UV radiation reduces epidermal cell expansion in leaves of Arabidopsis thaliana

A dose-dependent effect of UV-328 on photosynthesis: Exploring light harvesting and UV-B sensing mechanisms

Benzotriazole ultraviolet (UV) stabilizers (BUVs) have emerged as significant environmental contaminants, frequently detected in various ecosystems. While the toxicity of BUVs to aquatic organisms is well-documented, studies on their impact on plant life are scarce. Plants are crucial as they provide the primary source of energy and organic matter in ecosystems through photosynthesis. This study investigated the effects of UV-328 (2-(2-hydroxy-4',6'-di-tert-amylphenyl) benzotriazole) on plant growth indices and photosynthesis processes, employing conventional physiological experiments, RNA sequencing (RNA-seq) analysis, and computational methods. Results demonstrated a biphasic response in plant biomass and the maximum quantum yield of PS II (Fv/Fm), showing improvement at a 50 μM UV-328 treatment but reduction under 150 μM UV-328 exposure. Additionally, disruption in thylakoid morphology was observed at the higher concentration. RNA-seq and qRT-PCR analysis identified key differentially expressed genes (light-harvesting chlorophyll-protein complex Ⅰ subunit A4, light-harvesting chlorophyll b-binding protein 3, UVR8, and curvature thylakoid 1 A) related to photosynthetic light harvesting, UV-B sensing, and chloroplast structure pathways, suggesting they may contribute to the observed alterations in photosynthesis activity induced by UV-328 exposure. Molecular docking analyses further supported the binding affinity between these proteins and UV-328. Overall, this study provided comprehensive physiological and molecular insights, contributing valuable information to the evaluation of the potential risks posed by UV-328 to critical plant physiological processes.

Ultraviolet-B radiation in relation to agriculture in the context of climate change: a review

Over the past few decades, the amount of ultraviolet-B radiation (UV-B) reaching the earth's surface has been altered due to climate change and stratospheric ozone dynamics. This narrow but highly biologically active spectrum of light (280-320 nm) can affect plant growth and development. Depletion of ozone and climate change are interlinked in a very complicated manner, i.e., significantly contributing to each other. The interaction of climate change, ozone depletion, and changes in UV-B radiation negatively affects the growth, development, and yield of plants. Furthermore, this interaction will become more complex in the coming years. The ozone layer reduction is paving a path for UV-B radiation to impact the surface of the earth and interfere with the plant's normal life by negatively affecting the plant's morphology and physiology. The nature and degree of the future response of the agricultural ecosystem to the decreasing or increasing UV-B radiation in the background of climate change and ozone dynamics are still unclear. In this regard, this review aims to elucidate the effects of enhanced UV-B radiation reaching the earth's surface due to the depletion of the ozone layer on plants' physiology and the performance of major cereals.

E2Fb and E2Fa transcription factors independently regulate the DNA damage response after ultraviolet B exposure in Arabidopsis

Ultraviolet (UV)B radiation affects plant growth inhibiting cell proliferation. This inhibition is in part controlled by the activity of transcription factors from the E2F family. In particular, the participation of E2Fc and E2Fe in UV-B responses in Arabidopsis plants was previously reported. However, the E2Fa and E2Fb contribution to these processes has still not been investigated. Thus, in this work, we provide evidence that, in Arabidopsis, both E2Fa and E2Fb control leaf size under UV-B conditions without participating in the repair of cyclobutane pyrimidine dimers in the DNA. Nevertheless, in UV-B-exposed seedlings, E2Fa, but not E2Fb, regulates primary root elongation, cell proliferation, and programmed cell death in the meristematic zone. Using e2fa mutants that overexpress E2Fb, we showed that the role of E2Fa in the roots could not be replaced by E2Fb. Finally, our results show that E2Fa and E2Fb differentially regulate the expression of genes that activate the DNA damage response and cell cycle progression, both under conditions without UV-B and after exposure. Overall, we showed that both E2Fa and E2Fb have different and non-redundant roles in developmental and DNA damage responses in Arabidopsis plants exposed to UV-B.

Comparative Growth, Photosynthetic Pigments, and Osmolytes Analysis of Hemp (Cannabis sativa L.) Seedlings under an Aeroponics System with Different LED Light Sources

The performance of hemp seedlings was evaluated through morphological traits, photosynthetic pigments, and osmolytes under 11 light treatments (10 LED light compositions + natural light) in an aeroponics system. The seedlings were brought under treatment at 25 days of age, where the light intensity was 300 µmol m−2s−1 and duration was 20 days. A higher leaf number and node number were observed in L10 (R4:B2:W2:FR1:UV1) and L11 (R2:B2:G2:W2:FR1:UV1), and a higher leaf length and leaf width were recorded in the L2 (white), L3 (R8:B2), and L5 (R7:B2:FR1) treatments. Furthermore, a higher shoot length was recorded in L3 (R8:B2), L6 (R6:B2:G1:FR1), and L9 (R6:B2:FR1:UV1) while roots developed more in the L1 (natural light), L5 (R7:B2:FR1), and L9 (R6:B2:FR1:UV1) treatments. On the other hand, the L3 (R8:B2) treatment manifested higher chlorophyll a, chlorophyll b, and photosynthetic quantum yield (Fv/Fm). The hierarchical clustering and heatmap analysis revealed that higher leaf numbers and node numbers resulted in bushy plants with shorter shoots and longer roots. A negative correlation was also observed in photosynthetic traits (pigments and fluorescence) with osmolytes and root length. Importantly, the treatments L4 (R7:B2:G1), L6 (R6:B2:G1:FR1), L8 (R5:B2:G1:FR1:UV1), and L11 (R2:B2:G2:W2:FR1:UV1) manifested higher nodes with a higher osmolyte content, such as proline, ascorbic acid, total soluble carbohydrate, and sucrose, which may be a helpful indicator for higher branches and inflorescences, and ultimately higher cannabinoids accumulation in the plants. The approach and findings of this study could provide future research with the baseline information on optimizing the light composition to produce hemp plants with ideal phenotypes.

