.). (

A

) shiny adaxial surface under UV‐B; (

B

) UV‐B adaxial – brittle and dead; (

C

) UV‐B adaxial – multiseriate epidermis; (

D

) UV‐B adaxial – broken trichome.

Figure 12.3 Epidermal and anatomical characteristics of first fully expanded leaves of

Vigna unguiculata

(L.). (

A

) control adaxial – normal stomata; (

B

) UV‐B adaxial – abnormal stomata; (

C

) control abaxial – normal stomata; (

D

) UV‐B abaxial – abnormal stomata.

Figure 12.4 Transverse section of potato leaves. In comparison with control (

A

), UV‐B exposed leaf (

B

) appeared thicker, but the gross anatomy was maintained (scale bar for

A

and

B

: 1 μm).

Figure 12.5 Heat map comparing the expression of solar UV‐induced genes in wild‐type Ler exposed for 12 hours to solar UV, with microarray data of photoreceptor mutants available in the

Genevestigator

database (Hruz

et al

., 2008). The gene expression responses are calculated as log2 ratios between the signal intensities from treated genotypes vs. controls. Red and green colours are used to indicate upregulation and downregulation of genes, respectively.

Figure 12.6 Mechanism of DNA damage and repair

Figure 12.7 Diagram showing UV‐B‐induced changes in leaf and plant morphology. Part (a) is the control; part (b) is a plant exposed to supplementary UV‐B

Figure 12.8 Possible role of pectin as a redox regulator during UV‐B response in cells of the root apex transition zone. AsA indicates ascorbic acid; for other details, see the text above

Figure 12.9 Multi equilibria of anthocyanins in aqueous solution, illustrated with pelargonin.

Figure 12.10 Molecular structures of pelargonin, cyanin and malvin.

Chapter 13

Figure 13.1 A generalized model for UV‐B irradiation effects on photosynthetic machinery.

Chapter 14

Figure 14.1 UVR8 structure and distinct groups of Trps.(

A

) The arrangement of all UVR8 tryptophan residues (Trp) (except W400) in the monomer (side view). Trps in the protein core and at the dimer interaction surface are shown in blue and red, respectively.(

B

) The Trps in the core (dimer interaction surface view). Each Trp is associated with a different propeller blade (numbered).(

C

) The Trps at the dimer interaction surface. (Triad‐magenta)(

D

) Two pyramid clusters of excitonically coupled Trps in UVR8 dimer, each consisting of the triad Trps together with W94 on the opposing monomer.

Chapter 15

Figure 15.1 Diagrammatic representation of UVR8‐mediated UVB perception, signalling and physiological responses

Figure 15.2 Working model of molecular mechanism of UVB perception, depicting formation of monomer from UVBR8 dimer, followed by interaction with the WD40 domain of COP1 (violet colour) and C terminal C27 amino acid fragment of UVR8 (blue colour). UVR8‐COP1 complex stabilizes the bZIP transcription factor HY5 (orange in colour), which further induces expression of RUP1 and RUP2 genes (pink in colour) and leads to feedback regulation (represented by red cross)

Figure 15.3 Hypothetical model depicting integration of UVR8 with other metabolic pathways leading to different physiological responses.

Guide

Cover

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UV‐B Radiation: From Environmental Stressor to Regulator of Plant Growth

Edited by

Vijay Pratap Singh

Chhattisgarh, India

Samiksha Singh

Allahabad, India

Sheo Mohan Prasad

Allahabad, India

Parul Parihar

Allahabad, India

This edition first published 2017 © 2017 by John Wiley & Sons Ltd

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Raja Ramanuj Pratap Singhdev (1901–1954)

