1. Introduction
Light energy from the sun and the ability of photoautotrophs to fix that energy in the presence of chlorophyll has promoted life on earth. Apart from being an energy source, light acts as a major environmental cue for plants to adjust to the surrounding conditions. In nature, solar radiation comprises different wavelengths of electromagnetic radiation and is broadly classified as Ultraviolet Radiation (UV < 400 nm), Photosynthetically Active Radiation (PAR~400–700 nm) and Far Red Radiation (FR~700–780 nm). Approximately 7%–9% of the total solar radiation reaching the earth’s surface is in the UV range (200–400 nm) [1–4]. UV radiation is broadly classified based on wavelength as UV-C radiation (200–280 nm), which is the most hazardous range of UV light but physiologically insignificant since these wavelengths are completely absorbed by the atmosphere; UV-B radiation (280–320 nm), which is filtered through the stratospheric ozone layer and, therefore, only a small proportion reaches the earth surface; and UV-A radiation (320–400 nm), which cannot be absorbed by the ozone layer and is fully transmitted to the earth’s surface. Among these three types of UV radiation, UV-B is of prime importance despite its small proportion i.e., 1.5% of total radiation, because of its severe damaging effects on plant growth and development. UV-A radiation, although it represents 6.3% of total radiation, is less harmful than the other wavelengths of UV radiation [3].
The flux of solar radiation is strongly modified when transmitted through the earth’s atmosphere and it greatly varies due to different factors like solar angle (time of the day, season, latitude), altitude, stratospheric ozone, cloud cover and shading [5, 6]. The ozone layer acts as a key component protecting living beings from the damaging UV radiation. Anthropogenic factors, such as the release of chlorofluorocarbons into the atmosphere, can result in a depletion of the ozone layer of ~5% [7]. In general, a 1% reduction in the ozone layer results in a 1.3%–1.8% increase in the amount of biologically active UV-B radiation [3]. The adoption of the international Montreal Protocol, which seeks to reduce factors that deplete the ozone layer, is a possible solution to this complex problem. Despite such efforts, the global ozone level is still lower than in the 1970s and a return to that state is not expected for several decades [8]. Moreover, there has been a significant increase in UV-B radiation reaching the earth’s surface between 1979 and 2008, over all latitudes except the equatorial zone, with the largest increments taking place at mid to high latitudes in the Southern Hemisphere [9, 10]. Thus, this scenario of changing climate suggests that life on earth will be continuously exposed to higher dosages of UV radiation. It is therefore important to study in detail the effect of UV radiation on plants.
The current knowledge regarding ecophysiological impact of UV radiation on plants has come largely through field experiments using natural or moderately higher levels of UV-B radiation. The results from all these experiments can be summarized as UV-B radiation has very small (<20%) inhibitory effect on plant growth, on activation of acclimation response and on interference between the interaction of plants and herbivores [11–13]. Apart from field condition experiments, data from laboratory controlled conditions has helped us in the understanding of UV-B perception, signaling and UV induced damage. It has been documented that plants respond differentially to UV fluence rate as well as wavelength [4, 14]. Lower doses of UV-B stimulate photomorphogenesis in etiolated plants while higher doses of UV-B or UV-C result in cellular damage [4, 15]. It has been reported that UV-C overexposure can induce programmed cell death (PCD) by activation of caspase-like proteases, oligonucleosomal DNA fragmentation, and appearance of apoptotic nuclear morphology in Arabidopsis thaliana[16–18]. Moreover, UV-B overexposure can also induce PCD in a BY-2 tobacco cell line [19]. The current review focuses on the detailed mechanism of PCD in response to higher UV radiation and its physiological significance.
