Ethylene inhibits ABA-induced stomatal closure via regulating NtMYB184-mediated flavonol biosynthesis in tobacco.

Stomatal movement can be regulated by ABA signaling through synthesis of reactive oxygen species (ROS) in guard cells. By contrast, ethylene triggers the biosynthesis of antioxidant flavonols to suppress ROS accumulation and prevent ABA-induced stomatal closure; however, the underlying mechanism remains largely unknown. In this study, we isolated and characterized the tobacco (Nicotiana tabacum) R2R3-MYB transcription factor NtMYB184, which belongs to the flavonol-specific SG7 subgroup. RNAi suppression and CRISPR/Cas9 mutation (myb184) of NtMYB184 in tobacco caused down-regulation of flavonol biosynthetic genes and decreased the concentration of flavonols in the leaves. Yeast one-hybrid assays, transactivation assays, EMSAs, and ChIP-qPCR demonstrated that NtMYB184 specifically binds to the promoters of flavonol biosynthetic genes via MYBPLANT motifs. NtMYB184 regulated flavonol biosynthesis in guard cells to modulate ROS homeostasis and stomatal aperture. ABA-induced ROS production was accompanied by the suppression of NtMYB184 and flavonol biosynthesis, which may accelerate ABA-induced stomatal closure. Furthermore, ethylene stimulated NtMYB184 expression and flavonol biosynthesis to suppress ROS accumulation and curb ABA-induced stomatal closure. In myb184, however, neither the flavonol and ROS concentrations nor the stomatal aperture varied between the ABA and ABA+ethylene treatments, indicating that NtMYB184 was indispensable for the antagonism between ethylene and ABA via regulating flavonol and ROS concentrations in the guard cells.

Melatonin functions in priming of stomatal immunity in Panax notoginseng and Arabidopsis thaliana.

Melatonin (MT) plays important roles in plant disease response, but the mechanisms are largely unknown. Here, we show that MT functions in stomatal immunity in Panax notoginseng and Arabidopsis thaliana. Biochemical analyses showed that MT-induced stomatal closure plays a prominent role in preventing invasion of bacteria Pseudomonas syringe pv. tomato (Pst) DC3000 via activation of mitogen-activated protein kinase (MAPK) and NADPH oxidase-mediated reactive oxygen species production in P. notoginseng. The first putative phytomelatonin receptor 1 (PMTR1) is a plasma membrane protein required for perceiving MT signaling in stomatal closure and activation of MAPK. Biochemical and genetic tests found PMTR1 is essential for flg22- and MT-induced MAPK activation in a heterotrimeric GTP-binding protein Gα subunit GPA1-independent manner. GPA1 functions in the same genetic pathways of FLS2/BAK1 (Flagellin Sensing 2/Brassinosteroid Insensitive 1-associated kinase 1)- as well as PMTR1-mediated flg22 and MT signaling in stomatal closure. The stomata in pmtr1 are insensitive to MT and flg22, but the application of MT induces stomatal closure and reduces the bacterial growth in fls2 and bak1 plants, indicating that PMTR1 might be a downstream signaling component in FLS2- and BAK1-mediated stomatal immunity. In summary, our results (i) demonstrate that phytomelatonin functions in the priming of stomatal immunity and (ii) provide insights into the phytomelatonin signaling transduction pathway.

Magnesium reduces cadmium accumulation by decreasing the nitrate reductase-mediated nitric oxide production in Panax notoginseng roots.

Panax notoginseng is a traditional medicinal herb in China. However, the high capacity of its roots to accumulate cadmium (Cd) poses a potential risk to human health. Our previous study showed that nitrate reductase (NR)-dependent nitric oxide (NO) production promoted Cd accumulation in P. notoginseng root cell walls. In this study, the role of Mg in the regulation of NO production and Cd accumulation in P. notoginseng roots was characterized. Exposure of P. notoginseng roots to increasing concentrations of Cd resulted in a linear increase in NO production. The application of 2 mM Mg for 24 h significantly alleviated Cd-induced NO production and Cd accumulation in roots, which coincided with a significant decrease in the NR activity. Western analysis suggested that Mg increased the interaction between the 14-3-3 protein and NR, which might have been a reason for the Mg-mediated decrease in NR activity and NO production under Cd stress. These results suggested that Mg-mediated alleviation of Cd-induced NO production and Cd accumulation is achieved by enhancement of the interaction between the 14-3-3 protein and NR in P. notoginseng roots.

Daily rhythms of phytomelatonin signaling modulate diurnal stomatal closure via regulating reactive oxygen species dynamics in Arabidopsis.

Melatonin is a well-studied neurohormone oscillating in a 24-h cycle in vertebrates. Phytomelatonin is widespread in plant kingdom, but it remains elusive whether this newly characterized putative hormone underlies the regulation by daily rhythms. Here, we report phytomelatonin signaling, as reflected by changes in endogenous concentrations of phytomelatonin and expression of genes associated with biosynthesis of phytomelatonin (AtSNAT1, AtCOMT1, and AtASMT) and its receptor (AtPMTR1), shows 24-h oscillations in Arabidopsis. The variation of reactive oxygen species (ROS) production and scavenging and expression of ROS-related genes significantly decrease in pmtr1 and snat and increase in PMTR1-OE seedlings, indicating the rhythmicity in phytomelatonin signaling is required for maintenance of ROS dynamics. Additionally, the ROS signaling feedback influences the expression of AtSNAT1, AtCOMT1, AtASMT, and AtPMTR1, suggesting the phytomelatonin and ROS signaling are coordinately interrelated. The pmtr1 mutant plants lose diurnal stomatal closure, with stomata remaining open during daytime as well as nighttime and mutants showing more water loss and drought sensitivity when compared with the wild-type Col-0 plants. Taken together, our results suggest that PMTR1-regulated ROS signaling peaks in the afternoon and may transmit the darkness signals to trigger stomatal closure, which might be essential for high water-use efficiency and drought tolerance.