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List of Contributors

Chhavi AgrawalMolecular Biology SectionCentre of Advanced Study in BotanyBanaras Hindu UniversityVaranasi, IndiaNamira ArifDD Pant Interdisciplinary Research Lab Department of BotanyUniversity of AllahabadAllahabad, IndiaNeelam AtriMMV, Banaras Hindu University Varanasi, IndiaGausiya BashriRanjan Plant Physiology and Biochemistry Laboratory Department of BotanyUniversity of AllahabadAllahabad, IndiaSoumya ChatterjeeDefence Research Laboratory, DRDO Tezpur, Assam, IndiaAntra ChatterjeeMolecular Biology SectionCentre of Advanced Study in BotanyBanaras Hindu UniversityVaranasi, IndiaDevendra Kumar ChauhanDD Pant Interdisciplinary Research LabDepartment of BotanyUniversity of AllahabadAllahabad, IndiaSibnarayan DattaDefence Research Laboratory, DRDO Tezpur, Assam, IndiaFarah DeebaPlant Ecology & Environmental Science CSIR‐National Botanical Research InstituteRana Pratap MargLucknow, IndiaNawal Kishor DubeyCenter of Advanced Studies Department of Botany Banaras Hindu University Varanasi, IndiaSunil K GuptaPlant Ecology & EnvironmentalScience CSIR‐National Botanical ResearchInstituteRana Pratap MargLucknow, IndiaAran IncharoensakdiLaboratory of Cyanobacterial BiotechnologyDepartment of BiochemistryFaculty of ScienceChulalongkorn UniversityBangkok, ThailandJuhie JoshiPhotobiology LabSchool of Life SciencesDAVVIndore, IndiaPadmanava JoshiFormerly Reader in Physics‐cum‐PrincipalAnchal CollegePadampur, RajborasambarBargarh, Odisha, IndiaSonja Veljović JovanovićInstitute for Multidisciplinary Research, University of Belgrade, Kneza Višeslava, 1, 11000, Belgrade, SerbiaSunita KatariaPhotobiology LabSchool of Life SciencesDAVVIndore, IndiaDatta MadamwarBRD School of Biosciences Vadtal Road, Satellite CampusSardar Patel UniversityVallabh Vidyanagar, AnandGujarat, IndiaRohit Kumar MishraRanjan Plant Physiology and Biochemistry LaboratoryDepartment of BotanyUniversity of AllahabadAllahabad, IndiaFilis MorinaInstitute for Multidisciplinary Research University of Belgrade, Kneza Višeslava, 1, 11000, Belgrade, SerbiaVivek PandeyPlant Ecology & Environmental ScienceCSIR‐National Botanical Research InstituteRana Pratap MargLucknow, IndiaParul PariharRanjan Plant Physiology and Biochemistry LaboratoryDepartment of BotanyUniversity of AllahabadAllahabad, IndiaSheo Mohan PrasadRanjan Plant Physiology andBiochemistry LaboratoryDepartment of BotanyUniversity of AllahabadAllahabad, IndiaLC RaiMolecular Biology SectionCentre of Advanced Study in BotanyBanaras Hindu UniversityVaranasi, IndiaRuchi RaiMolecular Biology SectionCentre of Advanced Study in BotanyBanaras Hindu UniversityVaranasi, IndiaShweta RaiMolecular Biology SectionCentre of Advanced Study in Botany Banaras Hindu UniversityVaranasi, IndiaRajesh P RastogiBRD School of BiosciencesVadtal Road, Satellite CampusSardar Patel UniversityVallabh Vidyanagar, AnandGujarat, IndiaSonia SenMolecular Biology SectionCentre of Advanced Study in BotanyBanaras Hindu UniversityVaranasi, IndiaMarisha SharmaPlant Ecology & Environmental ScienceCSIR‐National Botanical Research InstituteRana Pratap MargLucknow, IndiaSonika SharmaDefence Research Laboratory DRDO, Tezpur Assam, IndiaAlok Kumar ShrivastavaMolecular Biology SectionCentre of Advanced Study in BotanyBanaras Hindu UniversityVaranasi, IndiaAnita SinghRanjan Plant Physiology and Biochemistry LaboratoryDepartment of BotanyUniversity of AllahabadAllahabad, IndiaMPVVB SinghRanjan Plant Physiology and Biochemistry LaboratoryDepartment of BotanyUniversity of AllahabadAllahabad, IndiaRachana SinghRanjan Plant Physiology and Biochemistry LaboratoryDepartment of BotanyUniversity of AllahabadAllahabad, IndiaSamiksha SinghRanjan Plant Physiology and Biochemistry LaboratoryDepartment of BotanyUniversity of AllahabadAllahabad, IndiaShilpi SinghMolecular Biology SectionCentre of Advanced Study in BotanyBanaras Hindu UniversityVaranasi, IndiaShwetaDD Pant Interdisciplinary Research Lab Department of BotanyUniversity of AllahabadAllahabad, IndiaShweta SinghDD Pant Interdisciplinary Research LabDepartment of BotanyUniversity of AllahabadAllahabad, IndiaSwati SinghDD Pant Interdisciplinary Research LabDepartment of BotanyUniversity of AllahabadAllahabad, IndiaVijay Pratap SinghGovernment Ramanuj Pratap Singhdev Post Graduate CollegeBaikunthpur, KoriyaChhattisgarh, IndiaRavi R SonaniBRD School of BiosciencesVadtal Road, Satellite CampusSardar Patel UniversityVallabh Vidyanagar, AnandGujarat, IndiaSanjesh TiwariRanjan Plant Physiology and Biochemistry LaboratoryDepartment of BotanyUniversity of AllahabadAllahabad, IndiaDurgesh Kumar TripathiCenter of Advanced StudiesDepartment of BotanyBanaras Hindu UniversityVaranasi, IndiaMohan G VairaleDefence Research Laboratory, DRDO Tezpur, Assam, IndiaVijay VeerDefence Research Laboratory, DRDO Tezpur, Assam, IndiaMarija VidovićInstitute for Multidisciplinary ResearchUniversity of BelgradeKneza Višeslava, 1, 11000Belgrade, SerbiaVaishali YadavDD Pant Interdisciplinary Research LabDepartment of BotanyUniversity of AllahabadAllahabad, IndiaShivam YadavMolecular Biology SectionCentre of Advanced Study in BotanyBanaras Hindu UniversityVaranasi, IndiaKrystyna Żuk‐GołaszewskaDepartment of Agrotechnology Agricultural Production Management and AgribusinessUniversity of Warmia and Mazury in OlsztynPoland