2. Perception and Signaling of UV Light in Plants
Light regulates different growth stages and vital physiological process throughout the lifecycle of plants. Owing to the importance of light, plants evolved different photoreceptors for perception of light through which they can sense the quality (wavelength), intensity, duration (including day length) and direction of light [20]. Plants have evolved complex and sophisticated transcriptional networks involving different photoreceptors that mediate developmental changes in response to light. These developmental changes, which include seed germination, de-etiolation, gravitropic orientation, plant architecture, stomatal movement and reproductive development at time of flowering, have been extensively reviewed [21–23]. In Arabidopsis, three classes of UV-A/blue photoreceptors viz., cryptochromes (CRY1, CRY2 and CRY3), phototropins (PHOT1, PHOT2) and members of the Zeitlupe family (ZTL, FKF1, and LKP2) [24–27] have been identified, while red (R) and far-red (FR) light is maximally absorbed by the phytochromes (PHYA, PHYB, PHYC, PHYD and PHYE) [28, 29]. Different studies have shown that UV-B radiation evokes diverse phenotypic responses, including hypocotyl growth inhibition, cotyledon expansion, phototropic curvature and induction of UV-B-protecting pigmentation, in plants [15, 30–34]. In the last decade, whole genome expression profiling has identified specific pathways activated in response to low-level UV-B [35]. These photomorphogenic UV-B responses have been shown to be mediated through UV RESISTANCE LOCUS8 (UVR8) protein [20,34, 36] which has been characterized as a UV-B photoreceptor [37–39]. UVR8 is a β-propeller protein showing similarity with human guanine nucleotide exchange factor Regulator of Chromatin Condensation 1 (RCC1) and its crystal structure is maintained by salt bridge interactions between charged amino acids Arg, Asp and Glu at the dimeric interaction surface [38]. Four tryptophans (W94, W233, W285, and W337) located adjacent to salt-bridge amino acids are arranged to form two pyramids across the dimeric interface [38]. Absorption of UV-B light by one or more tryptophans results in a loss of excitation coupling and leads to disruption of salt-bridges and hence monomerization. Unlike other photoreceptors UVR8 lacks a specific chromophore or rather intrinsic tryptophans W285 and W233 which are important for UV-B absorbance [38,39].
Interestingly, the UV-B specific pathway is devoid of known light photoreceptors but involves two known central components of light signaling, viz., ELONGATED HYPOCOTYL (HY5) and CONSTITUTIVE PHOTOMORPHOGENESIS 1 (COP1). The UVR8 interacts with COP1 in UV-B dependant manner to activate expression of a set of genes that protects against potential damage by UV-B exposure, including genes encoding flavonoid biosynthesis enzymes, DNA repair enzymes, and also proteins involved in mitigating oxidative stress as shown in Figure 1[34]. The downstream components in the UVR8-COP1-HY5 pathways are summarized in Table 1.
The importance of UVR8 mediated response in the acclimatization of plants against UV-B radiation is well known from the fact that the Arabidopsis mutant does not accumulate flavanoids and other phenolic compounds showing that they are UV sensitive. It was reported that flavanoids also possess free-radical scavenging activity that might offer additional cellular protection [40]. Apart from induction of UV absorbing pigments, there is upregulation of DNA repair machineries such as photolyase genes. The summary of genes impaired in UV sunscreen pigments causing hyper or hypo- sensitivity to UV radiation is given in Table 2.
