Light Regulates Plant Alternative Splicing through the Control of Transcriptional Elongation.

Light makes carbon fixation possible, allowing plant and animal life on Earth. We have previously shown that light regulates alternative splicing in plants. Light initiates a chloroplast retrograde signaling that regulates nuclear alternative splicing of a subset of Arabidopsis thaliana transcripts. Here, we show that light promotes RNA polymerase II (Pol II) elongation in the affected genes, whereas in darkness, elongation is lower. These changes in transcription are consistent with elongation causing the observed changes in alternative splicing, as revealed by different drug treatments and genetic evidence. The light control of splicing and elongation is abolished in an Arabidopsis mutant defective in the transcription factor IIS (TFIIS). We report that the chloroplast control of nuclear alternative splicing in plants responds to the kinetic coupling mechanism found in mammalian cells, providing unique evidence that coupling is important for a whole organism to respond to environmental cues.

Efficient evaluation of a gene containment system for poplar through early flowering induction.

KEY MESSAGE: The early flowering system HSP::AtFT allowed a fast evaluation of a gene containment system based on the construct PsEND1::barnase-barstar for poplar. Transgenic lines showed disturbed pollen development and sterility. Vertical gene transfer through pollen flow from transgenic or non-native plant species into their crossable natural relatives is a major concern. Gene containment approaches have been proposed to reduce or even avoid gene flow among tree species. However, evaluation of genetic containment strategies for trees is very difficult due to the long-generation times. Early flowering induction would allow faster evaluation of genetic containment in this case. Although no reliable methods were available for the induction of fertile flowers in poplar, recently, a new early flowering approach was developed. In this study, early flowering poplar lines containing the gene construct PsEND1::barnase-barstar were obtained. The PsEND1 promoter was chosen due to its early expression pattern, its versality and efficiency for generation of male-sterile plants fused to the barnase gene. RT-PCRs confirmed barnase gene activity in flowers, and pollen development was disturbed, leading to sterile flowers. The system developed in this study represents a valuable tool for gene containment studies in forest tree species.

Light-Induced Artemisinin Biosynthesis Is Regulated by the bZIP Transcription Factor AaHY5 in Artemisia annua.

Artemisinin, the frontline drug against malaria, is a sesquiterpenoid extracted from Artemisia annua. Light has been proposed to play an important role in the activation of artemisinin biosynthesis. Here, we report the basic leucine zipper transcription factor (TF) AaHY5 as a key regulator of light-induced biosynthesis of artemisinin. We show that AaHY5 transcription overlaps with that of artemisinin biosynthesis genes in response to light and in A. annua tissues. Analysis of AaHY5 overexpression and RNAi-suppression lines suggests that AaHY5 is a positive regulator of the expression of artemisinin biosynthesis genes and accumulation of artemisinin. We show that AaHY5 complements the hy5 mutant in Arabidopsis thaliana. Our data further suggest that AaHY5 interacts with AaCOP1, the ubiquitin E3 ligase CONSTITUTIVE PHOTOMORPHOGENIC1 in A. annua. In yeast one-hybrid and transient expression assays, we demonstrate that AaHY5 acts via the TF GLANDULAR TRICHOME-SPECIFIC WRKY 1 (AaGSW1) in artemisinin regulation. In summary, we present a novel regulator of artemisinin gene expression and propose a model in which AaHY5 indirectly controls artemisinin production in response to changing light conditions.

MdCOL4 Interaction Mediates Crosstalk Between UV-B and High Temperature to Control Fruit Coloration in Apple.

In many plants, anthocyanin biosynthesis is affected by environmental conditions. Ultraviolet-B (UV-B) radiation promotes anthocyanin accumulation and fruit coloration in apple skin, whereas high temperature suppresses these processes. In this study, we characterized a B-box transcription factor, MdCOL4, from 'Fuji' apple, and identified its role in anthocyanin biosynthesis by overexpressing its encoding gene in apple red callus. The expression of MdCOL4 was reduced by UV-B, but promoted by high temperature. We explored the regulatory relationship between heat shock transcription factors (HSFs) and MdCOL4, and found that MdHSF3b and MdHSF4a directly bound to the heat shock element cis-element of the MdCOL4 promoter. MdCOL4 interacted with MdHY5 to synergistically inhibit the expression of MdMYB1, and MdCOL4 directly bound to the promoters of MdANS and MdUFGT, which encode genes in the anthocyanin biosynthetic pathway, to suppress their expression. Our findings shed light on the molecular mechanism by which MdCOL4 suppresses anthocyanin accumulation in apple skin under UV-B and high temperature.