Isotopic and morphologic proxies for reconstructing light environment and leaf function of fossil leaves: a modern calibration in the Daintree Rainforest, Australia

PREMISE

Within closed-canopy forests, vertical gradients of light and atmospheric CO₂ drive variations in leaf carbon isotope ratios, leaf mass per area (LMA), and the micromorphology of leaf epidermal cells. Variations in traits observed in preserved or fossilized leaves could enable inferences of past forest canopy closure and leaf function and thereby habitat of individual taxa. However, as yet no calibration study has examined how isotopic, micro- and macromorphological traits, in combination, reflect position within a modern closed-canopy forest or how these could be applied to the fossil record.

METHODS

Leaves were sampled from throughout the vertical profile of the tropical forest canopy using the 48.5 m crane at the Daintree Rainforest Observatory, Queensland, Australia. Carbon isotope ratios, LMA, petiole metric (i.e., petiole-width² /leaf area, a proposed proxy for LMA that can be measured from fossil leaves), and leaf micromorphology (i.e., undulation index and cell area) were compared within species across a range of canopy positions, as quantified by leaf area index (LAI).

RESULTS

Individually, cell area, δ¹³ C, and petiole metric all correlated with both LAI and LMA, but the use of a combined model provided significantly greater predictive power.

CONCLUSIONS

Using the observed relationships with leaf carbon isotope ratio and morphology to estimate the range of LAI in fossil floras can provide a measure of canopy closure in ancient forests. Similarly, estimates of LAI and LMA for individual taxa can provide comparative measures of light environment and growth strategy of fossil taxa from within a flora.

Effect of UV radiation and altitude characteristics on the functional traits and leaf optical properties in Saxifraga hostii at the alpine and montane sites in the Slovenian Alps

UV radiation affects the biochemical, physiological and morphological responses of plants. The effect is most pronounced at high altitude, such as alpine regions, and low latitude environments. The effect of UV radiation is impacted by different environmental conditions including temperature. We examined the response of the alpine plant Saxifraga hostii Tausch subsp. hostii growing at two altitudes (montane, 1100 m a.s.l. and alpine, 1500 m a.s.l.) in the Slovenian Alps. Selected ecophysiological, anatomical and pigment analyses along with measurements of the leaf optical properties were carried out during the growing season from July to September. Plants were grown under two different UV levels, near-ambient UV (UV) and reduced UV (UV-) radiation, and temperature conditions were monitored at both altitudes. Saxifraga hostii exhibited high photochemical efficiency of photosystem II and stomatal conductance under near-ambient UV radiation in August, which indicates that it is a well-acclimated plant. In September, photochemical efficiency was higher under reduced UV at the alpine altitude which together with a lower photosynthetic pigment content indicate delayed senescence for plants growing under reduced UV. Most leaf tissue thicknesses were not affected by UV radiation and altitude difference. There was a trend of increased stomatal density and reduced stomatal length on both leaf surfaces under near-ambient UV in August. However, there was no effect of UV attenuation or location at the alpine or montane site on the content of UV-B absorbing compounds, which implies the plant's tolerance of UV-B radiation. Saxifraga hostii leaves showed high absorption in the UV spectrum at higher altitudes, as shown by their optical properties. This study shows that Saxifraga hostii is well-acclimated to ambient UV radiation and to the environmental conditions at both altitudes. The effect of UV radiation is impacted by site conditions and this produces diverse plant responses, which contribute to the specific functional traits of Saxifraga hostii in the high-altitude environment.

AtCAF-1 mutants show different DNA damage responses after ultraviolet-B than those activated by other genotoxic agents in leaves

Chromatin assembly factor-1 (CAF-1) is a histone H3/H4 chaperone that participates in DNA and chromatin interaction processes. In this manuscript, we show that organs from CAF-1 deficient plants respond differently to ultraviolet-B (UV-B) radiation than to other genotoxic stresses. For example, CAF-1 deficient leaves tolerate better UV-B radiation, showing lower cyclobutane pyrimidine dimer (CPD) accumulation, lower inhibition of cell proliferation, increased cell wall thickness, UV-B absorbing compounds, and ploidy levels, whereas previous data from different groups have shown that CAF-1 mutants show shortening of telomeres, loss of 45S rDNA, and increased homologous recombination, phenotypes associated to DNA breaks. Interestingly, CAF-1 deficient roots show increased inhibition of primary root elongation, with decreased meristem size due to a higher inhibition of cell proliferation after UV-B exposure. The decrease in root meristem size in CAF-1 mutants is a consequence of defects in programmed cell death after UV-B exposure. Together, we provide evidence demonstrating that root and shoot meristematic cells may have distinct protection mechanisms against CPD accumulation by UV-B, which may be linked with different functions of the CAF-1 complex in these different organs.

Single-dose β-aminobutyric acid treatment modifies tobacco (Nicotiana tabacum L.) leaf acclimation to consecutive UV-B treatment

β-Aminobutyric acid (BABA) pre-treatment has been shown to alter both biotic and abiotic stress responses. The present study extends this observation to acclimative UV-B-response, which has not been explored in this context so far. A single soil application of 300 ppm BABA modified the non-enzymatic antioxidant capacities and the leaf hydrogen peroxide levels in tobacco (Nicotiana tabacum L.) leaves in response to a 9-day treatment with 5.4 kJ m-2 d-1 biologically effective supplementary UV-B radiation in a model experiment that was performed in a growth chamber. BABA decreased leaf hydrogen peroxide levels both as a single factor and in combination with UV-B, but neither BABA nor UV-B affected leaf photochemistry significantly. The total antioxidant capacities were increased by either BABA or UV-B, and this response was additive in BABA pre-treated leaves. These results together with the observed changes in hydroxyl radical neutralising ability and non-enzymatic hydrogen peroxide antioxidant capacities show that BABA pre-treatment (i) has a long-term effect on leaf antioxidants even in the absence of other factors and (ii) modifies acclimative readjustment of prooxidant-antioxidant balance in response to UV-B. BABA-inducible antioxidants do not include phenolic compounds as a UV-B-induced increase in the adaxial leaf flavonoid index and total leaf extract UV absorption were unaffected by BABA.