Although, in the last decade we have made much progress regarding the understanding of the UV-B photoreceptor and its structure in the model plant Arabidopsis, there are still certain points to be considered. The fluence response curve experiment with uvr8-2 and hy5 lines in etiolated plants does not show significant differences from the wild type suggesting the presence of some other photoreceptors in plants [50]. It has been speculated that longer wavelengths and shorter wavelengths of UV-B radiation trigger two pathways and the shorter wavelength pathway negatively interferes with the longer wavelength pathway. The longer wavelength response can be mediated by UV-B photoreceptors and the short wavelength may represent indirect effects of UV-B exposure through general cellular stress pathways or distinct photoreceptors [14, 50]. Due to the lack of well-defined phenotypes and the accompanying damage caused by UV-B it has made it difficult to determine a UV-B photoreceptor apart from UVR8 which can respond to short wavelength radiation and regulate stress response. The UV-B induced stress pathway also known as “UV response”, is conserved in yeast, mammals and plants and involves the activation of mitogen-activated protein kinases (MAPKs) [51–53]. Recently, in Arabidopsis, MPK3 and MPK6 have been shown to be activated in response to UV-B stress and their mis-regulation in mkp1 mutant results in UV-B hypersensitivity phenotypes [46]. The MPK6 has been known as a positive regulator of programmed cell death (PCD) in developmental processes like senescence [54, 55] and also in biotic stresses like pathogen attacks [56]. Thus, it has been suggested that MKP1 provides protection against UV-B-induced cell death by inhibiting UV-induced MPK3 and MPK6 activities [46]. The two UV-B response pathways namely UV-B induced photomorphogenesis regulated by UVR8 and stress induced MAPK pathway have been shown to be independent of each other and coordinately determine plant UV-B tolerance (Figure 2).
3. UV Induced Damage and Cell Death
Recently the UV-B light specific photoreceptors were reviewed and it was explained there how low fluence UV-B induces protective response [20]. In the current review, we discuss in detail UV light perception, signaling and important UV targets in the cell with a focus on how high intensity UV induces plant stress. The response to such stress independent of the UV photoreceptor and plants can activate the cell death pathway. Programmed cell death (PCD), is a genetically controlled self destruction mechanism in all eukaryotic multi-cellular organisms, and can be induced either as part of normal development or in response to stress. PCD is also known as apoptosis and has been well studied in animal systems. Apoptosis is characterized by specific hallmarks such as cell shrinkage, nuclear condensation and fragmentation, and eventually the breakup of the cell into “apoptotic bodies” that are eventually engulfed by phagocytes [57, 58]. The progress of plant PCD is much slower but interestingly shares many conserved components with animal apoptosis. Despite the similarities between cell death pathways in plants and animals, there is no evidence for apoptotic bodies, as well as classical caspases in plant systems. Plant cell death is described as an apoptotic-like PCD (AL-PCD) [59]. AL-PCD is now accepted as a fundamental cellular process since it plays an essential role during development, under stress conditions, in the senescence process and in response to pathogen infection in plants [60, 61]. Apart from developmental stimuli, biotic stresses, such as the pathogen induced hypersensitivity response (HR) and abiotic stresses such as heat stress and high fluence UV radiation, have been shown to induce AL-PCD [16, 62– 65]. Genes involved in cell death pathway in plants in response to UV radiation are summarized in Table 3.
3.1. DNA Damage, Cell Cycle Arrest and Cell Death
DNA is one of the key targets for UV-induced damage in both prokaryotic and eukaryotic cells. The adverse effects of solar radiation on living organisms are mostly due to the small amount of UV-B reaching the earth’s surface that is absorbed by cellular DNA. UV-A wavelengths are less efficient in inducing DNA damage as they are not absorbed by native DNA. It has been documented that the increase in solar UV-B radiation, due to a long-term depletion of the stratospheric ozone layer, may influence the genomic stability of plant populations [69]. The different types of DNA damage in response to UV-B or UV-C radiation includes the formation of cyclobutyl pyrimidine dimers (CPDs), (6-4) photoproducts ((6-4) PPs) [70], Inter/Intra Cross Link (ICL), 8-oxoG or even DNA Double-Strand Breaks (DSBs) [71]. The formation of CPDs represents approximately 75% of the total DNA damage by UV light [70]. The plants adopt three different strategies towards minimizing the DNA damage namely plant-specific photo reactivation pathway [70], global genome repair- nucleotide excision repair (GGR-NER) pathway [71, 72] and homologous recombination [73]. The photo reactivation pathway includes different photolyases for example in Arabidopsis, the UVR2 gene encodes a photolyase (PHR1) that acts only on CPDs [74] whereas the UVR3 gene encodes a photolyase specific for 6-4 photoproducts [75]. CPDs and 6-4 PPs can also be removed in the dark through nucleotide excision repair, endonucleolytic cleavage, release of the damaged nucleotides and strand re-synthesis [72]. The genes mentioned above involved in DNA repair pathways are summarized in Table 4.
