Distinct early transcriptional regulations by turgor and osmotic potential in the roots of Arabidopsis.

In a context of climate change, deciphering signaling pathways driving plant adaptation to drought, changes in water availability, and salt is key. A crossing point of these plant stresses is their impact on plant water potential (Ψ), a composite physico-chemical variable reflecting the availability of water for biological processes such as plant growth and stomatal aperture. The Ψ of plant cells is mainly driven by their turgor and osmotic pressures. Here we investigated the effect of a variety of osmotic treatments on the roots of Arabidopsis plants grown in hydroponics. We used, among others, a permeating solute as a way to differentiate variations on turgor from variations in osmotic pressure. Measurement of cortical cell turgor pressure with a cell pressure probe allowed us to monitor the intensity of the treatments and thereby preserve the cortex from plasmolysis. Transcriptome analyses at an early time point (15 min) showed specific and quantitative transcriptomic responses to both osmotic and turgor pressure variations. Our results highlight how water-related biophysical parameters can shape the transcriptome of roots under stress and provide putative candidates to explore further the early perception of water stress in plants.

FtMYB6, a Light-Induced SG7 R2R3-MYB Transcription Factor, Promotes Flavonol Biosynthesis in Tartary Buckwheat (Fagopyrum tataricum).

Tartary buckwheat (Fagopyrum tataricum) is rich in flavonols, which are thought to be highly beneficial for human health. However, little is known about the regulatory mechanism of flavonol biosynthesis in Tartary buckwheat. In this study, we identified and characterized a novel SG7 R2R3-MYB transcription factor in Tartary buckwheat, FtMYB6. We showed that FtMYB6 is located in the nucleus and acts as a transcriptional activator. The FtMYB6 promoter showed strong spatiotemporal specificity and was induced by light. The expression of FtMYB6 showed a significant correlation with rutin accumulation in the roots, stems, leaves, and flowers. Overexpression of FtMYB6 in transgenic Tartary buckwheat hairy roots and tobacco (Nicotiana tabacum) plants significantly increased the accumulation of flavonols. In transient luciferase (LUC) activity assay, FtMYB6 promoted the activity of FtF3H and FtFLS1 promoters and inhibited the activity of the Ft4CL promoter. Collectively, our results suggest that FtMYB6 promotes flavonol biosynthesis by activating FtF3H and FtFLS1 expression.

FtMYB18 acts as a negative regulator of anthocyanin/proanthocyanidin biosynthesis in Tartary buckwheat.

KEY MESSAGE: FtMYB18 plays a role in the repression of anthocyanins and proanthocyanidins accumulation by strongly down-regulating the CHS and DFR genes in Tartary buckwheat, and the C5 motif plays an important role in this process. Anthocyanins and proanthocyanidins (PAs) are important flavonoids in Tartary buckwheat (Fagopyrum tataricum Gaertn.), which provides various vibrant color and stronge abiotic stress resistance. Their synthesis is generally regulated by MYB transcription factors at transcription level. However, the negative regulations of MYB and their effects on flavonol metabolism are poorly understood. A SG4-like MYB subfamily TF, FtMYB18, containing C5 motif was identified from Tartary buckwheat. The expression of FtMYB18 was not only showed a negative correlation with anthocyanins and PAs content but also strongly respond to MeJA and ABA. As far as the transgenic lines with FtMYB18 overexpression, anthocyanins and PAs accumulations were decreased through down-regulating expression levels of NtCHS and NtDFR in tobacco, AtDFR and AtTT12 in Arabidopsis, FtCHS, FtDFR and FtANS in Tartary buckwheat hairy roots, respectively. However, FtMYB18 showed no effect on the FLS gene expression and the metabolites content in flavonol synthesis branch. The further molecular interaction analysis indicated FtMYB18 could mediate the inhibition of anthocyanins and PAs synthesis by forming MBW transcriptional complex with FtTT8 and FtTTG1, or MYB-JAZ complex with FtJAZ1/-3/-4/-7. Importantly, in FtMYB18 mutant lines with C5 motif deletion (FtMYB18-C), both of anthocyanins and PAs accumulations had recovered to the similar level as that in wild type, which was attributed to the weakened MBW complex activity or the deficient molecular interaction between FtMYB18ΔC5 with FtJAZ3/-4. The results showed that FtMYB18 could suppress anthocyanins and PAs synthesis at transcription level through the specific interaction of C5 motif with other proteins in Tartary buckwheat.