Physiological function of photoreceptor UVR8 in UV-B tolerance in the liverwort Marchantia polymorpha.

MAIN CONCLUSION: The physiological importance of MpUVR8 in UV-B resistance and translocation in a UV-B-dependent manner from the cytosol into the nucleus is characterized in Marchantia polymorpha. UV RESISTANCE LOCUS 8 (UVR8) is an ultraviolet-B (UV-B) light receptor functioning for UV-B sensing and tolerance in Arabidopsis thaliana and other species. It is unclear whether UVR8 physiologically functions in UV-B-induced defense responses in Marchantia polymorpha, which belongs to the earliest diverging group of embryophyte lineages. Here, we demonstrate that UVR8 has a physiological function in UV-B tolerance and that there is a UVR8-dependent pathway involved. In addition, a UVR8-independent pathway is revealed. We examine the tissue-specific expression pattern of M. polymorpha UVR8 (MpUVR8), showing that it is highly expressed in the apical notch in thalli and gametangiophores, as well as in antheridial and archegonial heads. Furthermore, Mpuvr8KO plant transformants, in which the MpUVR8 locus was disrupted, were produced and analyzed to understand the physiological and molecular function of MpUVR8. Analysis using these plants indicates the important roles of MpUVR8 and MpUVR8-regulated genes, and of MpUVR8-independent pathways in UV-B tolerance. Subcellular localization of Citrine-fused MpUVR8 in M. polymorpha cells was also investigated. It was found to translocate from the cytosol into the nucleus in response to UV-B irradiation. Our findings indicate strong conservation of the physiological function of UVR8 and the molecular mechanisms for UVR8-dependent signal transduction through regulation of gene expression in embryophytes.

Low-fluence blue light-induced phosphorylation of Zmphot1 mediates the first positive phototropism.

Phototropin1 (phot1) perceives low- to high-fluence blue light stimuli and mediates both the first and second positive phototropisms. High-fluence blue light is known to induce autophosphorylation of phot1, leading to the second positive phototropism. However, the phosphorylation status of phot1 by low-fluence blue light that induces the first positive phototropism had not been observed. Here, we conducted a phosphoproteomic analysis of maize coleoptiles to investigate the fluence-dependent phosphorylation status of Zmphot1. High-fluence blue light induced phosphorylation of Zmphot1 at several sites. Notably, low-fluence blue light significantly increased the phosphorylation level of Ser291 in Zmphot1. Furthermore, Ser291-phosphorylated and Ser369Ser376-diphosphorylated peptides were found to be more abundant in the low-fluence blue light-irradiated sides than in the shaded sides of coleoptiles. The roles of these phosphorylation events in phototropism were explored by heterologous expression of ZmPHOT1 in the Arabidopsis thaliana phot1phot2 mutant. The first positive phototropism was restored in wild-type ZmPHOT1-expressing plants; however, plants expressing S291A-ZmPHOT1 or S369AS376A-ZmPHOT1 showed significantly reduced complementation rates. All transgenic plants tested in this study exhibited a normal second positive phototropism. These findings provide the first indication that low-fluence blue light induces phosphorylation of Zmphot1 and that this induced phosphorylation is crucial for the first positive phototropism.

Regulation of flowering by green light depends on its photon flux density and involves cryptochromes.