Preface

In the course of acquiring knowledge about UV‐B research in plant systems from the past up to the present day, we have found a considerable gap between the availability of books and emerging areas of research. This book has been written to bridge the gap between researches being conducted from the past up to today, and the direction these researches might take in the future with respect to UV‐B.

The title itself indicates that this book has mapped UV‐B research from past up to recent times. It is a book of theoretical knowledge, and the compilation has been done on the basis of practical work done by the researchers and scientists. We have briefed out the historical backgrounds of UV‐B namely, how it reaches the earth’s surface, its action spectra and its interaction with living systems, using the research work conducted by researchers in the past, to recent studies that show how research in UV‐B has taken a U‐turn with the discovery of UVR8.

A good book is one that includes knowledge for all readers, including students, and of course we are indebted to the many authors who have contributed to it. This book includes chapters which cover several aspects of UV‐B, starting from the basics of UV‐B research and going on to the present date, and a brief outline has been provided below.

The first chapter gives an overview of the ozone layer and the reasons for its depletion and UV‐B reaching the earth’s surface, and it also offers a brief introduction to action spectra and biologically effective irradiance. In later sections, the authors also discuss the impact of UV‐B on plants by analysing the researches performed in the past.

The second chapter gives a brief historical background for the effect of ambient UV‐B on plants, with special reference to accumulation of secondary metabolites, such as phenolic compounds, alkaloids and terpenoids. The authors have also discussed recent studies regarding phenolics under ecologically relevant UV‐B radiation, and changes in the content of secondary metabolites, with reference to species variation, changes in the UV‐B : UV‐A : PAR ratio, UV‐B doses and UV‐B spectral quality.