Diverse biological effects of glycosyltransferase genes from Tartary buckwheat.

BACKGROUND: Tartary buckwheat (Fagopyrum tataricum) is an edible cereal crop whose sprouts have been marketed and commercialized for their higher levels of anti-oxidants, including rutin and anthocyanin. UDP-glucose flavonoid glycosyltransferases (UFGTs) play an important role in the biosynthesis of flavonoids in plants. So far, few studies are available on UFGT genes that may play a role in tartary buckwheat flavonoids biosynthesis. Here, we report on the identification and functional characterization of seven UFGTs from tartary buckwheat that are potentially involved in flavonoid biosynthesis (and have varying effects on plant growth and development when overexpressed in Arabidopsis thaliana.) RESULTS: Phylogenetic analysis indicated that the potential function of the seven FtUFGT proteins, FtUFGT6, FtUFGT7, FtUFGT8, FtUFGT9, FtUFGT15, FtUFGT40, and FtUFGT41, could be divided into three Arabidopsis thaliana functional subgroups that are involved in flavonoid biosynthesis of and anthocyanin accumulation. A significant positive correlation between FtUFGT8 and FtUFGT15 expression and anthocyanin accumulation capacity was observed in the tartary buckwheat seedlings after cold stress. Overexpression in Arabidopsis thaliana showed that FtUFGT8, FtUFGT15, and FtUFGT41 significantly increased the anthocyanin content in transgenic plants. Unexpectedly, overexpression of FtUFGT6, while not leading to enhanced anthocyanin accumulation, significantly enhanced the growth yield of transgenic plants. When wild-type plants have only cotyledons, most of the transgenic plants of FtUFGT6 had grown true leaves. Moreover, the growth speed of the oxFtUFGT6 transgenic plant root was also significantly faster than that of the wild type. At later growth, FtUFGT6 transgenic plants showed larger leaves, earlier twitching times and more tillers than wild type, whereas FtUFGT15 showed opposite results. CONCLUSIONS: Seven FtUFGTs were isolated from tartary buckwheat. FtUFGT8, FtUFGT15, and FtUFGT41 can significantly increase the accumulation of total anthocyanins in transgenic plants. Furthermore, overexpression of FtUFGT6 increased the overall yield of Arabidopsis transgenic plants at all growth stages. However, FtUFGT15 shows the opposite trend at later growth stage and delays the growth speed of plants. These results suggested that the biological function of FtUFGT genes in tartary buckwheat is diverse.

FtMYB8 from Tartary buckwheat inhibits both anthocyanin/Proanthocyanidin accumulation and marginal Trichome initiation.

BACKGROUND: Because flavonoids and trichomes play crucial roles in plant defence, their formation requires fine transcriptional control by multiple transcription factor families. However, little is known regarding the mechanism of the R2R3-MYB transcription factors that regulate both flavonoid metabolism and trichome development. RESULTS: Here, we identified a unique SG4-like-MYB TF from Tartary buckwheat, FtMYB8, which harbours the C2 repression motif and an additional TLLLFR repression motif. The expression profiles of FtMYB8 combined with the transcriptional activity of PFtMYB8 promoter showed that FtMYB8 mRNA mainly accumulated in roots during the true leaf stage and flowering stage and in bud trichomes and flowers, and the expression of this gene was markedly induced by MeJA, ABA and UV-B treatments but repressed by dark treatment. Overexpression of FtMYB8 in Arabidopsis reduces the accumulation of anthocyanin/proanthocyanidin by specifically inhibiting TT12 expression, which may depend on the interaction between FtMYB8 and TT8. Interestingly, this interaction may also negatively regulate the marginal trichome initiation in Arabidopsis leaves. CONCLUSIONS: Taken together, our results suggest that FtMYB8 may fine-tune the accumulation of anthocyanin/proanthocyanidin in the roots and flowers of Tartary buckwheat by balancing the inductive effects of transcriptional activators, and probably regulate trichome distribution in the buds of Tartary buckwheat.