Arabidopsis E2Fc is required for the DNA damage response under UV-B radiation epistatically over the microRNA396 and independently of E2Fe

UV-B radiation inhibits plant growth, and this inhibition is, to a certain extent, regulated by miR396-mediated repression of Growth Regulating Transcription factors (GRFs). Moreover, E2Fe transcription factor also modulates Arabidopsis leaf growth. Here, we provide evidence that, at UV-B intensities that induce DNA damage, E2Fc participates in the inhibition of cell proliferation. We demonstrate that E2Fc-deficient plants show a lower inhibition of leaf size under UV-B conditions that damage DNA, decreased cell death after exposure and altered SOG1 and ATR expression. Interestingly, the previously reported participation of E2Fe in UV-B responses, which is a transcriptional target of E2Fc, is independent and different from that described for E2Fc. Conversely, we here demonstrate that E2Fc has an epistatic role over the miR396 pathway under UV-B conditions. Finally, we show that inhibition of cell proliferation by UV-B is independent of the regulation of class II TCP transcription factors. Together, our results demonstrate that E2Fc is required for miR396 activity on cell proliferation under UV-B, and that its role is independent of E2Fe, probably modulating DNA damage responses through the regulation of SOG1 and ATR transcript levels.

Extrapolation of significant genes and transcriptional regulatory networks involved in Zea mays in response in UV-B stress

A wide range of plant species growth influenced when they exposed to solar UV-B radiation. Leaves of the plant are highly affected by UV-B radiation lead to the reduction in the growth of the plant. Current work demonstrates the comparative transcriptional changes and visible symptoms occurred in the maize leaf growth zone (GZ). Primary objective of this study was to identify differentially expressed genes (DEGs) responsible for leaf growth and their association in the transcriptional regulatory network under UV-B stress. Whole transcriptomic data was analysed and the quality check was tested for each sample and further genome-wide mapping and DEGs were performed. Gene Ontology (GO) based functional annotation, associated transcriptional networks and molecular pathways were annotated. Reduction in cell production due to UV-B stress causes a decrease in leaf's length and size was observed. Further, the specific role of the DEGs, in UV-B signalling pathways and other molecular functions responsible for leaf cell death was discovered. Results also infer that the major changes occurred in the cell cycle, transcriptional regulation, post-transcriptional modification, phytohormones, flavonoids biosynthesis, and chromatin remodeling. UV-B signalling pathways and the transcriptional regulatory networks infer the different molecular steps along with downstream transcriptional and post-transcriptional control of metabolic enzymes used in long-term memory adoption and attainment resistance to UV-B stress identified. Effects of UV-B radiation on leaf growth was noted in this study. UV-B stress response genes and associated transcriptional regulatory networks were identified, can be used in developing the marker assist UB-B stress tolerant genotypes of the maize.

An ultraviolet B condition that affects growth and defense in Arabidopsis

Ultraviolet B light (UV-B, 280-315 nm) is the shortest wavelength of the solar spectrum reaching the surface of the Earth. It has profound effects on plants, ranging from growth regulation to severe metabolic changes. Low level UV-B mainly causes photomorphogenic effects while higher levels can induce stress, yet these effects tend to overlap. Here we identified a condition that allows growth reduction without obvious detrimental stress in wild type Arabidopsis rosette plants. This condition was used to study the effects of a daily UV-B dose on plant characteristics of UV-B adapted plants in detail. Exploration of the transcriptome of developing leaves indicated downregulation of genes involved in stomata formation by UV-B, while at the same time genes involved in photoprotective pigment biosynthesis were upregulated. These findings correspond with a decreased stomatal density and increased UV-B absorbing pigments. Gene ontology analysis revealed upregulation of defense related genes and meta-analysis showed substantial overlap of the UV-B regulated transcriptome with transcriptomes of salicylate and jasmonate treated as well as herbivore exposed plants. Feeding experiments showed that caterpillars of Spodoptera littoralis are directly affected by UV-B, while performance of the aphid Myzus persicae is diminished by a plant mediated process.

Developmental reprogramming by UV-B radiation in plants

Plants are extremely plastic organisms with the ability to adapt and respond to the changing environmental conditions surrounding them. Sunlight is one of the main resources for plants, both as a primary energy source for photosynthesis and as a stimulus that regulates different aspects of their growth and development. UV-B comprises wavelengths that correspond to a high energy region of the solar spectrum capable of reaching the biosphere, influencing plant growth. It is currently believed that plants are able to acclimate when growing under the influence of this radiation and perceive it as a signal, without stress signs. Nonetheless, many UV-B induced changes are elicited after DNA damage occurs as a consequence of exposure. In this review we focus on the influence of UV-B on leaf, flower and root development and emphasize the limited understanding of the molecular mechanisms for most of this developmental processes affected by UV-B documented over the years of research in this area.