In the next few chapters, authors discuss risk arising due to the interaction of UV‐B with the components of plants, and biological effects arising due to absorption of UV radiation, whether from UV‐A or UV‐B, by important biomolecules like nucleic acids, lipids and proteins. They also examine the impact on the phytochrome system and photosynthetic machinery. In addition, the authors also discuss the effects of UV‐B radiation in terms of oxidative stress, and the responses generated by plants to combat from the stress arising due to UV‐B induced toxicity, which includes accumulation of sun‐screen molecules. These chapters basically focus on the past researches that have been performed with UV‐B. With technology and research advancement, the introduction of photomorphogenic responses came into existence, which compelled researchers to gain a deeper insight into this phenomenon, and this curiosity for innovation led to the discovery of UVR8.

In later chapters, authors have very well documented the history of photomorphogenic responses and how UVR8 was discovered – and all the regulators, whether positive or negative, involved with this component. In the last chapter, the authors discuss the mechanism of regulatory action by UVR8 and its integration with other pathways.

In concluding, it is a pleasure to express our thanks to all the authors for contributing chapters that have helped us in giving a clear picture of the changing scenario of research in UV‐B. We hope that this book will be of special value to environmentalists, researchers and students seeking knowledge on UV‐B, which has not yet been assimilated in textbooks.

Editors:Vijay Pratap SinghSamiksha SinghSheo Mohan PrasadParul Parihar

1An Introduction to UV‐B Research in Plant Science

Rachana Singh1, Parul Parihar1, Samiksha Singh1, MPVVB Singh1, Vijay Pratap Singh2and Sheo Mohan Prasad1

1Ranjan Plant Physiology and Biochemistry Laboratory, Department of Botany, University of Allahabad, Allahabad, India

2Government Ramanuj Pratap Singhdev Post Graduate College, Baikunthpur, Koriya, Chhattisgarh, India

1.1 The Historical Background

About 3.8 × 109 years ago, during the early evolutionary phase, the young earth was receiving a very high amount of UV radiation and it is estimated that, at that time, the sun was behaving like young T‐Tauristars and was emitting 10,000 times greater UV than today (Canuto et al., 1982). Then, the radiance of the sun became lower than it is in the present day, thereby resulting in temperatures below freezing. On the other hand, due to high atmospheric carbon dioxide (CO2) level, which was 100–1000 times greater than that of present values, liquid water did occur and absorbed infrared (IR) radiation, and this shaped an obvious greenhouse effect (Canuto et al., 1982). Due to the photosynthesis of photosynthetic bacteria, cyanobacteria and eukaryotic algae, oxygen (O2) was released for the first time into the environment, which led to an increase of atmospheric O2 and a simultaneous decrease of atmospheric CO2.

About 2.7 × 109 years ago, due to the absence of oxygenic photosynthesis, oxygen was absent from the atmosphere. About 2.7 × 109 years ago, with the deposition of iron oxide (Fe2O3) in Red Beds, aerobic terrestrial weathering occurred and, at that time, O2 was approximately about 0.001% of the present level (Rozema et al., 1997). In proportion with gradual atmospheric O2 increase, the accumulation of stratospheric ozone might have been slow. Alternatively, about 3.5 × 108 years ago, due to a sheer rise in atmospheric oxygen, it might have reached close to the present levels of 21% (Kubitzki, 1987; Stafford, 1991). Nevertheless, terrestrial plant life was made possible by the development of the stratospheric ozone (O3) layer, which absorbs solar UV‐C completely and a part of UV‐B radiation, thereby reducing the damaging solar UV flux on the earth’s surface (Caldwell, 1997).