In mammals, it has been reported that DNA damage caused by UV radiation results in cell cycle arrest and apoptosis [76] but very little is known about such mechanisms in plants. The transcriptional responses of maize to UV-B radiation under field conditions indicates that UV-B might affect the cell cycle [77]. Recently, it has been shown that UV-B radiation modulates cell cycle regulatory genes involved in G1 -to- S transition using HU (Hydroxy Urea) synchronized root tips of Arabidopsis. The delayed induction of CYCD3;1 transcripts under UV-B radiation results in delayed G1-to-S transition [78]. It was also confirmed that the G1-to-S arrest induced by UV-B in root tips was a consequence of DNA damage which has been shown using uvh1 mutant impaired in removal of CPDs [78,79]. G1-to-S arrest induced by UV-B has been suggested as a protective mechanism against UV-B-induced DNA damage, which allows time for repairing DNA damages before replication. This study concludes that the DNA damage is a possible reason behind the plant growth inhibition after UV exposure. In Arabidopsis, there are two proteins which can acts as sensors of DNA damage, namely, ATM (ataxia-telangiectasia mutated) and ATR (ataxia-telangiectasia and Rad3-related). The role of Arabidopsis ATM is more prominent under gamma radiation induced DNA damage while ATR deficient plants showed hypersensitivity to UV-B radiation and exhibited altered G2-phase cell cycle checkpoints [80, 81]. Different genes involved in cell cycle regulation impairment causing UV-induced hypersensitivity response are summarized in Table 4.
The higher doses of UV-B or UV-C radiation can induce oligonucleosomal DNA fragmentation, as a physiological response, which shows nucleosomal fragments in multiples of 180 bp like a typical apoptotic DNA ladder [16, 19]. DNA laddering is an integral part of programmed cell death (PCD) in plant systems [86, 87], confirming the role of UV radiation in induction of PCD. DNA damage is quantified by using terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling reaction (TUNEL), which detects in situ free 3′-OH DNA breaks [16, 88]. Although the doses of UV radiation in the above mentioned studies are physiologically irrelevant, they are still important for making experimental set ups to study PCD responses in plants.
3.2. Proteins and Lipids Damage
Proteins and lipids are direct targets of UV-B radiation. Since proteins strongly absorb ~280 nm or higher wavelengths, UV-B can affect the aromatic amino acids such as tyrosine, phenylalanine, and tryptophan [89]. Proteins can undergo photomodification directly through photooxidation reactions or indirectly by photosensitized production of active oxygen species and free radicals. UV-B exposures lead to damage of photosynthetic machinery including RUBISCO [90]. These direct effects are often observed in high UV-B fluence and low accompanying PAR. However, the induced loss of photosynthetic activity, often observed under lab or greenhouse conditions might not translate to the fields or natural conditions [91]. UV-B damages the ribosomes by crosslinking with the cytosolic ribosomal proteins and chloroplast ribosomal proteins to RNA, thereby transiently inhibiting translation in vivo. Although there is a small and significant decline in photosynthesis rate, recovery is complete by the next day and also there are no changes in the levels of chlorophylls a and b, carotenoids, or total proteins [92]. Thus, despite the presence of ribosome damage and a decrease in translation, physiological parameters were not significantly affected by the UV-B treatments, demonstrating that the treatment applied was not lethal.