Environmental plasticity of Pinot noir grapevine leaves: A trans-European study of morphological and biochemical changes along a 1,500-km latitudinal climatic gradient

A 2-year study explored metabolic and phenotypic plasticity of sun-acclimated Vitis vinifera cv. Pinot noir leaves collected from 12 locations across a 36.69-49.98°N latitudinal gradient. Leaf morphological and biochemical parameters were analysed in the context of meteorological parameters and the latitudinal gradient. We found that leaf fresh weight and area were negatively correlated with both global and ultraviolet (UV) radiation, cumulated global radiation being a stronger correlator. Cumulative UV radiation (sumUVR) was the strongest correlator with most leaf metabolites and pigments. Leaf UV-absorbing pigments, total antioxidant capacities, and phenolic compounds increased with increasing sumUVR, whereas total carotenoids and xanthophylls decreased. Despite of this reallocation of metabolic resources from carotenoids to phenolics, an increase in xanthophyll-cycle pigments (the sum of the amounts of three xanthophylls: violaxanthin, antheraxanthin, and zeaxanthin) with increasing sumUVR indicates active, dynamic protection for the photosynthetic apparatus. In addition, increased amounts of flavonoids (quercetin glycosides) and constitutive β-carotene and α-tocopherol pools provide antioxidant protection against reactive oxygen species. However, rather than a continuum of plant acclimation responses, principal component analysis indicates clusters of metabolic states across the explored 1,500-km-long latitudinal gradient. This study emphasizes the physiological component of plant responses to latitudinal gradients and reveals the physiological plasticity that may act to complement genetic adaptations.

HAC1 and HAF1 Histone Acetyltransferases Have Different Roles in UV-B Responses in Arabidopsis

Arabidopsis has 12 histone acetyltransferases grouped in four families: the GNAT/HAG, the MYST/HAM, the p300/CBP/HAC and the TAFII250/HAF families. We previously showed that ham1 and ham2 mutants accumulated higher damaged DNA after UV-B exposure than WT plants. In contrast, hag3 RNA interference transgenic plants showed less DNA damage and lower inhibition of plant growth by UV-B, and increased levels of UV-B-absorbing compounds. These results demonstrated that HAM1, HAM2, and HAG3 participate in UV-B-induced DNA damage repair and signaling. In this work, to further explore the role of histone acetylation in UV-B responses, a putative function of other acetyltransferases of the HAC and the HAF families was analyzed. Neither HAC nor HAF acetyltrasferases participate in DNA damage and repair after UV-B radiation in Arabidopsis. Despite this, haf1 mutants presented lower inhibition of leaf and root growth by UV-B, with altered expression of E2F transcription factors. On the other hand, hac1 plants showed a delay in flowering time after UV-B exposure and changes in FLC and SOC1 expression patterns. Our data indicate that HAC1 and HAF1 have crucial roles for in UV-B signaling, confirming that, directly or indirectly, both enzymes also have a role in UV-B responses.

Alteration in Auxin Homeostasis and Signaling by Overexpression Of PINOID Kinase Causes Leaf Growth Defects in Arabidopsis thaliana

In plants many developmental processes are regulated by auxin and its directional transport. PINOID (PID) kinase helps to regulate this transport by influencing polar recruitment of PIN efflux proteins on the cellular membranes. We investigated how altered auxin levels affect leaf growth in Arabidopsis thaliana. Arabidopsis mutants and transgenic plants with altered PID expression levels were used to study the effect on auxin distribution and leaf development. Single knockouts showed small pleiotropic growth defects. Contrastingly, several leaf phenotypes related to changes in auxin concentrations and transcriptional activity were observed in PID overexpression (PID^OE ) lines. Unlike in the knockout lines, the leaves of PID^OE lines showed an elevation in total indole-3-acetic acid (IAA). Accordingly, enhanced DR5-visualized auxin responses were detected, especially along the leaf margins. Kinematic analysis revealed that ectopic expression of PID negatively affects cell proliferation and expansion rates, yielding reduced cell numbers and small-sized cells in the PID^OE leaves. We used PID^OE lines as a tool to study auxin dose effects on leaf development and demonstrate that auxin, above a certain threshold, has a negative affect on leaf growth. RNA sequencing further showed how subtle PID^OE -related changes in auxin levels lead to transcriptional reprogramming of cellular processes.

REPRESSOR OF ULTRAVIOLET-B PHOTOMORPHOGENESIS function allows efficient phototropin mediated ultraviolet-B phototropism in etiolated seedlings

Ultraviolet B (UV-B) light is a part of the solar radiation which has significant effects on plant morphology, even at low doses. In Arabidopsis, many of these morphological changes have been attributed to a specific UV-B receptor, UV resistance locus 8 (UVR8). Recent findings showed that next to phototropin regulated phototropism, UVR8 mediated signaling is able of inducing directional bending towards UV-B light in etiolated seedlings of Arabidopsis, in a phototropin independent manner. In this study, kinetic analysis of phototropic bending was used to evaluate the relative contribution of each of these pathways in UV-B mediated phototropism. Diminishing UV-B light intensity favors the importance of phototropins. Molecular and genetic analyses suggest that UV-B is capable of inducing phototropin signaling relying on phototropin kinase activity and regulation of NPH3. Moreover, enhanced UVR8 responses in the UV-B hypersensitive rup1rup2 mutants interferes with the fast phototropin mediated phototropism. Together the data suggest that phototropins are the most important receptors for UV-B induced phototropism in etiolated seedlings, and a RUP mediated negative feedback pathway prevents UVR8 signaling to interfere with the phototropin dependent response.

