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.
Melatonin plays important roles in plant disease response, but the mechanisms are largely unknown. Here, we show that melatonin functions in stomatal immunity in Panax notoginseng and Arabidopsis (Arabidopsis thaliana). Biochemical analyses showed that melatonin-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. Putative phytomelatonin receptor 1 (PMTR1) is a plasma membrane protein required for perceiving melatonin signaling in stomatal closure and activation of MAPK. Biochemical and genetic tests found PMTR1 is essential for flg22- and melatonin-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 melatonin signaling in stomatal closure. The stomata in pmtr1 are insensitive to melatonin and flg22, but the application of melatonin 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 priming of stomatal immunity and (ii) provide insights into the phytomelatonin signaling transduction pathway.
Aluminum (Al) toxicity and magnesium (Mg) deficiency often coexist in acidic soils. Nitric oxide (NO) is an important signaling molecule involved in diverse physiological processes and stress responses in plants. In this study, we investigated the role of NO and Mg availability in promotion of root growth and Al tolerance in Arabidopsis. The results showed that both Al toxicity-and Mg deficiency-induced NO production contributed to inhibition of primary root growth and the root cell cycle progression. Additionally, the NO production and root growth inhibition were aggravated in the absence of Mg under Al stress conditions. In contrast, Mg supply promoted root growth associated with decreasing NO production under Al stress and/or Mg deficiency conditions. Magnesium decreased the activities and expression of the genes related to NO biosynthesis enzymes in wild type Col-0, but not in the Mg transporter mutant seedlings (mgt1) in the presence or absence of Al toxicity. Accordingly, the mgt1 mutant plants exhibited high Al accumulation, NO concentration and Al sensitivity in comparison with the Col-0 plants under Al stress. The NO-associated protein 1 mutant noa1 and the nitrate reductase mutant nia1nia2 with impaired NO production showed Al toxicity-and Mg deficiency-insensitive phenotypes, further confirming reduction of NO production was involved in Mg-mediated root growth promotion under Al toxicity and/or Mg deficiency conditions. Taken together, our results suggested that Mg-mediated enhancement of root growth and Al tolerance is associated with altering NO production in Arabidopsis.
BackgroundAcidic soils with a pH water below 5.5 occupy up to 40% of world's arable land. Aluminum (Al) toxicity and magnesium (Mg) deficiency often coexist in acidic soils, limiting crop production. In this study, we investigated a role of Mg in alleviation of Al stress in maize (Zea mays) by characterizing the growth responses, Al accumulation and the antioxidant properties of plants cultured hydroponically. MethodsPhysiological and molecular analyses were used to investigate the mechanisms of Mg governing the alleviation of Al-induced ROS production and root growth inhibition. ResultsAluminum (50 µM) decreased root growth and induced oxidative stress. Exogenously added millimolar concentrations of Mg significantly alleviated Al toxicity as evidenced by restoration of plant growth, suppression of Al uptake, and a decline in root H 2 O 2 concentration. Furthermore, the addition of Mg to the Al treatment solution enhanced the activities and expression of genes encoding superoxide dismutase, catalase and peroxidase compared to the Al-only treatment. ConclusionsThe results indicate that Mg plays a role in alleviation of Al toxicity, reduction of Al accumulation and protection from Al-induced oxidative stress through activation of antioxidative enzymes.
Stomatal movement could be regulated by ABA signaling through synthesis of reactive oxygen species (ROS) in guard cells. By contrast, ethylene triggers biosynthesis of antioxidant flavonol to suppress ROS accumulation and prevent ABA-induced stomatal closure, but the underlying mechanism remains largely unknown. We isolated and characterized tobacco R2R3-MYB NtMYB184, belonging to the flavonol-specific SG7 subgroup. The RNAi suppression (KD line) and knockout (myb184) of NtMYB184 caused a down-regulation of flavonol biosynthetic genes and decreased the concentration of flavonol in tobacco leaves. Yeast one-hybrid assay, transactivation assay, EMSA and ChIP-qPCR demonstrated that NtMYB184 specifically binds to the promoters of flavonol biosynthetic genes via the MYBPLANT motifs. In tobacco, NtMYB184 regulates 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 the 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 guard cells.
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