Plant cell membranes contain unsaturated fatty acids, which are damaged by UV-radiation in the presence of oxygen [93, 94]. The peroxidation of membrane lipids leads to breakdown of their structure and function [95, 96]. The composition of membrane lipids such as mono- and digalactosyldiglicerides (MGDGs, DGDGs) in chloroplasts may change due to UV-radiation. The concentration of the MGDGs reduced without affecting the overall DGDG concentration and the total phospholipid levels. This reduction of the MGDGs is important for maintaining the optimal membrane structure in chloroplasts for photosynthesis under UV-B radiation [97].
3.3. Organelle Dysfunction in Response to UV and Cell Death
In plants, chloroplast and mitochondria are the two important cell organelles which act as sites of ROS production (Figure 2). Plant chloroplasts capture light energy during photosynthesis, a process that is very sensitive to stress conditions generating ROS or oxidative damage [98, 99]. It is known that chloroplasts are involved in different cell death processes such as leaf senescence [100]. Chloroplast involvement in PCD was confirmed by ectopic expression of mammalian anti-apoptotic Bcl-2 family members, which protect transgenic tobacco plants from PCD, induced by chloroplast-targeted herbicides [101]. The chloroplast specific ROS inhibitor 3-(3,4-dichlorophenyl1)-1,1-dimethylurea (DCMU) can retard cell death induced in response to UV overexposure. Moreover, after UV treatment and under continuous light, ROS, derived from chloroplasts, may further damage adjacent mitochondria [18].
Involvement of mitochondria in programmed cell death has been extensively studied in animals. Recently, a role for mitochondria in plant PCD was reported in response to different stimuli such as ceramide, protoporphyrin IX, the HR elicitor AvrRpt2, and UV-C radiation [18, 102]. Different stress conditions cause changes of the mitochondrial transmembrane potential (MTP) releasing cytochrome c into the intermembrane mitochondrial space, which activates caspase activity and subsequently cell death [102, 103].
3.4. UV-Induced Caspase Activity and Cell Death
In the animal systems, apoptotic cell death is controlled by caspases (cysteine-dependent aspartate-specific proteases), which show specific protease activity upon induction by cell death [104, 105]. In plants, caspase-like activities have been demonstrated during hypersensitive response (HR) [106] or after heat shock of suspension cells [107]. However, plants do not appear to have true caspases but rather a small family of proteins that share similarities with caspase-like domains, known as metacaspases [108]. There are nine metacaspases genes in the Arabidopsis genome, which can be divided into two types. Type I is comprised of three genes, while the Type II subfamily consists of six genes. Among these nine metacaspases, five genes prefer arginine rather than aspartate as a P1 amino acid at the cleavage site [109–111]. It has been hypothesized that the metacaspases are the functional homologs of animal caspases in plants.
A protease, cleaving the caspase substrate Asp-Glu-Val-Asp (DEVDase activity), is induced within 30 min and peaks at 1 h by UV overexposure under light conditions [17]. Broad-range cysteine protease inhibitors did not affect the DEVDase activity. However, caspase-1 and caspase-3 inhibitors and the pan-caspase inhibitor p35 were able to block this activity. Strong, ectopic expression of AtDAD1 and AtDAD2, A. thaliana homologs of Defender against Apoptotic Death-1, can suppress the DNA fragmentation caused by DEVDase activity and, in turn increase cell survivability under stress conditions (Figure 2). The activation of caspase-3-like protease in UV-C-induced PCD has been documented using FRET technique which is a powerful tool for monitoring key events of PCD in living cells [112]. These data confirm that the caspase-like activities observed during HR, heat stress and UV exposure function in a similar manner to animal caspases. Apart from this indirect evidence, it has been demonstrated that there is induction of Arabidopsis metacaspase-8 (AtMC8), in response to oxidative stresses caused by UV-C, H2O2, or methyl viologen [67]. The induction of AtMC8 depends on AtRCD1 and thus, mutant defective in either atmc8-1/2 or rcd 1-1 lines display tolerance to cell death induced by ultraviolet light. Interestingly, the induction of AtMC8 transcripts in response to UV treatments also depends upon light showing it to be an integral part of plant specific PCD. However, it is not clear how light promotes cell death in response to stress.