Integration and scaling of UV-B radiation effects on plants: from molecular interactions to whole plant responses

A process based model integrating the effects of UV-B radiation to molecular level processes and their consequences to whole plant growth and development was developed from key parameters in the published literature. Model simulations showed that UV-B radiation induced changes in plant metabolic and/or photosynthesis rates can result in plant growth inhibitions. The costs of effective epidermal UV-B radiation absorptive compounds did not result in any significant changes in plant growth, but any associated metabolic costs effectively reduced the potential plant biomass. The model showed significant interactions between UV-B radiation effects and temperature and any factor leading to inhibition of photosynthetic production or plant growth during the midday, but the effects were not cumulative for all factors. Vegetative growth were significantly delayed in species that do not exhibit reproductive cycles during a growing season, but vegetative growth and reproductive yield in species completing their life cycle in one growing season did not appear to be delayed more than 2-5 days, probably within the natural variability of the life cycles for many species. This is the first model to integrate the effects of increased UV-B radiation through molecular level processes and their consequences to whole plant growth and development.

Integration and scaling of UV-B radiation effects on plants: from DNA to leaf

A process-based model integrating the effects of UV-B radiation through epidermis, cellular DNA, and its consequences to the leaf expansion was developed from key parameters in the published literature. Enhanced UV-B radiation-induced DNA damage significantly delayed cell division, resulting in significant reductions in leaf growth and development. Ambient UV-B radiation-induced DNA damage significantly reduced the leaf growth of species with high relative epidermal absorbance at longer wavelengths and average/low pyrimidine cyclobutane dimers (CPD) photorepair rates. Leaf expansion was highly dependent on the number of CPD present in the DNA, as a result of UV-B radiation dose, quantitative and qualitative absorptive properties of epidermal pigments, and repair mechanisms. Formation of pyrimidine-pyrimidone (6-4) photoproducts (6-4PP) has no effect on the leaf expansion. Repair mechanisms could not solely prevent the UV-B radiation interference with the cell division. Avoidance or effective shielding by increased or modified qualitative epidermal absorptance was required. Sustained increased UV-B radiation levels are more detrimental than short, high doses of UV-B radiation. The combination of low temperature and increased UV-B radiation was more significant in the level of UV-B radiation-induced damage than UV-B radiation alone. Slow-growing leaves were more affected by increased UV-B radiation than fast-growing leaves.

Re-interpreting plant morphological responses to UV-B radiation

There is a need to reappraise the effects of UV-B radiation on plant morphology in light of improved mechanistic understanding of UV-B effects, particularly elucidation of the UV RESISTANCE LOCUS 8 (UVR8) photoreceptor. We review responses at cell and organismal levels, and explore their underlying regulatory mechanisms, function in UV protection and consequences for plant fitness. UV-induced morphological changes include thicker leaves, shorter petioles, shorter stems, increased axillary branching and altered root:shoot ratios. At the cellular level, UV-B morphogenesis comprises changes in cell division, elongation and/or differentiation. However, notwithstanding substantial new knowledge of molecular, cellular and organismal UV-B responses, there remains a clear gap in our understanding of the interactions between these organizational levels, and how they control plant architecture. Furthermore, despite a broad consensus that UV-B induces relatively compact architecture, we note substantial diversity in reported phenotypes. This may relate to UV-induced morphological changes being underpinned by different mechanisms at high and low UV-B doses. It remains unproven whether UV-induced morphological changes have a protective function involving shading and decreased leaf penetration of UV-B, counterbalancing trade-offs such as decreased photosynthetic light capture and plant-competitive abilities. Future research will need to disentangle seemingly contradictory interactions occurring at the threshold UV dose where regulation and stress-induced morphogenesis overlap.

How does solar ultraviolet-B radiation improve drought tolerance of silver birch (Betula pendula Roth.) seedlings?

We hypothesized that solar ultraviolet (UV) radiation would protect silver birch seedlings from the detrimental effects of water stress through a coordinated suite of trait responses, including morphological acclimation, improved control of water loss through gas exchange and hydraulic sufficiency. To better understand how this synergetic interaction works, plants were grown in an experiment under nine treatment combinations attenuating ultraviolet-A and ultraviolet-B (UVB) from solar radiation together with differential watering to create water-deficit conditions. In seedlings under water deficit, UV attenuation reduced height growth, leaf production and leaf length compared with seedlings receiving the full spectrum of solar radiation, whereas the growth and morphology of well-watered seedlings was largely unaffected by UV attenuation. There was an interactive effect of the treatment combination on water relations, which was more apparent as a change in the water potential at which leaves wilted or plants died than through differences in gas exchange. This suggests that changes occur in the cell wall elastic modulus or accumulation of osmolites in cells under UVB. Overall, the strong negative effects of water deficit are partially ameliorated by solar UV radiation, whereas well-watered silver birch seedlings are slightly disadvantaged by the solar UV radiation they receive.

Acclimation to UV-B radiation and visible light in Lactuca sativa involves up-regulation of photosynthetic performance and orchestration of metabolome-wide responses

UV-B radiation is often viewed as a source of stress for higher plants. In particular, photosynthetic function has been described as a common target for UV-B impairment; yet as our understanding of UV-B photomorphogenesis increases, there are opportunities to expand the emerging paradigm of regulatory UV response. Lactuca sativa is an important dietary crop species and is often subjected to rapid sunlight exposure at field transfer. Acclimation to UV-B and visible light conditions in L. sativa was dissected using gas exchange and chlorophyll fluorescence measurements, in addition to non-destructive assessments of UV epidermal shielding (SUV ). After UV-B treatment, seedlings were subjected to wide-range metabolomic analysis using liquid chromatography hybrid quadrupole time-of-flight high-resolution mass spectrometry (LC-QTOF-HRMS). During the acclimation period, net photosynthetic rate increased in UV-treated plants, epidermal UV shielding increased in both subsets of plants transferred to the acclimatory conditions (UV+/UV- plants) and Fv /Fm declined slightly in UV+/UV- plants. Metabolomic analysis revealed that a key group of secondary compounds was up-regulated by higher light conditions, yet several of these compounds were elevated further by UV-B radiation. In conclusion, acclimation to UV-B radiation involves co-protection from the effects of visible light, and responses to UV-B radiation at a photosynthetic level may not be consistently viewed as damaging to plant development.

