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Phytomelatonin is a universal signal molecule that regulates plant growth and stress responses; however, only one receptor that can directly bind with and perceive melatonin signaling has been identified so far, namely AtPMTR1/CAND2 in Arabidopsis. Whether other plants contain a similar receptor and, if so, how it functions is still unknown. In this study, we identified a new phytomelatonin receptor in the monocot maize (Zea mays), and investigated its role in plant responses to osmotic and drought stress. Using homology searching, we identified a plasma membrane-localized protein, Zm00001eb214610/ZmPMTR1, with strong binding activity to melatonin as a potential phytomelatonin receptor in maize. Overexpressing ZmPMTR1 in Arabidopsis Col-0 promoted osmotic stress tolerance, and rescued osmotic stress sensitivity of the Arabidopsis cand2-1 mutant. Furthermore, ZmPMTR1 also largely rescued defects in melatonin-induced stomatal closure in the cand2-1 mutant, thereby reducing water loss rate and increasing tolerance to drought stress. In addition, we identified a maize mutant of ZmPMTR1 , EMS4-06e2fl , with a point-mutation causing premature termination of protein translation, and found that this mutant had lower leaf temperatures, increased rate of water loss, and enhanced drought stress sensitivity. Thus, we present ZmPMTR1 as the first phytomelatonin receptor to be identified and examined in a monocot plant, and our results indicate that it plays an important function in the response of maize to drought stress. [ABSTRACT FROM AUTHOR]

SUMMARY: Low phosphate (LP) in soil is a common nutrient stress that severely restricts agricultural production, but the role, if any, of the major stress phytohormone abscisic acid (ABA) in plant phosphate (Pi) starvation responses remains elusive. Here, we report that LP‐induced ABA accumulation promotes Pi uptake in an ABA INSENSITIVE5 (ABI5)‐dependent manner in Arabidopsis thaliana. LP significantly activated plant ABA biosynthesis, metabolism, and stress responses, suggesting a role of ABA in the plant response to Pi availability. LP‐induced ABA accumulation and expression of two major high‐affinity phosphate transporter genes PHOSPHATE TRANSPORTER1;1/1;4 (PHT1;1/1;4) were severely impaired in a mutant lacking BETA‐GLUCOSIDASE1 (BG1), which converts conjugated ABA to active ABA, and the mutant had shorter roots and less Pi content than wild‐type plants under LP conditions. Moreover, a mutant of ABI5, which encodes a central transcription factor in ABA signaling, also exhibited suppressed root elongation and had reduced Pi content under LP conditions. ABI5 facilitated Pi acquisition by activating the expression of PHT1;1 by directly binding to its promoter, while overexpression of PHT1;1 completely rescued its Pi content under LP conditions. Together, our findings illustrate a molecular mechanism by which ABA positively modulates phosphate acquisition through ABI5 in the Arabidopsis response to phosphate deficiency. Significance Statement: Low phosphate (LP) is a common nutrient stress that severely suppresses plant growth and development, while whether and how the major stress phytohormone abscisic acid (ABA) functions in plant phosphate (Pi) starvation responses remains elusive. Our study showed that LP‐induced ABA promotes Pi uptake by activating PHOSPHATE TRANSPORTER1;1 expression in an ABA INSENSITIVE5‐dependent manner in Arabidopsis thaliana under LP. [ABSTRACT FROM AUTHOR]