Before focusing on the various aspects of UV‐B radiation, we should firstly understand the electromagnetic spectrum. The electromagnetic spectrum consists of ultraviolet (UV) and visible (VIS) radiations (i.e. also PAR). The wavelength ranges of UV and visible radiation are listed in Table 1.1. Solar radiations, with a longer wavelength, are called infrared (IR) radiations. The spectral range between 200 and 400 nm, which borders on the visible range, is called UV radiation, and is divided into three categories: UV‐C (100–280 nm), UV‐B (280–315 nm) and UV‐A (315–400 nm). The shorter wavelengths of UV get filtered out by stratospheric O3, and less than 7% of the sun’s radiation range between 280 and 400 nm (UV‐A and UV‐B) reaches the Earth’s surface.

Table 1.1 Regions of the electromagnetic spectrum together with colours, modified from Iqbal (1983) and Eichler et al. (1993).

Wavelength (nm)

Frequency (THz)

Colour

50 000–10

6

6–0.3

far IR

3000–50 000

100–6

mid IR

770–3000

390–100

near IR

622–770

482–390

red

597–622

502–482

Orange

577–597

520–502

yellow

492–577

610–520

Green

455–492

660–610

blue

390–455

770–660

violet

315–400

950–750

UV‐A

280–315

1070–950

UV‐B

100–280

3000–1070

UV‐C

The level of UV‐B radiation over temperate regions is lower than it is in tropical latitudes, due to higher atmospheric UV‐B absorption, primarily caused by changes in solar angle and the thickness of the ozone layer. Therefore, the intensity of UV‐B radiation is relatively low in the polar regions and high in the tropical areas. Over 35 years ago, it was warned that man‐made compounds (e.g. CFCs, HCFCs, halons, carbon tetrachloride, etc.) cause the breakdown of large amounts of O3 in the stratosphere (Velders et al., 2007) thereby increasing the level of UV‐B reaching the Earth’s surface. Increase in the UV‐B radiation has been estimated since the 1980s (UNEP, 2002), and projections like the Kyoto protocol estimate that, even after the implementation of these protocols, returning to pre‐1980 levels will be possible by 2050–2075 (UNEP, 2002).

1.2 Biologically Effective Irradiance

The term ‘biologically effective irradiance’ means the effectiveness of different wavelengths in obtaining a number of photobiological outcomes when biological species are irradiated with ultraviolet radiations (UVR). The UV‐B, UV‐A and photosynthetically active radiations (PAR; 400–700 nm) have a significant biological impact on organisms (Vincent and Roy, 1993; Ivanov et al., 2000). Ultraviolet irradiation results into a number of biological effects that are initiated by photochemical absorption by biologically significant molecules. Among these molecules, the most important are nucleic acids, which absorb the majority of ultraviolet photons, and also proteins, which do so to a much lesser extent (Harm, 1980).

Nucleic acids (a necessary part of DNA) are nucleotide bases that have absorbing centres (i.e. chromophores). In DNA, the absorption spectra of purine (adenine and guanine) and pyrimidine derivatives (thymine and cytosine), are slightly different, but an absorption maximum between 260–265 nm, with a fast reduction in the absorption at longer wavelengths, is common (Figure 1.1). In contrast with nucleic acids solutions of equal concentration, the absorbance of proteins is lower. Proteins with absorption maxima of about 280 nm most strongly absorb in the UV‐B and UV‐C regions (Figure 1.1). The other biologically significant molecules that absorb UVR are caratenoids, porphyrins, quinones and steroids.

Figure 1.1 Absorption spectra of protein and DNA at equal concentrations

(adapted from Harm, 1980).

1.3 UV‐B‐induced Effects in Plants

In the past few decades, a lot of studies have been made on the role of UV‐B radiation. Due to the fact that sunlight necessity for their survival, plants are inevitably exposed to solar UV‐B radiation reaching the earth’s surface. From the point of view of ozone depletion, this UV‐B radiation should be considered as an environmental stressor for photosynthetic organisms (Caldwell et al., 2007). However, according to the evolutionary point of view, this assumption is questionable.