Three-dimensional patterns of cell division and expansion throughout the development of Arabidopsis thaliana leaves

Variations in size and shape of multicellular organs depend on spatio-temporal regulation of cell division and expansion. Here, cell division and expansion rates were quantified relative to the three spatial axes in the first leaf pair of Arabidopsis thaliana. The results show striking differences in expansion rates: the expansion rate in the petiole is higher than in the leaf blade; expansion rates in the lateral direction are higher than longitudinal rates between 5 and 10 days after stratification, but become equal at later stages of leaf blade development; and anticlinal expansion co-occurs with, but is an order of magnitude slower than periclinal expansion. Anticlinal expansion rates also differed greatly between tissues: the highest rates occurred in the spongy mesophyll and the lowest in the epidermis. Cell division rates were higher and continued for longer in the epidermis compared with the palisade mesophyll, causing a larger increase of palisade than epidermal cell area over the course of leaf development. The cellular dynamics underlying the effect of shading on petiole length and leaf thickness were then investigated. Low light reduced leaf expansion rates, which was partly compensated by increased duration of the growth phase. Inversely, shading enhanced expansion rates in the petiole, so that the blade to petiole ratio was reduced by 50%. Low light reduced leaf thickness by inhibiting anticlinal cell expansion rates. This effect on cell expansion was preceded by an effect on cell division, leading to one less layer of palisade cells. The two effects could be uncoupled by shifting plants to contrasting light conditions immediately after germination. This extended kinematic analysis maps the spatial and temporal heterogeneity of cell division and expansion, providing a framework for further research to understand the molecular regulatory mechanisms involved.

Footprints of the sun: memory of UV and light stress in plants

Sunlight provides the necessary energy for plant growth via photosynthesis but high light and particular its integral ultraviolet (UV) part causes stress potentially leading to serious damage to DNA, proteins, and other cellular components. Plants show adaptation to environmental stresses, sometimes referred to as "plant memory." There is growing evidence that plants memorize exposure to biotic or abiotic stresses through epigenetic mechanisms at the cellular level. UV target genes such as CHALCONE SYNTHASE (CHS) respond immediately to UV treatment and studies of the recently identified UV-B receptor UV RESISTANCE LOCUS 8 (UVR8) confirm the expedite nature of UV signaling. Considering these findings, an UV memory seems redundant. However, several lines of evidence suggest that plants may develop an epigenetic memory of UV and light stress, but in comparison to other abiotic stresses there has been relatively little investigation. Here we summarize the state of knowledge about acclimation and adaptation of plants to UV light and discuss the possibility of chromatin based epigenetic memory.

Role of Arabidopsis UV RESISTANCE LOCUS 8 in plant growth reduction under osmotic stress and low levels of UV-B

In high-light environments, plants are exposed to different types of stresses, such as an excess of UV-B, but also drought stress which triggers a common morphogenic adaptive response resulting in a general reduction of plant growth. Here, we report that the Arabidopsis thaliana UV RESISTANCE LOCUS 8 (UVR8) gene, a known regulator of the UV-B morphogenic response, was able to complement a Saccharomyces cerevisiae osmo-sensitive mutant and its expression was induced after osmotic or salt stress in Arabidopsis plants. Under low levels of UV-B, plants overexpressing UVR8 are dwarfed with a reduced root development and accumulate more flavonoids compared to control plants. The growth defects are mainly due to the inhibition of cell expansion. The growth inhibition triggered by UVR8 overexpression in plants under low levels of UV-B was exacerbated by mannitol-induced osmotic stress, but it was not significantly affected by ionic stress. In contrast, uvr8-6 mutant plants do not differ from wild-type plants under standard conditions, but they show an increased shoot growth under high-salt stress. Our data suggest that UVR8-mediated accumulation of flavonoid and possibly changes in auxin homeostasis are the underlying mechanism of the observed growth phenotypes and that UVR8 might have an important role for integrating plant growth and stress signals.

Footprints of the sun: memory of UV and light stress in plants

Sunlight provides the necessary energy for plant growth via photosynthesis but high light and particular its integral ultraviolet (UV) part causes stress potentially leading to serious damage to DNA, proteins, and other cellular components. Plants show adaptation to environmental stresses, sometimes referred to as "plant memory." There is growing evidence that plants memorize exposure to biotic or abiotic stresses through epigenetic mechanisms at the cellular level. UV target genes such as CHALCONE SYNTHASE (CHS) respond immediately to UV treatment and studies of the recently identified UV-B receptor UV RESISTANCE LOCUS 8 (UVR8) confirm the expedite nature of UV signaling. Considering these findings, an UV memory seems redundant. However, several lines of evidence suggest that plants may develop an epigenetic memory of UV and light stress, but in comparison to other abiotic stresses there has been relatively little investigation. Here we summarize the state of knowledge about acclimation and adaptation of plants to UV light and discuss the possibility of chromatin based epigenetic memory.

Review on shape formation in epidermal pavement cells of the Arabidopsis leaf

Epidermal pavement cells appear with a fascinating irregular wavy shape in the Arabidopsis thaliana leaf. This review addresses the questions of why this particular shape is produced during leaf development and how this is accomplished. To answer the first question most probably waviness offers some biomechanical benefits over other organisations. Different positions of lobe-formation are therefore explored and discussed. At the moment, however, no hard evidence that favours any one morphology is available. The latter question comprises the biomechanical accomplishment of shape and refers to the cell wall and cytoskeletal involvement herein. A current model for pavement cell development is discussed but remaining questions and pitfalls are put forward. Moreover, an overview of the genetic and biochemical regulatory pathways that are described up to date in the literature is presented.

