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Phytohormones, epigenetic regulation and environmental factors regulate fruit ripening but their interplay during strawberry fruit ripening remains to be determined. In this study, bagged strawberry fruit exhibited delayed ripening compared with fruit grown in normal light, correlating with reduced abscisic acid (ABA) accumulation. Transcription of the key ABA catabolism gene, ABA 8′‐hydroxylase FaCYP707A4, was induced in bagged fruit. With light exclusion whole genome DNA methylation levels were up‐regulated, corresponding to a delayed ripening process, while DNA methylation levels in the promoter of FaCYP707A4 were suppressed, correlating with increases in transcript and decreased ABA content. Experiments indicated FaCRY1, a blue light receptor repressed in bagged fruit and FaAGO4, a key protein involved in RNA‐directed DNA methylation, could bind to the promoter of FaCYP707A4. The interaction between FaCRY1 and FaAGO4, and an increased enrichment of FaAGO4 directed to the FaCYP707A4 promoter in fruit grown under light suggests FaCRY1 may influence FaAGO4 to modulate the DNA methylation status of the FaCYP707A4 promoter. Furthermore, transient overexpression of FaCRY1, or an increase in FaCRY1 transcription by blue light treatment, increases the methylation level of the FaCYP707A4 promoter, while transient RNA interference of FaCRY1 displayed opposite phenotypes. These findings reveal a mechanism by which DNA methylation influences ABA catabolism, and participates in light‐mediated strawberry ripening. [ABSTRACT FROM AUTHOR]

Environmental extremes, such as drought and flooding, are becoming more common with global warming, resulting in significant crop losses. Understanding the mechanisms underlying the plant water stress response, regulated by the abscisic acid (ABA) pathway, is crucial to building resilience to climate change. Potted kiwifruit plants (two cultivars) were exposed to contrasting watering regimes (water logging and no water). Root and leaf tissues were sampled during the experiments to measure phytohormone levels and expression of ABA pathway genes. ABA increased significantly under drought conditions compared with the control and waterlogged plants. ABA-related gene responses were significantly greater in roots than leaves. ABA responsive genes, DREB2 and WRKY40, showed the greatest upregulation in roots with flooding, and the ABA biosynthesis gene, NCED3, with drought. Two ABA-catabolic genes, CYP707A i and ii were able to differentiate the water stress responses, with upregulation in flooding and downregulation in drought. This study has identified molecular markers and shown that water stress extremes induced strong phytohormone/ABA gene responses in the roots, which are the key site of water stress perception, supporting the theory kiwifruit plants regulate ABA to combat water stress. [ABSTRACT FROM AUTHOR]

Abscisic acid (ABA) hydroxylation is an important pathway for ABA inactivation and homeostasis maintenance. Here, we discover a new downstream catabolite of neophaseic acid (neoPA) in the ABA 9′-hydroxyl pathway and identify it as epi-neodihydrophaseic acid (epi-neoDPA) by comparing its accurate mass, retention time, and MSn spectra with those of our chemically synthesized epi-neoDPA. Analyses of Arabidopsis seed germination and ABA-related gene expression reveal that neoPA rather than epi-neoDPA possesses ABA-like hormonal activity. In vitro enzyme activity tests of prokaryotic recombinant protein reveal that NeoPAR1 (neoPA reductase 1) identified from Arabidopsis converts neoPA into epi-neoDPA. Site-directed mutation at Tyr163 in the conserved motif of NeoPAR1 abolishes the catalytic activity of NeoPAR1. Accelerated seed germination was observed in NeoPAR1 knockdown and knockout mutants, whereas retarded seed germination and the accumulation of epi-neoDPA and ABA were observed in NeoPAR1 overexpression lines, suggesting that NeoPAR1 is involved in seed germination and maintenance of ABA homeostasis.

