Tissue- and isoform-specific phytochrome regulation of light-dependent anthocyanin accumulation in Arabidopsis thaliana

Phytochromes regulate light- and sucrose-dependent anthocyanin synthesis and accumulation in many plants. Mesophyll-specific phyA alone has been linked to the regulation of anthocyanin accumulation in response to far-red light in Arabidopsis thaliana. However, multiple mesophyll-localized phytochromes were implicated in the photoregulation of anthocyanin accumulation in red-light conditions. Here, we report a role for mesophyll-specific phyA in blue-light-dependent regulation of anthocyanin levels and novel roles for individual phy isoforms in the regulation of anthocyanin accumulation under red illumination. These results provide new insight into spatial- and isoform-specific regulation of pigmentation by phytochromes in A. thaliana.Key words: anthocyanin, Arabidopsis, photomorphogenesis, photoreceptor, phytochrome     

Arabidopsis ROOT HAIR DEFECTIVE3 is involved in nitrogen starvation-induced anthocyanin accumulation

Anthocyanin accumulation is a common phenom-enon seen in plants under environmental stress. In this study, we identified a new allele of ROOT HAIR DEFECTIVE3 (RHD3) showing an anthocyanin overaccumulat...     

Leaf developmental stage modulates metabolite accumulation and photosynthesis contributing to acclimation of Arabidopsis thaliana to water deficit

We examined whether young and mature leaves of Arabidopsis thaliana in their response to mild water deficit (MiWD) and moderate water deficit (MoWD), behave differentially, and whether photosynthetic acclimation to water deficit correlates with increased proline and sugar accumulation. We observed that with increasing water deficit, leaf relative water content decreased, while proline and sugar accumulation increased in both leaf-developmental stages. Under both MiWD and MoWD, young leaves showed less water loss and accumulated higher level of metabolites compared to mature leaves. This, leaf age-related increase in metabolite accumulation that was significantly higher under MoWD, allowed young leaves to cope with oxidative damage by maintaining their base levels of lipid peroxidation. Thus, acclimation of young leaves to MoWD, involves a better homeostasis of reactive oxygen species (ROS), that was achieved among others by (1) increased sugar accumulation and (2) either increased proline synthesis and/or decreased proline catabolism, that decrease the NADPH/NADP⁺ ratio, resulting in a higher level of oxidized state of quinone A and thus in a reduced excitation pressure, and by (3) stimulation of the photoprotective mechanism of non-photochemical quenching, that reflects the dissipation of excess excitation energy in the form of harmless heat, thus protecting the plant from the damaging effects of ROS.     

Plasticity to wind is modular and genetically variable in Arabidopsis thaliana

Thigmomorphogenesis, the characteristic phenotypic changes by which plants react to mechanical stress, is a widespread and probably adaptive type of phenotypic plasticity. However, little is known about its genetic basis and population variation. Here, we examine genetic variation for thigmomorphogenesis within and among natural populations of the model system Arabidopsis thaliana. Offspring from 17 field-collected European populations was subjected to three levels of mechanical stress exerted by wind. Overall, plants were remarkably tolerant to mechanical stress. Even high wind speed did not significantly alter the correlation structure among phenotypic traits. However, wind significantly affected plant growth and phenology, and there was genetic variation for some aspects of plasticity to wind among A. thaliana populations. Our most interesting finding was that phenotypic traits were organized into three distinct and to a large degree statistically independent covariance modules associated with plant size, phenology, and growth form, respectively. These phenotypic modules differed in their responsiveness to wind, in the degree of genetic variability for plasticity, and in the extent to which plasticity affected fitness. It is likely, therefore, that thigmomorphogenesis in this species evolves quasi-independently in different phenotypic modules.     