Plant microtubules reorganization under the indirect UV-B exposure and during UV-B-induced programmed cell death

The role of microtubules in cellular pathways of UV-B signaling in plants as well as in related structural cell response become into focus of few last publications. As microtubules in plant cell reorient/reorganize (become randomized, fragmented or depolymerized) in a response to direct UV-B exposure, these cytoskeletal components could be involved into UV-B signaling pathways as highly responsive players. In the current addendum, indirect UV-B-induced microtubules reorganization in cells of shielded Arabidopsis thaliana (GFP-MAP4) primary roots and the correspondence of microtubules depolymerization with the typical hallmarks of the programmed cell death in Nicotiana tabacum BY-2 (GFP-MBD) cells are discussed.

From ozone depletion to agriculture: understanding the role of UV radiation in sustainable crop production

Largely because of concerns regarding global climate change, there is a burgeoning interest in the application of fundamental scientific knowledge in order to better exploit environmental cues in the achievement of desirable endpoints in crop production. Ultraviolet (UV) radiation is an energetic driver of a diverse range of plant responses and, despite historical concerns regarding the damaging consequences of UV-B radiation for global plant productivity as related to stratospheric ozone depletion, current developments representative of a range of organizational scales suggest that key plant responses to UV-B radiation may be exploitable in the context of a sustainable contribution towards the strengthening of global crop production, including alterations in secondary metabolism, enhanced photoprotection, up-regulation of the antioxidative response and modified resistance to pest and disease attack. Here, we discuss the prospect of this paradigm shift in photobiology, and consider the linkages between fundamental plant biology and crop-level outcomes that can be applied to the plant UV-B response, in addition to the consequences for related biota and many other facets of agro-ecosystem processes.

Acclimation and interaction between drought and elevated UV-B in A. thaliana: Differences in response over treatment, recovery and reproduction

Here, a factorial experiment was used to investigate the interactive effects of a UV-B episode and concurrent progressive drought on the growth, chemistry, and reproductive success of A. thaliana. Both drought and UV-B negatively affected rosette growth, although UV-B had the greater effect. Acclimation to UV-B involved adjustment of leaf morphology, while drought induced accumulation of soluble sugars and phenolics. All plants recovered from treatments, but the cost of recovery was a developmental delay resulting in alteration in phenological timings. Combined treatments interacted causing additive negative effects on growth following exposure. This may be linked with inhibition of soluble sugar accumulation by UV-B, restricting the capacity for osmotic adjustment in response to drought. Following cessation of treatments, relative growth rate (RGR) and net assimilation rate (NAR) were significantly stimulated in plants treated with combined drought and UV-B. This interaction alleviated subsequent impacts of elevated UV-B on silique yield and reproductive timings. This study demonstrates the potential for interaction between these two common environmental factors. Furthermore, it shows the changeable nature of these interactions over the course of exposure and recovery through to reproduction, highlighting the need for sustained assessment of such interactions over a plant's lifecycle.

Soybeans grown in the Chernobyl area produce fertile seeds that have increased heavy metal resistance and modified carbon metabolism

Plants grow and reproduce in the radioactive Chernobyl area, however there has been no comprehensive characterization of these activities. Herein we report that life in this radioactive environment has led to alteration of the developing soybean seed proteome in a specific way that resulted in the production of fertile seeds with low levels of oil and β-conglycinin seed storage proteins. Soybean seeds were harvested at four, five, and six weeks after flowering, and at maturity from plants grown in either non-radioactive or radioactive plots in the Chernobyl area. The abundance of 211 proteins was determined. The results confirmed previous data indicating that alterations in the proteome include adaptation to heavy metal stress and mobilization of seed storage proteins. The results also suggest that there have been adjustments to carbon metabolism in the cytoplasm and plastids, increased activity of the tricarboxylic acid cycle, and decreased condensation of malonyl-acyl carrier protein during fatty acid biosynthesis.

UV-B induced morphogenesis: four players or a quartet?

Low levels of ultraviolet (UV)-radiation alter the morphology of plants. UV-B exposure can lead to shorter petioles and shorter, narrower and/or thicker leaf blades. The resulting decrease in leaf area has been associated with inhibitory UV-B effects on biomass accumulation. In Arabidopsis, UV-B effects on leaf area have variously been attributed to altered cell division, cell expansion or combinations of these two processes. A dedicated UV-B sensory system, crosstalk between flavonoids and auxins, endoreduplication and generic Stress Induced Morphogenic Responses (SIMR) have all been proposed to contribute to the UV-B phenotype. Here, we propose that UV-mediated morphogenesis, rather than being controlled by a single regulatory pathway, is controlled by a regulatory blur involving multiple compensatory molecular and physiological feedback interactions.

Nitric oxide as a critical factor for perception of UV-B irradiation by microtubules in Arabidopsis

Influence of ultraviolet-B (UV-B) as an abiotic stress factor on plant microtubules (MTs) and involvement of nitric oxide (NO) as a secondary messenger mediating plant cell response to environmental stimuli were investigated in this study. Taking into account that endogenous NO content in plant cells has been shown to be increased under a broad range of abiotic stress factors, the effects of UV-B irradiation and also the combined action of UV-B and NO donor sodium nitroprusside (SNP) or NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO) on the MTs organization in different root cells of Arabidopsis thaliana were tested. Subsequently, realization of the MT-mediated processes such as root growth and development was studied under these conditions. Arabidopsis thaliana seedlings expressing the chimeric gene gfp-map4 were exposed to the enhanced UV-B with or without SNP or c-PTIO pretreatment. The UV-B irradiation alone led to a dose-dependent root growth inhibition and to morphological alterations of the primary root manifested in their swelling and excessive root hair formation. Moreover, dose-dependent randomization and depolymerization of MTs in both epidermal and cortical cells under the enhanced UV-B were found. However, SNP pretreatment of the UV-B irradiated A. thaliana seedlings recovered the UV-B inhibited root growth as compared to c-PTIO pretreatment. It has been shown that in 24 h after UV-B irradiation the organization of MTs in root epidermal cells of SNP-pretreated A. thaliana seedlings was partially recovered, whereas in c-PTIO-pretreated ones the organization of MTs has not been distinctly improved. Therefore, we suppose that the enhanced NO levels in plant cells can protect MTs organization as well as MT-related processes of root growth and development against disrupting effects of UV-B.