Photoperiodic lighting can promote flowering of long-day plants (LDPs) and inhibit flowering of short-day plants (SDPs). Red (R) and far-red (FR) light regulate flowering through phytochromes, whereas blue light does so primarily through cryptochromes. In contrast, the role of green light in photoperiodic regulation of flowering has been inconsistent in previous studies. We grew four LDP species (two petunia cultivars, ageratum, snapdragon and Arabidopsis) and two SDP species (three chrysanthemum cultivars and marigold) in a greenhouse under truncated 9-h short days with or without 7-h day-extension lighting from green light (peak = 521 nm) at 0, 2, 13 or 25 µmol m-2  s-1 or R + white (W) + FR light at 2 µmol m-2  s-1 . Increasing the green photon flux density from 0 to 25 µmol m-2  s-1 accelerated flowering of all LDPs and delayed flowering of all SDPs. Petunia flowered similarly fast under R + W + FR light and moderate green light but was shorter and developed more branches under green light. To be as effective as R + W + FR light, saturation green photon flux densities were 2 µmol m-2  s-1 for LDP ageratum and SDP marigold and 13 µmol m-2  s-1 for LDP petunia. Snapdragon was the least sensitive to green light. In Arabidopsis, cryptochrome 2 mediated promotion of flowering under moderate green light, whereas both phytochrome B and cryptochrome 2 mediated that under R + W + FR light. We conclude that 7-h day-extension lighting from green light-emitting diodes can control flowering of photoperiodic ornamentals and that in Arabidopsis, cryptochrome 2 mediates promotion of flowering under green light.

Light affects salt stress-induced transcriptional memory of P5CS1 in Arabidopsis.

To cope with environmental stresses, plants often adopt a memory response upon primary stress exposure to facilitate a quicker and stronger reaction to recurring stresses. However, it remains unknown whether light is involved in the manifestation of stress memory. Proline accumulation is a striking metabolic adaptation of higher plants during various environmental stresses. Here we show that salinity-induced proline accumulation is memorable and HY5-dependent light signaling is required for such a memory response. Primary salt stress induced the expression of Δ1-pyrroline-5-carboxylate synthetase 1 (P5CS1), encoding a proline biosynthetic enzyme and proline accumulation, which were reduced to basal level during the recovery stage. Reoccurring salt stress-induced stronger P5CS1 expression and proline accumulation were dependent upon light exposure during the recovery stage. Further studies demonstrated that salt-induced transcriptional memory of P5CS1 is associated with the retention of increased H3K4me3 level at P5CS1 during the recovery stage. HY5 binds directly to light-responsive element, C/A-box, in the P5CS1 promoter. Deletion of the C/A-box or hy5 hyh mutations caused rapid reduction of H3K4me3 level at P5CS1 during the recovery stage, resulting in impairment of the stress memory response. These results unveil a previously unrecognized mechanism whereby light regulates salt-induced transcriptional memory via the function of HY5 in maintaining H3K4me3 level at the memory gene.

Genome-Wide Identification and Expression Analysis of the Ascorbate Oxidase Gene Family in Gossypium hirsutum Reveals the Critical Role of GhAO1A in Delaying Dark-Induced Leaf Senescence.

Ascorbate oxidase (AO) plays important roles in plant growth and development. Previously, we reported a cotton AO gene that acts as a positive factor in cell growth. Investigations on Gossypium hirsutum AO (GhAO) family genes and their multiple functions are limited. The present study identified eight GhAO family genes and performed bioinformatic analyses. Expression analyses of the tissue specificity and developmental feature of GhAOs displayed their diverse expression patterns. Interestingly, GhAO1A demonstrated the most rapid significant increase in expression after 1 h of light recovery from the dark. Additionally, the transgenic ao1-1/GhAO1A Arabidopsis lines overexpressing GhAO1A in the Arabidopsis ao1-1 late-flowering mutant displayed a recovery to the normal phenotype of wild-type plants. Moreover, compared to the ao1-1 mutant, the ao1-1/GhAO1A transgenic Arabidopsis presented delayed leaf senescence that was induced by the dark, indicating increased sensitivity to hydrogen peroxide (H2O2) under normal conditions that might be caused by a reduction in ascorbic acid (AsA) and ascorbic acid/dehydroascorbate (AsA/DHA) ratio. The results suggested that GhAOs are functionally diverse in plant development and play a critical role in light responsiveness. Our study serves as a foundation for understanding the AO gene family in cotton and elucidating the regulatory mechanism of GhAO1A in delaying dark-induced leaf senescence.