Adaptation of Arabidopsis to nitrogen limitation involves induction of anthocyanin synthesis which is controlled by the NLA gene

Plants can survive a limiting nitrogen (N) supply by developing a set of N limitation adaptive responses. However, the Arabidopsis nla (nitrogen limitation adaptation) mutant fails to produce such responses, and cannot adapt to N limitation. In this study, the nla mutant was utilized to understand further the effect of NLA on Arabidopsis adaptation to N limitation. Grown with limiting N, the nla mutant could not accumulate anthocyanins and instead produced an N limitation-induced early senescence phenotype. In contrast, when supplied with limiting N and limiting phosphorus (Pi), the nla mutants accumulated abundant anthocyanins and did not show the N limitation-induced early senescence phenotype. These results support the hypothesis that Arabidopsis has a specific pathway to control N limitation-induced anthocyanin synthesis, and the nla mutation disrupts this pathway. However, the nla mutation does not affect the Pi limitation-induced anthocyanin synthesis pathway. Therefore, Pi limitation induced the nla mutant to accumulate anthocyanins under N limitation and allowed this mutant to adapt to N limitation. Under N limitation, the nla mutant had a significantly down-regulated expression of many genes functioning in anthocyanin synthesis, and an enhanced expression of genes involved in lignin production. Correspondingly, the nla mutant grown with limiting N showed a significantly lower production of anthocyanins (particularly cyanidins) and an increase in lignin contents compared with wild-type plants. These data suggest that NLA controls Arabidopsis adaptability to N limitation by channelling the phenylpropanoid metabolic flux to the induced anthocyanin synthesis, which is important for Arabidopsis to adapt to N limitation.     

Transgenerational response to stress in Arabidopsis thaliana

Plants exposed to stress pass the memory of exposure to stress to the progeny. Previously, we showed that the phenomenon of transgenerational memory of stress is of epigenetic nature and depends on the function of Dicer-like (DCL) 2 and DCL3 proteins. Here, we discuss a possible role of DNA methylation and function of small RNAs in establishing and maintaining transgenerational responses to stress. Our new data report that memory of stress is passed to the progeny predominantly through the female rather than male gamete. Possible evolutionary advantages of this mechanism are also discussed.Key words: transgenerational response to stress, Arabidopsis thaliana, maternal inheritance, methylation changes, homologous recombination frequency, genome instability, adaptive response, dcl2, dcl3Plants are sedentary organisms and thus can not respond to rapidly changing growth conditions by escaping to new environments as animals usually do. Moreover, since seed dispersal is rather limited in the vast majority of plants, the progeny is very likely to grow under the same environmental growth conditions as its parents did. The memory of pre-existing growth conditions can be advantageous for plant survival. The environmental experience of parents can be recorded in the form of induced epigenetic modifications that occur in somatic cell lineages. The very late, almost at the end of plant development, separation of germline cells from somatic tissues enables incorporation of acquired epigenetic changes in the gametes. Indeed, previous reports suggested that the progeny of exposed plants might have an advantage while growing in the same environment as its parents.^1–3 Despite a growing number of experimental evidences that support the existence of the phenomenon of memory of stress, the data on adaptive changes in the progeny of stressed plants are scarce.Parental exposure to stress may not only lead to adaptive effects in progeny but also introduce a certain degree of changes in genome stability.^4–9 Our early report showed that the progeny of tobacco plants infected with tobacco mosaic virus had an increased meiotic recombination frequency.⁸ A more recent report demonstrated that these progeny plants had a higher frequency of rearrangements at the loci carrying the homology to N-gene-like R-gene loci, allowing speculations about a possible role of these rearrangements in pathogen resistance evolution.⁹ Similarly, a study of Molinier et al. (2006) showed that the progeny of plants exposed to UVC or flagellin had an increased frequency of somatic homologous recombination events (HRF).⁴ The authors demonstrated that an increase in HRF triggered by a single exposure to UVC was maintained for five consecutive generations in the absence of stress. In contrast, our most recent reports demonstrated that maintaining an increase in HRF caused by ancestral exposure to heat, cold, flood, UVC or salt required exposure to stress in subsequent generations: if F1 plants were propagated for one more generation without stress, the effect diminished and HRF returned back to the level observed in the progeny of untreated plants.^6,7 This scenario seems to be more probable from an evolutionary point of view. Within a given environmental niche, plants establish certain genetic and epigenetic traits needed to cope with the expected growth conditions. Drastic environmental changes or new unusual stresses may trigger a cascade of gene expression changes in attempt to survive and adapt to new conditions. Some of these potentially advantageous changes are most probably recorded in the form of DNA methylation and chromatin modifications and are passed to progeny as memory of stress exposure.It can be further hypothesized that if these new environmental conditions are no longer present during the lifespan of future generations, the newly established methylation patterns and chromatin organization will return to the original epigenetic landscape that was the most adequate fit for this environmental niche. If the same new stresses occur in consecutive generations, the newly established epigenetic changes will be maintained and possibly stabilized after many generations of exposure.     