The phytohormone auxin is a component of the regulatory system that controls UV-mediated accumulation of flavonoids and UV-induced morphogenesis

In plants, ultraviolet (UV)-B acclimation is a complex, dynamic process that plays an essential role in preventing UV-B damage to targets such as DNA and the photosynthetic machinery. In this study we tested the hypothesis that the phytohormone auxin is a component of the regulatory system that controls both UV-mediated accumulation of flavonoids and UV-induced morphogenesis. We found that the leaf area of Arabidopsis thaliana Col-0 plants raised under a low dose of UV radiation (0.56 kJ m(-2) daily dose) was, on average, decreased by 23% relative to plants raised in the absence of UV-B, and this was accompanied by a decrease (P = 0.063) in free auxin in young leaf tissues. Compared to Col-0, both the auxin influx mutant axr4-1 and the auxin biosynthesis mutant nit1-3 displayed significantly stronger morphogenic responses, i.e. relative decreases in leaf area were greater for these two mutants. UV exposure also induced accumulation of flavonoids. In Col-0, increases in the concentrations of specific kaempferol derivatives ranged from 2.1- to 19-fold. Thus, UV induces complex changes in flavonoid-glycosylation patterns. Compared to Col-0, three auxin mutants displayed significantly different flavonoid profiles. Thus, based on mutant analysis, it is concluded that the phytohormone auxin plays a role in UV acclimation by regulating flavonoid concentration, flavonoid-glycosylation pattern and by controlling UV-induced morphogenic responses.

Species-specific effect of UV-B radiation on the temporal pattern of leaf growth

Recent molecular and physiological studies have demonstrated that ultraviolet-B radiation (UV-B) can affect some of the processes involved in leaf growth, but the phases of leaf growth affected have not been clearly delimited. We used functional growth analysis to assess the effects of UV-B radiation on the time course of leaf growth in seedlings of two birch species (Betula pendula and Betula pubescens). Our aim was to identify the phase(s) of leaf development affected by UV-B radiation. In a greenhouse study, 1-year-old birch seedlings were subjected to three daily doses of supplemental UV-B radiation treatments (UV-B⁺) and no UV-B radiation controls (UV-B⁻). Leaf growth measurements every 2 days were complemented by assessment of other functional traits over a 4-week period at the start of the growing season. Using fitted curves, we were able to determine that the rate of leaf expansion was slowed by the UV-B⁺ treatment in leaves of B. pendula because of a slower maximum leaf growth rate compared with plants under the UV-B⁻ controls, but that compensation toward the end of the period of expansion negated this difference when leaves reached their final size. UV-B⁺ had little effect on the rate of B. pubescens leaf growth despite a larger reduction in leaf final size due to UV-B⁺ than occurred in B. pendula leaves. In conclusion, effective regulation ameliorated the effects of UV-B radiation on leaf and seedling growth in B. pendula, whereas in B. pubescens, reductions in leaf final size under UV-B⁺ were consistent with a slightly reduced rate of height growth.

Response of Trifolium repens to UV-B radiation: morphological links to plant productivity and water availability

This study used nine populations of Trifolium repens L. (white clover) to investigate possible relationships between plant morphological attributes and responses to ultraviolet-B (UV-B) radiation. Plants were exposed to 0 or 13.3 kJ·m(-2) ·day(-1) UV-B for 12 weeks. Drought was applied in parallel to these treatments during the last 4 weeks of the experiment to test whether limited moisture availability would alter morphological UV-B responses. UV-B affected plant morphology under well-watered conditions, reducing leaf size by 15%, leaf number by 5% and stolon elongation by 19%. The number of leaf primordia in the apical bud was decreased by 4% under UV-B, and by 12% under drought. In drought-exposed plants, leaf size was reduced by 50%, leaf number by 30% and stolon elongation by 60%. In addition, drought reduced specific leaf area (SLA) by 33% and increased leaf percentage dry mass (PDM) by 40%. UV-B-induced reduction in plant biomass in the T. repens populations was associated with higher plant productivity and this was further linked to larger leaf size as well as to lower PDM. In conclusion, the findings suggest that morphological attributes conferring fast potential growth under productive conditions carry a cost in the form of lower biomass accumulation under UV-B.

UV radiation reduces epidermal cell expansion in Arabidopsis thaliana leaves without altering cellular microtubule organization

Upon chronic UV treatment pavement cell expansion in Arabidopsis leaves is reduced, implying alterations in symplastic and apoplastic properties of the epidermal cells. In this study, the effect of UV radiation on microtubule patterning is analysed, as microtubules are thought to serve as guiding rails for the cellulose synthase complexes depositing cellulose microfibrils. Together with hemicelluloses, these microfibrils are regarded as the load-bearing components of the cell wall. Leaves of transgenic plants with fluorescently tagged microtubules (GFP-TUA6) were as responsive to UV as wild type plants. Despite the UV-induced reduction in cell elongation, confocal microscopy revealed that cellular microtubule arrangements were seemingly not affected by the UV treatments. This indicates an unaltered deposition of cellulose microfibrils in the presence of UV radiation. Therefore, we surmise that the reduction in cell expansion in UV-treated leaves is most probably due to changes in cell wall loosening and/or turgor pressure.