Global spectroscopic analysis to study the regulation of the photosynthetic proton motive force: A critical reappraisal.

In natural variable environments, plants rapidly adjust photosynthesis for optimum balance between photochemistry and photoprotection. These adjustments mainly occur via changes in their proton motive force (pmf). Recent studies based on time resolved analysis of the Electro Chromic Signal (ECS) bandshift of photosynthetic pigments in the model plant Arabidopsis thaliana have suggested an active role of ion fluxes across the thylakoid membranes in the regulation of the pmf. Among the different channels and transporters possibly involved in this phenomenon, we previously identified the TPK3 potassium channel. Plants silenced for TPK3 expression displayed light stress signatures, with reduced Non Photochemical Quenching (NPQ) capacity and sustained anthocyanin accumulation, even at moderate intensities. In this work we re-examined the role of this protein in pmf regulation, starting from the observation that both TPK3 knock-down (TPK3 KD) and WT plants display enhanced anthocyanin accumulation in the light under certain growth conditions, especially in old leaves. We thus compared the pmf features of young "green" (without anthocyanins) and old "red" (with anthocyanins) leaves in both genotypes using a global fit analysis of the ECS. We found that the differences in the ECS profile measured between the two genotypes reflect not only differences in TPK3 expression level, but also a modified photosynthetic activity of stressed red leaves, which are present in a larger amounts in the TPK3 KD plants.

The causes of altered chlorophyll fluorescence quenching induction in the Arabidopsis mutant lacking all minor antenna complexes.

Non-photochemical quenching (NPQ) of chlorophyll fluorescence is the process by which excess light energy is harmlessly dissipated within the photosynthetic membrane. The fastest component of NPQ, known as energy-dependent quenching (qE), occurs within minutes, but the site and mechanism of qE remain of great debate. Here, the chlorophyll fluorescence of Arabidopsis thaliana wild type (WT) plants was compared to mutants lacking all minor antenna complexes (NoM). Upon illumination, NoM exhibits altered chlorophyll fluorescence quenching induction (i.e. from the dark-adapted state) characterised by three different stages: (i) a fast quenching component, (ii) transient fluorescence recovery and (iii) a second quenching component. The initial fast quenching component originates in light harvesting complex II (LHCII) trimers and is dependent upon PsbS and the formation of a proton gradient across the thylakoid membrane (ΔpH). Transient fluorescence recovery is likely to occur in both WT and NoM plants, but it cannot be overcome in NoM due to impaired ΔpH formation and a reduced zeaxanthin synthesis rate. Moreover, an enhanced fluorescence emission peak at ~679 nm in NoM plants indicates detachment of LHCII trimers from the bulk antenna system, which could also contribute to the transient fluorescence recovery. Finally, the second quenching component is triggered by both ΔpH and PsbS and enhanced by zeaxanthin synthesis. This study indicates that minor antenna complexes are not essential for qE, but reveals their importance in electron stransport, ΔpH formation and zeaxanthin synthesis.

Functional characterization of a constitutively active kinase variant of Arabidopsis phototropin 1.

Phototropins (phots) are plasma membrane-associated serine/threonine kinases that coordinate a range of processes linked to optimizing photosynthetic efficiency in plants. These photoreceptors contain two light-, oxygen-, or voltage-sensing (LOV) domains within their N terminus, with each binding one molecule of flavin mononucleotide as a UV/blue light-absorbing chromophore. Although phots contain two LOV domains, light-induced activation of the C-terminal kinase domain and subsequent receptor autophosphorylation is controlled primarily by the A'α-LOV2-Jα photosensory module. Mutations that disrupt interactions between the LOV2 core and its flanking helical segments can uncouple this mode of light regulation. However, the impact of these mutations on phot function in Arabidopsis has not been explored. Here we report that histidine substitution of Arg-472 located within the A'α-helix of Arabidopsis phot1 constitutively activates phot1 kinase activity in vitro without affecting LOV2 photochemistry. Expression analysis of phot1 R472H in the phot-deficient mutant confirmed that it is autophosphorylated in darkness in vivo but unable to initiate phot1 signaling in the absence of light. Instead, we found that phot1 R472H is poorly functional under low-light conditions but can restore phototropism, chloroplast accumulation, stomatal opening, and leaf positioning and expansion at higher light intensities. Our findings suggest that Arabidopsis can adapt to the elevated phosphorylation status of the phot1 R472H mutant in part by reducing its stability, whereas the activity of the mutant under high-light conditions can be attributed to additional increases in LOV2-mediated photoreceptor autophosphorylation.