Regulation of proline accumulation in Arabidopsis thaliana (L.) Heynh during development and in response to desiccation

Significant differences were observed in the amount and proportion of free amino acids in different organs of Arabidopsis thaliana (L.) Heynh, ecotype Columbia. The most notable were found for proline, which formed 17–26% of the total free amino acid concentration in reproductive tissues (floret and seed), but only 1–3% of the total free amino acid concentration in vegetative tissues (rosette leaf and root). Proline accumulation was associated with tissues that had relatively low water contents. Tissues which displayed high water contents, such as rosette leaves, contained low levels of proline. A significant increase in the levels of proline accumulation occurred in plants subjected to experimentally induced low water potentials as compared to unstressed plants. For instance, an 8–10-fold increase in proline was observed in the presence of 120 mmol kg^?1 NaCl or KCl, and a 20-fold increase was stimulated by 60 mmol kg^?1 PEG. However, in addition to the accumulation of proline, massive accumulation of Na⁺, K⁺ and Cl^? ions occurred in tissues of plants stressed with salt. No significant differences were observed in mineral ions in plants stressed with PEG. Isotope tracer experiments with ¹⁴C compounds established that glutamate, ornithine and arginine are precursors of the proline biosynthesis induced by PEG in response to low water potentials in Arabidopsis thaliana. We conclude that the accumulation of proline in response to PEG occurs through increased biosynthesis.     

Isolation and characterization of two cDNA clones that are rapidly induced during the wound response of Arabidopsis thaliana

Differential screening of a cDNA library of Arabidopsis thaliana constructed from the plant tissues harvested 1 h after wounding resulted in isolation of 2 wound-inducible cDNA clones. Kinetic analysis revealed that the corresponding genes are rapidly induced upon wounding. Expression of these clones reached the maximum level around 1–1.5 h after wounding and then were progressively reduced. The time by which expression returned to the control level was around 3 h after wounding. Partial sequence analysis revealed that the two clones are highly homologous to the S-adenosylmethionine synthetase and the glutathione-S-transferase gene, respectively.Abbreviations SAM S-adenosylmethionine - GST glutathione-S-transferase     

Adaptation and growth response of Arabidopsis thaliana to deuterium

Summary Arabidopsis seeds were sown aseptically on mineral media containing between 0 and 90% of heavy water (D₂O). Initially, a D₂O level of over 50% was lethal for the plants. However, after culture for six successive generations on 50% D₂O, plants were capable of growing marginally on media containing up to 70% D₂O, but not higher. With increasing concentration, deuterium progressively delays germination, slows growth, reduces survival, results in bleaching of the leaves and delays flowering. Pollen fertility is not affected measurably but seed set is reduced with increasing levels of deuteration so that at 70% D₂O few seeds were obtained. The viability of the seeds harvested from plants grown on deuterated media is low. No chlorophyll or morphological mutants were observed among a large number of plant progenies. Seeds from plants cultured on D₂O media for several generations grow normally on proteated media in the very first generation.     

