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Seed germination is a vital stage in the plant life-cycle that greatly contributes to plant establishment. Melatonin has been shown to promote seed germination under various environmental stresses; however, the mechanism remains largely underexplored. Here, we reported that melatonin antagonized abscisic acid (ABA) to promote seed germination by regulating ABA and gibberellic acid (GA 3 ) balance. Transcriptomic analysis revealed that such a role of melatonin was associated with Ca 2+ and redox signaling. Melatonin pretreatment induced Ca 2+ efflux accompanied by an up-regulation of vacuolar H+/Ca 2+ antiporter 3 (CAX3). AtCAX3 deletion in Arabidopsis exhibited reduced Ca 2+ efflux. Inhibition of Ca 2+ efflux in the seeds of melon and Arabidopsis mutant AtCAX3 compromised melatonin-induced germination under ABA stress. Melatonin increased H 2 O 2 accumulation, and H 2 O 2 pretreatment decreased ABA/GA 3 ratio and promoted seed germination under ABA stress. However, complete inhibition of H 2 O 2 accumulation abolished melatonin-induced ABA and GA 3 balance and seed germination. Our study reveals a novel regulatory mechanism in which melatonin counteracts ABA to induce seed germination that essentially involves CAX3-mediated Ca 2+ efflux and H 2 O 2 accumulation, which, in turn, regulate ABA and GA 3 balance by promoting ABA catabolism and/or GA 3 biosynthesis., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Precocious leaf senescence can reduce crop yield and quality by limiting the growth stage. Melatonin has been shown to delay leaf senescence; however, the underlying mechanism remains obscure. Here, we show that melatonin offsets abscisic acid (ABA) to protect photosystem II and delay the senescence of attached old leaves under the light. Melatonin induced H2O2 accumulation accompanied by an upregulation of melon respiratory burst oxidase homolog D (CmRBOHD) under ABA‐induced stress. Both melatonin and H2O2 induced the accumulation of cytoplasmic‐free Ca2+ ([Ca2+]cyt) in response to ABA, while blocking of Ca2+ influx channels attenuated melatonin‐ and H2O2‐induced ABA tolerance. CmRBOHD overexpression induced [Ca2+]cyt accumulation and delayed leaf senescence, whereas deletion of Arabidopsis AtRBOHD, a homologous gene of CmRBOHD, compromised the melatonin‐induced [Ca2+]cyt accumulation and delay of leaf senescence in Arabidopsis under ABA stress. Furthermore, melatonin, H2O2 and Ca2+ attenuated ABA‐induced K+ efflux and subsequent cell death. CmRBOHD overexpression and AtRBOHD deletion alleviated and aggravated the ABA‐induced K+ efflux, respectively. Taken together, our study unveils a new mechanism by which melatonin offsets ABA action to delay leaf senescence via RBOHD‐dependent H2O2 production that triggers [Ca2+]cyt accumulation and subsequently inhibits K+ efflux and delays cell death/leaf senescence in response to ABA. Summary statement: Melatonin has been shown to delay leaf senescence; however, the underlying mechanism remains obscure. This study shows that melatonin counteracts abscisic acid (ABA) to delay leaf senescence via RBOHD‐dependent H2O2 production that triggers cytoplasmic‐free Ca2+ accumulation and subsequently inhibits K+ efflux and delays cell death in response to ABA. [ABSTRACT FROM AUTHOR]

In the current study, we found that CaCl SB 2 sb pretreatment decreased the ABA/GA SB 3 sb ratio likely by promoting ABA catabolism and GA SB 3 sb biosynthesis and thus promoted seed germination under ABA stress, suggesting that cytoplasmic Ca SP 2+ sp signaling acts as a positive regulator in ABA-regulated seed germination. When compared to wild-type I Arabidopsis i seeds, seeds of the I at i I rbohd i and I at i I rbohf i mutants showed higher germination rates under ABA stress (Figure 6), suggesting that H SB 2 sb O SB 2 sb is involved in ABA-induced inhibition of seed germination. Consistently, we found that exogenous H SB 2 sb O SB 2 sb decreased the ABA level and increased the GA SB 3 sb level during seed germination under ABA stress, suggesting that H SB 2 sb O SB 2 sb -induced seed germination was closely associated with its regulatory role in balancing ABA/GA SB 3 sb (Figure 3). [Extracted from the article]

Grafting is widely used to increase plant defense responses to various stresses. Grafting-induced cold tolerance is associated with the increase of the antioxidant potential of plants; however, the underlying mechanisms remain unclear. Here, we found that pumpkin rootstocks promote antioxidant enzyme activities and alleviate cold-induced oxidative damage, accompanied by increased abscisic acid (ABA), melatonin, and methyl jasmonate (MeJA) levels in leaves. Increased ABA accumulation in leaves was attributed partly to the increased ABA levels in rootstocks. ABA induced antioxidant enzymes activities and the accumulation of melatonin and MeJA, while inhibition of ABA synthesis blocked the rootstock-induced antioxidant activity and the accumulation of melatonin and MeJA under cold stress. Melatonin and MeJA application also enhanced ABA accumulation in leaves after cold exposure, whereas inhibition of melatonin or MeJA synthesis attenuated the rootstock-induced increase of ABA. Moreover, melatonin and MeJA application alleviated cold-induced oxidative stress, but inhibition of melatonin or MeJA synthesis lowered the rootstock- or ABA-induced antioxidant potential and tolerance to cold. These findings indicate that ABA plays an important role in the grafting-induced cold tolerance by promoting the accumulation of melatonin and MeJA, which in turn, promote ABA accumulation, forming a positive feedback loop. [ABSTRACT FROM AUTHOR]

The carotenoid cleavage dioxygenase (CCD) gene family in plants comprises two subfamilies: CCD and 9-cis-epoxycarotenoid dioxygenase (NCED). Genes in the NCED subfamily are mainly involved in plant responses to abiotic stresses such as salt, low temperature, and drought. Members of the NCED subfamily are the most important rate-limiting enzymes in the biosynthesis of abscisic acid (ABA). In the present study, genome-wide analysis was performed to identify CCD gene members in six Cucurbitaceae species, including watermelon (Citrullus lanatus), melon (Cucumis melo), cucumber (C.sativus), pumpkin (Cucurbita moschata), bottle gourd (Lagenaria siceraria), and wax gourd (Benincasa hispida). A total of 10, 9, 9, 13, 8, 8 CCD genes were identified in the six species, respectively, and these genes were unevenly distributed in different chromosomes. Phylogenetic analysis showed that CCD genes of the six species clustered into two subfamilies: CCD and NCED, with five and three independent clades, respectively. The number of exons ranged from 1 to 15, and the number of motifs were set to 15 at most. The cis-acting elements analysis showed that a lot of the cis-acting elements were implicated in stress and hormone response. Melon seedlings were treated with salt, low temperature, drought, and ABA, and then tissue-specific analysis of CCDs expression were performed on the root, stem, upper leaf, middle leaf, female flower, male flower, and tendril of melon. The results showed that genes in CCD family exhibited various expression patterns. Different CCD genes of melon showed different degrees of response to abiotic stress. This study presents a comprehensive analysis of CCD gene family in six species of Cucurbitaceae, providing a strong foundation for future studies on specific genes in this family. [ABSTRACT FROM AUTHOR]