Rice Overexpressing OsNUC1-S Reveals Differential Gene Expression Leading to Yield Loss Reduction after Salt Stress at the Booting Stage.

Rice nucleolin (OsNUC1), consisting of two isoforms, OsNUC1-L and OsNUC1-S, is a multifunctional protein involved in salt-stress tolerance. Here, OsNUC1-S's function was investigated using transgenic rice lines overexpressing OsNUC1-S. Under non-stress conditions, the transgenic lines showed a lower yield, but higher net photosynthesis rates, stomatal conductance, and transpiration rates than wild type only in the second leaves, while in the flag leaves, these parameters were similar among the lines. However, under salt-stress conditions at the booting stage, the higher yields in transgenic lines were detected. Moreover, the gas exchange parameters of the transgenic lines were higher in both flag and second leaves, suggesting a role for OsNUC1-S overexpression in photosynthesis adaptation under salt-stress conditions. Moreover, the overexpression lines could maintain light-saturation points under salt-stress conditions, while a decrease in the light-saturation point owing to salt stress was found in wild type. Based on a transcriptome comparison between wild type and a transgenic line, after 3 and 9 days of salt stress, the significantly differentially expressed genes were enriched in the metabolic process of nucleic acid and macromolecule, photosynthesis, water transport, and cellular homeostasis processes, leading to the better performance of photosynthetic processes under salt-stress conditions at the booting stage.

Electron transfer between carotenoid and chlorophyll contributes to quenching in the LHCSR1 protein from Physcomitrella patens.

Plants harvest photons for photosynthesis using light-harvesting complexes (LHCs)-an array of chlorophyll proteins that can reversibly switch from harvesting to energy-dissipation mode to prevent over-excitation and damage of the photosynthetic apparatus. In unicellular algae and lower plants this process requires the LHCSR proteins which senses over-acidification of the lumen trough protonatable residues exposed to the thylakoid lumen to activate quenching reactions. Further activation is provided by replacement of the violaxanthin ligand with its de-epoxidized product, zeaxanthin, also induced by excess light. We have produced the ppLHCSR1 protein from Physcomitrella patens by over-expression in tobacco and purified it in either its violaxanthin- or the zeaxanthin-binding form with the aim of analyzing their spectroscopic properties at either neutral or acidic pH. Using femtosecond spectroscopy, we demonstrated that the energy dissipation is achieved by two distinct quenching mechanism which are both activated by low pH. The first is present in both ppLHCSR1-Vio and ppLHCSR1-Zea and is characterized by 30-40ps time constant. The spectrum of the quenching product is reminiscent of a carotenoid radical cation, suggesting that the pH-induced quenching mechanism is likely electron transfer from the carotenoid to the excited Chl a. In addition, a second quenching channel populating the S1 state of carotenoid via energy transfer from Chl is found exclusively in the ppLHCSR1-Zea at pH5. These results provide proof of principle that more than one quenching mechanism may operate in the LHC superfamily and also help understanding the photoprotective role of LHCSR proteins and the evolution of LHC antennae.

Specificity of root microbiomes in native-grown Nicotiana attenuata and plant responses to UVB increase Deinococcus colonization.