Thioredoxins m are major players in the multifaceted light-adaptive response in Arabidopsis thaliana

Thioredoxins (TRXs) are well-known redox signalling players, which carry out post-translational modifications in target proteins. Chloroplast TRXs are divided into different types and have central roles in light energy uptake and the regulation of primary metabolism. The isoforms TRX m1, m2, and m4 from Arabidopsis thaliana are considered functionally related. Knowing their key position in the hub of plant metabolism, we hypothesized that the impairment of the TRX m signalling would not only have harmful consequences on chloroplast metabolism but also at different levels of plant development. To uncover the physiological and developmental processes that depend on TRX m signalling, we carried out a comprehensive study of Arabidopsis single, double, and triple mutants defective in the TRX m1, m2, and m4 proteins. As light and redox signalling are closely linked, we investigated the response to high light (HL) of the plants that are gradually compromised in TRX m signalling. We provide experimental evidence relating the lack of TRX m and the appearance of novel phenotypic features concerning mesophyll structure, stomata biogenesis, and stomatal conductance. We also report new data indicating that the isoforms of TRX m fine-tune the response to HL, including the accumulation of the protective pigment anthocyanin. These results reveal novel signalling functions for the TRX m and underline their importance for plant growth and fulfilment of the acclimation/response to HL conditions.     

Stress-induced accumulation and tissue-specific localization of dehydrins in Arabidopsis thaliana

Stress-induced accumulation of five (COR47, LTI29, ERD14, LTI30 and RAB18) and tissue localization of four (LTI29, ERD14, LTI30 and RAB18) dehydrins in Arabidopsis were characterized immunologically with protein-specific antibodies. The five dehydrins exhibited clear differences in their accumulation patterns in response to low temperature, ABA and salinity. ERD14 accumulated in unstressed plants, although the protein level was up-regulated by ABA, salinity and low temperature. LTI29 mainly accumulated in response to low temperature, but was also found in ABA- and salt-treated plants. LTI30 and COR47 accumulated primarily in response to low temperature, whereas RAB18 was only found in ABA-treated plants and was the only dehydrin in this study that accumulated in dry seeds.Immunohistochemical localization of LTI29, ERD14 and RAB18 demonstrated tissue and cell type specificity in unstressed plants. ERD14 was present in the vascular tissue and bordering parenchymal cells, LTI29 and ERD14 accumulated in the root tip, and RAB18 was localized to stomatal guard cells. LTI30 was not detected in unstressed plants. The localization of LTI29, ERD14 and RAB18 in stress-treated plants was not restricted to certain tissues or cell types. Instead these proteins accumulated in most cells, although cells within and surrounding the vascular tissue showed more intense staining. LTI30 accumulated primarily in vascular tissue and anthers of cold-treated plants.This study supports a physiological function for dehydrins in certain plant cells during optimal growth conditions and in most cell types during ABA or cold treatment. The differences in stress specificity and spatial distribution of dehydrins in Arabidopsis suggest a functional specialization for the members of this protein family.     

Quantitative trait loci controlling light and hormone response in two accessions of Arabidopsis thaliana

We have mapped quantitative trait loci (QTL) responsible for natural variation in light and hormone response between the Cape Verde Islands (Cvi) and Landsberg erecta (Ler) accessions of Arabidopsis thaliana using recombinant inbred lines (RILs). Hypocotyl length was measured in four light environments: white, blue, red, and far-red light and in the dark. In addition, white light plus gibberellin (GA) and dark plus the brassinosteroid biosynthesis inhibitor brassinazole (BRZ) were used to detect hormone effects. Twelve QTL were identified that map to loci not previously known to affect light response, as well as loci where candidate genes have been identified from known mutations. Some QTL act in all environments while others show genotype-by-environment interaction. A global threshold was established to identify a significant epistatic interaction between two loci that have few main effects of their own. LIGHT1, a major QTL, has been confirmed in a near isogenic line (NIL) and maps to a new locus with effects in all light environments. The erecta mutation can explain the effect of the HYP2 QTL in the blue, BRZ, and dark environments, but not in far-red. LIGHT2, also confirmed in an NIL, has effects in white and red light and shows interaction with GA. The phenotype and map position of LIGHT2 suggest the photoreceptor PHYB as a candidate gene. Natural variation in light and hormone response thus defines both new genes and known genes that control light response in wild accessions.