Environmental extremes, such as drought and flooding, are becoming more common with global warming, resulting in significant crop losses. Understanding the mechanisms underlying the plant water stress response, regulated by the abscisic acid (ABA) pathway, is crucial to building resilience to climate change. Potted kiwifruit plants (two cultivars) were exposed to contrasting watering regimes (water logging and no water). Root and leaf tissues were sampled during the experiments to measure phytohormone levels and expression of ABA pathway genes. ABA increased significantly under drought conditions compared with the control and waterlogged plants. ABA-related gene responses were significantly greater in roots than leaves. ABA responsive genes, DREB2 and WRKY40, showed the greatest upregulation in roots with flooding, and the ABA biosynthesis gene, NCED3, with drought. Two ABA-catabolic genes, CYP707A i and ii were able to differentiate the water stress responses, with upregulation in flooding and downregulation in drought. This study has identified molecular markers and shown that water stress extremes induced strong phytohormone/ABA gene responses in the roots, which are the key site of water stress perception, supporting the theory kiwifruit plants regulate ABA to combat water stress.

Dopamine (DA) is a neurotransmitter which takes charge of brain activities about memory and self-stimulation behavior in animals. Interestingly, our results suggest that DA could also give maize an “excited state”. The results showed that 1 mmol L−1 DA promoted maize germination by 23.2% significantly, and accelerated the growth rate of roots and shoots by 21.4 and 24.7%, respectively. As we all known, abscisic acid (ABA) is the key hormone involved in seed dormancy. In our research, ABA levels in roots and shoots dramatically decreased by 16.45 and 57.57%, respectively. To further investigate how DA reduces the ABA level in budding seed, we studied ABA synthesis and catabolism pathway. Specific expression of key ABA-synthesis genes, such as ZmNCED1, ZmNCED3 and ZmZEP were down-regulated by DA. Simultaneously, the expression levels of ABA8OX1a and ABA8OX1b which are major transcripts of ABA 8′-hydroxylase in ABA catabolism were up-regulated at least 1.5- and 4.6-fold, respectively. Our results enriched the functions of animal hormones in plants.

In rice, OsABA8ox encodes abscisic acid (ABA) 8′-hydroxylase, which catalyzes the committed step of ABA catabolism. The contribution of ABA catabolism in root development remains unclear. We investigated the role of OsABA8ox2 in root growth and development and drought response. GUS staining results showed that OsABA8ox2 was expressed mainly in roots at seedling stage and was strongly expressed in the meristematic zone of the radicle. OsABA8ox2 expression in roots was markedly decreased after 0.5 h polyethylene glycol (PEG) treatment and increased after 0.5 h rehydration, implying that OsABA8ox2 is a drought-responsive gene. OsABA8ox2 knockout mediated by the CRISPR-Cas9 system increased drought-induced ABA and indole-3-acetic acid accumulation in roots, conferred increased ABA sensitivity, and promoted a more vertically oriented root system architecture (RSA) beneficial to drought tolerance. OsABA8ox2 overexpression suppressed root elongation and increased stomatal conductance and transpiration rate. Consequently, OsABA8ox2 knockout dramatically improved rice drought tolerance, whereas OsABA8ox2 overexpression seedlings were hypersensitive to drought stress, suggesting that OsABA8ox2 contributes to drought response in rice. Compared with wild type, functional leaves of OsABA8ox2 knockout seedlings showed higher ABA levels, whereas overexpression lines showed lower ABA levels, suggesting that OsABA8ox2, as an ABA catabolic gene, modulates ABA concentration through ABA catabolism. OsABA8ox2 and OsABA8ox3 were both localized in the endoplasmic reticulum. Together, these results indicate that OsABA8ox2 suppresses root elongation of rice seedlings, increases water transpiration, and contributes to drought response through ABA catabolism, and that OsABA8ox2 knockout dramatically improves rice drought tolerance. They highlight the key role of ABA catabolism mediated by OsABA8ox2 on root growth and development. OsABA8ox2, as a novel RSA gene, would be a potential genetic target for the improvement of rice drought tolerance.