Although UV‐B radiation comprises only a small part of the electromagnetic spectrum, the UV‐B reaching on earth’s surface is capable of producing several responses at molecular, cellular and whole‐organism level in plants (Jenkins, 2009). UV‐B radiation is readily absorbed by nucleic acids, lipids and proteins, thereby leading to their photo‐oxidation and resulting in promotional changes on multiple biological processes, either by regulating or damaging (Tian and Yu, 2009). In spite of the multiplicity of UV‐B targets in plants, it appears that the main action target of UV‐B is photosynthetic apparatus, leading to the impairment of the photosynthetic function (Lidon et al., 2012). If we talk about the negative impact of UV‐B, it inhibits chlorophyll biosynthesis, inactivates light harvesting complex II (LHCII), photosystem II (PSII) reaction centres functioning, as well as electron flux (Lidon et al., 2012).

The photosynthetic pathway responding to UV‐B may depend on various factors, including UV‐B dosage, growth stage and conditions, and flow rate, and also the interaction with other environmental stresses (e.g., cold, high light, drought, temperature, heavy metals, etc.) (Jenkins, 2009). The thylakoid membrane and oxygen evolving complex (OEC) are highly sensitive to UV‐B (Lidon et al., 2012). Since the Mn cluster of OEC is the most labile element of the electron transport chain, UV‐B absorption by the redox components or protein matrix may lead to conformational changes, as well as inactivation of the Mn cluster. The D1 and D2 are the main proteins of PSII reaction centres and the degradation and synthesis of D1 protein is in equilibrium under normal condition in light, however, its degradation rate becomes faster under UV‐B exposure thereby, equilibrium gets disturbed (Savitch et al., 2001; Lidon et al., 2012). In the OEC coupled to PSII, during light‐driven photosynthetic electron transport, tri‐molecular oxygen is produced continuously, which can be converted in the sequential reduction to superoxide radical (O2•–), hydrogen peroxide (H2O2) and hydroxyl radical (•OH) (Apel and Hirt, 2004). Furthermore, PSI and cytochrome b6/f complex are less affected by UV‐B radiation in comparison to PSII (Lidon et al., 2012).

Stomatal movement is an important regulatory process that limits the rate of photosynthesis. In Vicia faba, high UV‐B radiation stimulates either stomatal opening or closing, depending on the metabolic rate (Jansen and van‐den‐Noort, 2000). However, the stimulated reduction of stomatal conductance can be responsible for CO2 limitation, as reported in many plants (Zhao et al., 2003; Lidon and Ramalho, 2011), but the reduction in the stomatal conductance has a lesser extent than that of net photosynthetic rate. Additionally, UV‐B radiation strongly affects the activity as well as content of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) in plants (Correia et al., 1998; Savitch et al., 2001). Besides this, the intermediate stage of the Calvin cycle (i.e. sedoheptulose 1,7‐bisphosphatase), as well as the regeneration of RuBP, was found to be decreased upon exposure to UV‐B radiation (Allen et al., 1998).

UV‐B radiation has long been perceived as a stressor. Many studies have shown that it impedes photosynthetic activities, damages DNA, proteins and membranes, and impedes plant growth. Oxidative stress has been flagged as a pioneer factor in such UV‐B stress responses (Lidon et al., 2012). However, DNA damage, membrane degradation products, and ROS also play a role in mediating UV‐B protection, and have done so since the origin of the first plants. Cyanobacteria first evolved on the earth at a time when UV‐B levels were at their highest and no ozone layer existed. Under such high UV‐B radiation during the early evolution of photosynthetic organisms, they might have coevolved their genetic machinery along with the ambient UV‐B level, which might have also helped the transition to terrestrial life (Rozema et al., 1997). Therefore, it can be assumed that plants’ metabolic machinery must have all the compulsory elements for normal coexistence with present UV‐B levels, so the solar UV‐B radiation reaching the earth should not be considered to be an environmental stressor. Actually, the current ambient UV‐B radiation level should be considered as a signal factor which is capable of inducing the expression of genes related to the normal growth and development of plants (Jenkins, 2009).