Plants recruit microbial communities from the soil in which they germinate. Our understanding of the recruitment process and the factors affecting it is still limited for most microbial taxa. We analysed several factors potentially affecting root microbiome structure - the importance of geographic location of natural populations, the microbiome of native seeds as putative source of colonization and the effect of a plant's response to UVB exposure on root colonization of highly abundant species. The microbiome of Nicotiana attenuata seeds was determined by a culture-dependent and culture-independent approach, and the root microbiome of natural N. attenuata populations from five different locations was analysed using 454-pyrosequencing. To specifically address the influence of UVB light on root colonization by Deinococcus, a genus abundant and consistently present in N. attenuata roots, transgenic lines impaired in UVB perception (irUVR8) and response (irCHAL) were investigated in a microcosm experiment with/without UVB supplementation using a synthetic bacterial community. The seed microbiome analysis indicated that N. attenuata seeds are sterile. Alpha and beta diversities of native root bacterial communities differed significantly between soil and root, while location had only a significant effect on the fungal but not the bacterial root communities. With UVB supplementation, root colonization of Deinococcus increased in wild type, but decreased in irUVR8 and irCHAL plants compared to nontreated plants. Our results suggest that N. attenuata recruits a core root microbiome exclusively from soil, with fungal root colonization being less selective than bacterial colonization. Root colonization by Deinococcus depends on the plant's response to UVB.

Synthetic cold-inducible promoter enhances recombinant protein accumulation during Agrobacterium-mediated transient expression in Nicotiana excelsior at chilling temperatures.

OBJECTIVES: To exploit cold-inducible biochemical processes beneficial for foreign mRNA transcription, translation and storage, as well as protein product stability, during Agrobacterium-mediated transient expression. RESULTS: The efficiency of three different 5'-regulatory sequences to achieve transient expression of the GFP-based reporter gene under chilling conditions (6-8 °C since the 3rd day post inoculation) was compared. We studied the upstream sequences of a cold-inducible Arabidopsis thaliana cor15a gene, the core element of 35S CaMV promoter fused to the TMV omega 5'-UTR, and the synthetic promoter including the 35S core sequence and two binding sites for cold-inducible CBF transcription factors (P_DRE::35S). Cultivation of plants transiently expressing reporter gene under control of the synthetic P_DRE::35S promoter under chilling conditions since the 3rd dpi led to the reliably higher reporter accumulation as compared to the other tested regulatory sequences under chilling or greenhouse conditions. Reporter protein fluorescence under chilling conditions using P_DRE::35S reached 160% as compared to the transient expression in the greenhouse. Period of transient expression considerably extended if plants were cultivated at chilling temperature since the 3rd dpi: reporter protein fluorescence reached its maximum at the 20th dpi and was detected in leaves up to the 65th dpi. The enhanced protein accumulation at low temperature was accompanied by the prolonged period of corresponding mRNA accumulation. CONCLUSION: Transient expression under chilling conditions using synthetic cold-inducible promoter enhances target protein accumulation and may decrease greenhouse heating expenses.

Regulation of Arabidopsis leaf hydraulics involves light-dependent phosphorylation of aquaporins in veins.

The water status of plant leaves depends on the efficiency of the water supply, from the vasculature to inner tissues. This process is under hormonal and environmental regulation and involves aquaporin water channels. In Arabidopsis thaliana, the rosette hydraulic conductivity (Kros) is higher in darkness than it is during the day. Knockout plants showed that three plasma membrane intrinsic proteins (PIPs) sharing expression in veins (PIP1;2, PIP2;1, and PIP2;6) contribute to rosette water transport, and PIP2;1 can fully account for Kros responsiveness to darkness. Directed expression of PIP2;1 in veins of a pip2;1 mutant was sufficient to restore Kros. In addition, a positive correlation, in both wild-type and PIP2;1-overexpressing plants, was found between Kros and the osmotic water permeability of protoplasts from the veins but not from the mesophyll. Thus, living cells in veins form a major hydraulic resistance in leaves. Quantitative proteomic analyses showed that light-dependent regulation of Kros is linked to diphosphorylation of PIP2;1 at Ser-280 and Ser-283. Expression in pip2;1 of phosphomimetic and phosphorylation-deficient forms of PIP2;1 demonstrated that phosphorylation at these two sites is necessary for Kros enhancement under darkness. These findings establish how regulation of a single aquaporin isoform in leaf veins critically determines leaf hydraulics.

Cell growth defect factor1/chaperone-like protein of POR1 plays a role in stabilization of light-dependent protochlorophyllide oxidoreductase in Nicotiana benthamiana and Arabidopsis.