Abstract Terrestrial plants must cope with drought stress to survive. Under drought stress, plants accumulate the phytohormone abscisic acid (ABA) by increasing its biosynthesis and decreasing its catabolism. However, the regulatory pathways controlling ABA catabolism in response to drought remain largely unclear. Here, we report that the flowering repressor SHORT VEGETATIVE PHASE (SVP) is induced by drought stress and associates with the promoter regions of the ABA catabolism pathway genes CYP707A1 , CYP707A3 and AtBG1 , causing decreased expression of CYP707A1 and CYP707A3 but enhanced expression of AtBG1 in Arabidopsis leaves. Loss-of-function mutations in CYP707A1 and CYP707A3 or overexpression of AtBG1 could rescue the drought-hypersensitive phenotype of svp mutant plants by increasing cellular ABA levels. Collectively, our results suggest that SVP is a central regulator of ABA catabolism and that a regulatory pathway involving SVP , CYP707A1 / 3 , and AtBG1 plays a critical role in plant response to water deficit and plant drought resistance. This study reports that the flowering repressor SVP is a central regulator of ABA catabolism by regulating the expression of the ABA catabolism pathway genes CYP707A1 , CYP707A3 and AtBG1. Loss-of-function mutations in CYP707A1 and CYP707A3 or overexpression of AtBG1 can rescue the drought-hypersensitive phenotype of svp mutant plants by increasing cellular ABA levels. The results suggest that a regulatory pathway involving SVP , CYP707A1 / 3 , and AtBG1 plays a critical role in plant response to water deficit and plant drought resistance. [ABSTRACT FROM AUTHOR]

Chilling stress is an important constraint for maize seedling establishment in the field. To examine the role of salicylic acid (SA) and hydrogen peroxide (H2O2) in response to chilling stress, we investigated the effects of seed priming with SA, H2O2, and SACH2O2 combination on maize resistance under chilling stress (13°C). Priming with SA, H2O2, and especially SACH2O2 shortened seed germination time and enhanced seed vigor and seedling growth as compared with hydropriming and non-priming treatments under low temperature. Meanwhile, SACH2O2 priming notably increased the endogenous H2O2 and SA content, antioxidant enzymes activities and their corresponding genes ZmPAL, ZmSOD4, ZmAPX2, ZmCAT2, and ZmGR expression levels. The α-amylase activity was enhanced to mobilize starch to supply metabolites such as soluble sugar and energy for seed germination under chilling stress. In addition, the SACH2O2 combination positively up-regulated expressions of gibberellic acid (GA) biosynthesis genes ZmGA20ox1 and ZmGA3ox2, and down-regulated GA catabolism gene ZmGA2ox1 expression; while it promoted GA signaling transduction genes expressions of ZmGID1 and ZmGID2 and decreased the level of seed germination inhibitor gene ZmRGL2. The abscisic acid (ABA) catabolism gene ZmCYP707A2 and the expressions of ZmCPK11 and ZmSnRK2.1 encoding response receptors in ABA signaling pathway were all up-regulated. These results strongly suggested that priming with SA and H2O2 synergistically promoted hormones metabolism and signal transduction, and enhanced energy supply and antioxidant enzymes activities under chilling stress, which were closely relevant with chilling injury alleviation and chillingtolerance improvement in maize seed. [ABSTRACT FROM AUTHOR]