A conceptual U‐turn has been taken place, and UV‐B is rarely considered as a damaging factor. There is overpowering evidence that UV‐B is an environmental regulator that controls gene expression, cellular and metabolic activities, and also the growth and development (Jenkins, 2009). Under low UV‐B fluence rate, the regulatory role of UV‐B can be observed, and these effects are mediated by the UV‐B‐specific UV Resistance Locus 8 (UVR8) photoreceptor, which has opened the door to elucidate the UV‐B signalling pathways in plants (Christie et al., 2012; Wu et al., 2012; Singh et al., 2012; Srivastava et al., 2014).

The UVR8 photoreceptor exists as a homodimer that undergoes immediate monomerization following UV‐B exposure, and the process is dependent on an intrinsic tryptophan residue (Rizzini et al., 2011). Upon exposure to UV‐B, UVR8 accumulates rapidly, and interacts with Constitutively Photomorphogenic 1 (COP1) to initiate the molecular signalling pathway that leads to gene expression changes. UVR8 monomer is redimerized by the action of RUP1 and RUP2, which interrupts the UVR8‐COP1 interaction, thereby inactivating the signalling pathway and regenerating the UVR8 homodimer again, ready for UV‐B perception. This signalling leads to UVR8 dependent responses, such as UV‐B‐induced photomorphogenic responses, and also the accumulation of UV‐B‐absorbing flavonols (Tilbrook et al., 2013). Elongated Hypocotyl 5 (HY5) acts as a downstream effector, and is regulated by the negative feedback pathway.

Favory et al. (2009) hypothesized that during UVR8 interaction with COP1, COP1 might have been taken out from phytochrome (red light receptor) and cryptochrome (blue/UV‐A light receptor) under UV‐B exposure, and this fact was supported by the phenotype of the COP1 overexpressing line of UVR8. Conversely, Oravecz et al. (2006) and Favory et al. (2009) have noted that COP1 was excluded by the nucleus upon exposure to visible light, while UV‐B exposure results in nuclear accumulation and stabilization of COP1. In addition, being a repressor of photomorphogenesis, COP1 is dependent on SPA protein, which is not a part of the regulatory action by COP1 (Laubinger et al., 2004; Oravecz et al., 2006). Interestingly, SPA and Repressor of Photomorphogenesis (RUP) genes show similarity in their phylogeny while interacting with COP1 (Gruber et al., 2010; Fittinghoff et al., 2006). All these similarities suggest towards the evolution of complex photoreceptor UVR8 from the other photoreceptors, and the role of UVR8 as a signalling molecule.

1.4 Conclusion and Future Perspectives

Over recent years, significant progress has been made in identifying the molecular players, their early mechanisms and signalling pathway in UV‐B perception in plants, but there is more we have to do. Several questions remain to be uncovered, regarding the photochemistry, signal transduction and regulatory mechanisms of UVR8, that need to be addressed and, of course, this will open a new horizon in the field of UV‐B perception and signalling. Questions that remain to be traced out include: the primary responses of UVR8 after UV‐B perception; whether functioning at the chromatin level exists; sites of UVR8 functioning in the cell; crosstalk of UVR8 pathway with COP1 and visible light photoreceptors along with their signalling; whether UVR8 has evolved from other photoreceptors as a need of environmental changes and is now towards the degrading or evolutionary phase.

Now the stage is set to tackle these questions. No doubt, the answers will pave a new direction and a deep understanding of plant UV‐B responses. Of course, the future of UV‐B signalling will be more realistic after the preparation of a detailed molecular map of various signalling molecules regarding UV‐B.

Acknowledgements

The University Grants Commission, New Delhi is greatly acknowledged for financial support.