A R2R3-MYB transcription factor gene, FtMYB13, from Tartary buckwheat improves salt/drought tolerance in Arabidopsis.

Abiotic stress causes various negative impacts on plants, such as water loss, reactive oxygen species (ROS) accumulation and decreased photosynthesis. R2R3-MYB transcription factors (TFs) play crucial roles in the response of plants to abiotic stress. However, their functions in Tartary buckwheat, a strongly abiotic and resistant coarse cereal, haven't been fully investigated. In this paper, we report that a R2R3-MYB from Tartary buckwheat, FtMYB13, is not an activator of transcriptional activity but is located in the nucleus. Moreover, compared to the wild type (WT), transgenic Arabidopsis overexpressing FtMYB13 had a lower sensitivity to ABA and caused improved drought/salt tolerance, which was attributed to the higher proline content, greater photosynthetic efficiency, higher transcript abundance of some stress-related genes and the smaller amount of reactive oxygen species (ROS) and malondialdehyde (MDA) in the transgenic lines compared to WT. Consequently, our work indicates that FtMYB13 is involved in mediating plant responses to ABA, as well as salt and drought.

Overexpression of Fagopyrum tataricum FtbHLH2 enhances tolerance to cold stress in transgenic Arabidopsis.

bHLH transcription factors play important roles in the abiotic stress response in plants, but their characteristics and functions in Tartary buckwheat (Fagopyrum tataricum), a traditional coarse cereal with a strong stress tolerance, haven't been sufficiently studied. Here, we found that the expression of a bHLH gene, FtbHLH2, was induced significantly by cold treatments in Tartary buckwheat seedlings. Subcellular localization indicated that FtbHLH2 localized in nucleus. Its overexpression in Arabidopsis increased tolerance to cold. The Arabidopsis plants overexpressing FtbHLH2 displayed higher root length and photosynthetic efficiency, and had lower malondialdehyde (MDA) and reactive oxygen species (ROS) after cold treatment compared to wild type (WT) plants. Meanwhile, the expression levels of some stress-related genes in transgenic plants were remarkably higher than that in wild type under normal and/or stress conditions. Furthermore, transgenic Arabidopsis lines with the FtbHLH2 promoter had higher GUS activity after cold stress. On the whole, the results suggest that FtbHLH2 may play a positive regulatory in cold stress of Tartary buckwheat.

Tartary buckwheat FtMYB10 encodes an R2R3-MYB transcription factor that acts as a novel negative regulator of salt and drought response in transgenic Arabidopsis.

Tartary buckwheat is a strongly abiotic, resistant coarse cereal, but its tolerance mechanisms for stress are largely unknown. MYB transcription factors play key roles in various physiological, biochemical and molecular responses, which can both positively and negatively regulate the stress tolerance of plants. In this study, we report that the expression of FtMYB10, a R2R3-MYB gene from Tartary buckwheat, was induced significantly by ABA and drought treatments. A seed germination test under ABA treatment indicated that transgenic lines were less sensitive to ABA. The overexpression of FtMYB10 in Arabidopsis reduced drought and salt tolerance. Further studies showed that the proline contents in the transgenic plants are markedly decreased associated with reduced expression of the P5CS1 gene under both normal and stress conditions. Furthermore, the expression of some stress-responsive genes, including DREB1/CBFs, RD29B, RD22, and several genes of the DRE/CRT class, decreased in response to FtMYB10 overexpression in Arabidopsis. These results suggest that FtMYB10 may play a key role in ABA signaling feedback regulation and act as a novel negative regulator of salt and drought stress tolerance in plants.