3.5. UV-Induced ROS and Cell Death
In plants, ROS are generated as byproducts of essential energy generating processes such as photosynthesis and respiration. Therefore, some ROS production by chloroplasts and mitochondria is an unavoidable incidence in plants [113–115]. It was demonstrated that ROS are induced in response to different biotic and abiotic stresses. Recently, in addition to being a cell damaging agent, ROS were described as secondary signaling molecules [116]. ROS-mediated signaling is a complex mechanism that depends on the nature of the individual ROS species produced, the balance between ROS producing enzymes, and the oxidation–reduction states of various antioxidants [117]. Plants show elevated levels of ROS due to disruption of metabolic activities and increased activity of membrane-localized NADPH-oxidase in response to UV radiation [118, 119]. High UV-B radiation exposure to plants causes conditions similar to oxidative stress resulting from ROS and the gene expression pattern exhibited under both conditions is similar [120]. UVR8 mediated acclimatization is important for the survival of the plant against oxidative stress caused by UV- B radiation.
The defense responses to various pathogenic attacks [121] and other unfavorable conditions fails to control excess ROS accumulation leading to oxidative stress and may lead to cell death. Recently, the role of ROS as a trigger of UV-induced cell death processes in plants was studied. ROS acts as a signaling molecule leading to the opening of the permeability transition pore (PTP) in the mitochondrial membrane, which leads to the release of cytochrome c and the generation of more ROS, causing a feedback loop that amplifies the original PCD-inducing stress signal [58, 122]. ROS-dependent release of cyrochrome c from mitochondria during heat stress in tobacco cells suggested that the mitochondria may play a similar function in PCD in both plants and animals [123]. The role of mitochondria in the early stages of PCD induced by UV-C overexposure was confirmed using a protoplast system [18]. It was reported that before the onset of cell death, there was a loss of mitochondrial transmembrane potential (MTP). Subsequently, the mitochondria were found irregularly clumped around chloroplasts or aggregates in other places within the cytoplasm culminating in a loss of cellular movement of the mitochondria. The addition of cyclosporine, a known inhibitor of mitochondrial permeability transition pores (MPTP), effectively reduced the loss of MTP and in turn repressed cell death [18].
There are also roles for ROS and phytohormones [salicylic acid (SA), jasmonic acid (JA) and ethylene] in plant PCD [124]. In plants, the exposure to UV radiation results in transcriptional activation and repression of specific sets of genes. These regulatory pathways share common components involved in oxidative stress responses to pathogen attack, ozone exposure and wounding, including the involvement of ROS, SA, JA and ethylene. In response to UV-B exposure, SA levels increase, which triggers the induction of various pathogenesis related (PR) genes. Transgenic plants expressing NahG, a bacterial enzyme that degrades salicylate to catechol, have significantly lower levels of SA with a concomitant reduction in the expression of PR1 after UV-B exposure. These transgenic lines show increased sensitivity towards UV-B radiation [125, 126]. In field conditions, UV-B radiation does not induces JA levels but increases tissue sensitivity towards JA. UV-B induces the accumulation of phenolic compounds using jasmonate-dependent (polyamine conjugates) and jasmonate-independent (flavonoids and chlorogenic acid) pathways mediated by UVR8 [127, 128]. The JA insensitive mutant jar1 is defective in the recovery from UV-B treatment indicating that JA is important for tolerance to UV radiation [127, 129]. Like SA and JA, ethylene is also induced in response to diverse external stresses such as drought, ozone exposure and UV-B [97]. Although, ethylene is not required for induction of PR1 gene expression in response to pathogen infection, the ethylene insensitive mutant etr1 was unable to induce PR1 and PDF1.2 gene expression in response to UV-B radiation [130].