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2Stimulation of Various Phenolics in Plants Under Ambient UV‐B Radiation

Marija Vidović, Filis Morina and Sonja Veljović Jovanović

Institute for Multidisciplinary Research, University of Belgrade, Kneza Višeslava, 1, 11000, Belgrade, Serbia

2.1 Introduction

Under natural conditions, plants are constantly exposed to dynamic changes of solar radiation, which mainly consists of infrared (IR, >700 nm), photosynthetically active radiation (PAR, 400–700 nm) and minor portion of ultraviolet (UV) radiation (UV‐B, 290–315 nm and UV‐A, 315–400 nm). Besides being the primary source of energy in photosynthesis, sunlight is an important signal which regulates plant growth and development. In addition to light quantity, plants are able to monitor the quality, periodicity and direction of light (reviewed in Caldwell et al., 2007; Jiao et al., 2007). Plants perceive light signals through several protein photoreceptors: five phytochromes (PHY A‐E), which are sensitive to red and far red light (600–750 nm), and two cryptochromes (CRY1 and CRY2), two phototropins (PHOT1 and PHOT2) and zeitlupe proteins (ZTLs) for blue and UV‐A radiation (315–500 nm), while UV‐B radiation is sensed by UV Resistant Locus 8 (UVR8) (reviewed in Jiao et al., 2007; Heijde and Ulm, 2012).

During the period from the 1970s to 1990s, investigations on UV‐B effects on organisms were in the centre of attention, due to alarming depletion of stratospheric ozone layer and increased UV‐B radiation reaching the Earth’s surface. However, the results of numerous studies that explored the impact of high UV‐B radiation on plants were often contradictory. In following years, this was explained by different unrealistic UV‐B : UV‐A : PAR ratios, high UV‐B doses applied, different spectral distribution in the UV‐B region, as well as simultaneous effects of other environmental stressors (drought, high temperature, nutrient deprivation), and previous plant exposure to UV‐B radiation (plant history). Inconsistent reports on UV‐B effects on photosynthesis and stomata conductance were a result of different UV‐B doses applied, species‐specific, and even genotype‐specific responses, but also plant history and overall plant metabolism.

In the light of these findings, during the last decade, research on UV‐B radiation effects on biological systems has advanced towards more controlled conditions aiming to imitate ambient solar radiation. Using sun simulators with realistic balance of UV‐B, UV‐A and PAR, is a very good solution to achieve realistic and reproducible experimental conditions (Döhring et al., 1996; Aphalo et al., 2012). Contrary to previous widely accepted beliefs, in the last several years it has been demonstrated that UV‐B radiation, at low and ecologically relevant doses, presents an important regulator of plant growth and development (Jenkins, 2009; Hideg et al., 2013). Plants grown in the open field, exposed to natural UV‐B doses, have higher nutritional and pharmacological value than plants grown in polytunnels and glasshouses, which are non‐transparent to UV radiation (Jansen et al., 2008; Behn et al., 2010). Moreover, it has been shown that UV‐B radiation improves plant adaptive capacity to drought, high temperatures, pathogen and insect attack, and nutrient deficiency conditions (Schmidt et al., 2000; Caputo et al., 2006). These findings have a strong impact on the agricultural, pharmaceutical and food industries.

A hallmark of UV‐B response in plants is accumulation of secondary metabolites, such as phenolic compounds (particularly flavonoids and phenylpropanoids), alkaloids and terpenoids. Phenolics are the most abundant secondary metabolites in plants, and 20% of carbon fixed in photosynthesis is directed to their biosynthesis (Hernández and Van Breusegem, 2010). Phenolic compounds in plants are involved in many processes, from growth and development, to flowering, reproduction and seed dispersion, defence against pathogens, plant–insect interactions and protection against numerous abiotic stresses (Gould and Lister, 2005; Sedlarević et al., 2016). The most well‐studied mechanism of UV‐B induction of phenolic metabolism is certainly the UVR8 pathway, which will be discussed in detail in this chapter. However, regarding UV‐B and sunlight exposure in general, antioxidative