Angiosperms require light for chlorophyll biosynthesis because one reaction in the pathway, the reduction of protochlorophyllide (Pchlide) to chlorophyllide, is catalyzed by the light-dependent protochlorophyllide oxidoreductase (POR). Here, we report that Cell growth defect factor1 (Cdf1), renamed here as chaperone-like protein of POR1 (CPP1), an essential protein for chloroplast development, plays a role in the regulation of POR stability and function. Cdf1/CPP1 contains a J-like domain and three transmembrane domains, is localized in the thylakoid and envelope membranes, and interacts with POR isoforms in chloroplasts. CPP1 can stabilize POR proteins with its holdase chaperone activity. CPP1 deficiency results in diminished POR protein accumulation and defective chlorophyll synthesis, leading to photobleaching and growth inhibition of plants under light conditions. CPP1 depletion also causes reduced POR accumulation in etioplasts of dark-grown plants and as a result impairs the formation of prolamellar bodies, which subsequently affects chloroplast biogenesis upon illumination. Furthermore, in cyanobacteria, the CPP1 homolog critically regulates POR accumulation and chlorophyll synthesis under high-light conditions, in which the dark-operative Pchlide oxidoreductase is repressed by its oxygen sensitivity. These findings and the ubiquitous presence of CPP1 in oxygenic photosynthetic organisms suggest the conserved nature of CPP1 function in the regulation of POR.

Overexpression of soybean R2R3-MYB transcription factor, GmMYB12B2, and tolerance to UV radiation and salt stress in transgenic Arabidopsis.

MYB, v-myb avian myeloblastosis viral oncogene homolog, proteins play central roles in plant stress response. Previously, we identified a novel R2R3-MYB transcription factor, GmMYB12B2, which affected the expression levels of some key enzyme genes involved in flavonoid biosynthesis in transgenic Arabidopsis. In the present study, we analyzed the expression levels of GmMYB12B2 under salt, low temperature, drought, abscisic acid (ABA), and ultraviolet (UV) radiation treatments in soybean using semi-quantitative reverse transcription polymerase chain reaction. The expression of GmMYB12B2 was drastically induced by UV irradiation and salt treatment, but no response was detected under low temperature, drought, and ABA stresses. A detailed characterization of the GmMYB12B2 overexpression lines revealed that GmMYB12B2 might be involved in response of plants to UV radiation and salt stresses. Transgenic Arabidopsis lines constitutively expressing GmMYB12B2 showed an increased tolerance to salt and UV radiation treatment compared with wild-type plants. The expression levels of certain salt stress-responsive genes, such as DREB2A and RD17, were found to be elevated in the transgenic plants. These results indicate that GmMYB12B2 acts as a regulator in the plant stress response.

Arabidopsis thaliana thymidine kinase 1a is ubiquitously expressed during development and contributes to confer tolerance to genotoxic stress.

Thymidine kinase catalyzes the first step in the nucleotide salvage pathway by transferring a phosphate group to a thymidine molecule. In mammals thymidine kinase supplies deoxyribonucleotides for DNA replication and DNA repair, and the expression of the gene is tightly regulated during the cell cycle. Although this gene is phylogenetically conserved in many taxa, its physiological function in plants remains unknown. The genome of the model plant Arabidopsis thaliana has two thymidine kinase genes (AtTK1a and AtTK1b) and microarray data suggest they might have redundant roles. In this study we analyzed the TK1a function by evaluating its expression pattern during development and in response to genotoxic stress. We also studied its role in DNA repair by the characterization of a mutant that contained the T-DNA insertion in the promoter region of the TK1a gene. We found that TK1a is expressed in most tissues during plant development and it was differentially induced by ultraviolet-C radiation because TK1b expression was unaffected. In the mutant, the T-DNA insertion caused a 40 % rise in transcript levels and enzyme activity in Arabidopsis seedlings compared to wild-type plants. This elevation was enough to confer tolerance to ultraviolet-C irradiation in dark conditions, as determined by root growth, and meristem length and structure. TK1a overexpression also provided tolerance to genotoxins that induce double-strand break. Our results suggest that thymidine kinase contributes to several DNA repair pathways by providing deoxythymidine triphosphate that serve as precursors for DNA repair and to balance deoxyribonucleotides pools.