Abscisic acid (ABA) 8′-hydroxylase is the key enzyme in the oxidative catabolism of ABA. CYP707A genes encode (+)-ABA 8′-hydroxylases that are involved in fruit ripening and abiotic stress responses. However, there are limited reports on the functions of CYP707A genes in mulberry. This study analyzed the transcriptional expression of CYP707A genes during fruit development and their response to exogenous ABA and uniconazole (UNI) treatment. Additionally, the expression patterns of CYP707A under stress conditions were explored in mulberry. We obtained six CYP707A gene ( MnCYP707A1- MnCYP707A6) sequences from Morus notabilis genome database. Multiple sequence alignment results showed high conservation within the MnCYP707A genes. The expression profiles of these genes during fruit development and under stress conditions were analyzed by quantitative reverse transcriptase polymerase chain reaction (qRT-PCR). During fruit development, the expression levels of MaCYP707A1, MaCYP707A3, MaCYP707A5, and MaCYP707A6 were higher at the primary stage and then sharply declined to a lower level at the late stage. The expression of MaCYP707A1 in the fruit quickly declined at 1 d after 50 mmol/L UNI treatment. In addition, under Sodium chloride (NaCl) (0.3 % (w/v) at germination stage and 0.6 % (w/v) at seedling stage) or polyethylene glyco (PEG) 6000 (5 % (w/v) at germination stage and 20 % (w/v) at seedling stage) stress, most of the MaCYP707A genes were up-regulated, but displayed different patterns. Tissue-specific patterns of the CYP707A family appear to be universal under stress conditions. In a short, MaCYP707A genes play overlapping roles in ABA catabolism and MaCYP707A1 may be the key gene expressed during fruit maturation. Most of these genes were induced by abiotic stress, and various expression patterns were observed among the MaCYP707A in multiple mulberry tissues at different growth periods. [ABSTRACT FROM AUTHOR]

Main conclusion: The induction of dormancy release and germination of Avena fatua caryopses by KAR involves ABA degradation to phaseic acid. Both, KAR and GA , control the AsA-GSH cycle, DNA replication and accumulation of β-tubulin in embryos before caryopses germination. Abstract: Avena fatua caryopses cannot germinate in darkness at 20 °C because of dormancy, but karrikinolide-1 (KAR), a compound in plant-derived smoke, and gibberellic acid (GA) induced an almost complete germination. The radicle protrusion through the coleorhiza was preceded by increased water uptake, rupture of coat, increased embryo size and coleorhiza length as well as coleorhiza protrusion through covering structures. The stimulatory effect of KAR was correlated with the reduced content of abscisic acid (ABA) and an increase in phaseic acid (PA) in embryos from caryopses before coleorhiza protrusion. Two non-enzymatic antioxidants, ascorbate (AsA) and reduced glutathione (GSH), did not affect the germination of dormant caryopses, but in the presence of KAR or GA they only slightly delayed the germination. The stimulatory effect of KAR or GA on the final germination percentage was markedly antagonized by lycorine, an AsA biosynthesis inhibitor. KAR and GA applied during caryopses imbibition resulted in increases of AsA, dehydroascorbate (DHA) and GSH, but reduced the embryos' oxidized glutathione (GSSG) content. Furthermore, both KAR and GA induced an additional ascorbate peroxidase (APX) isoenzyme and increased the glutathione reductase (GR) activity. Both compounds stimulated β-tubulin accumulation in radicle+coleorhiza (RC) and plumule+coleoptile (PC), and enhanced the transition from G to S and also from S to G phases. The comparison of the effects produced by KAR and GA shows a similar action; thus the KAR effect may not be specific. The study provides new data regarding the mechanism with which KAR, a representative of a novel class of plant growth regulators, regulates dormancy and germination of caryopses. [ABSTRACT FROM AUTHOR]

Fruit set of apple can be reduced by cloudy weather, short-term shade treatments, or application of photosynthetic inhibitors when the young fruit are ≈8 to 15 mm in diameter, indicating that fruit are sensitive to a transient carbohydrate stress during this period. We investigated the potential for S-abscisic acid (ABA) and an ABA analog [(+)-8'-acetylene ABA) to chemically thin apple fruit by causing a stomatal limitation of photosynthesis. Stomatal conductance (gs) of 'Imperial Gala'/M.7 was reduced by 60% 3 h after application of 250 mg⋅L-1 ABA or 25 mg⋅L-1 (+)-8'-acetylene ABA. Stomatal conductance began to recover 4 days after application but did not return to control levels until 19 days after treatment. Application of 250 mg⋅L-1 ABA combined with 100 mg⋅L-1 6-benzyladenine (6-BA) when mean fruit diameter was ≈ 10 mm reduced fruit set of 'Gala'/M.7 but not 'Pink Lady™'/M.7 or 'Morganspur Delicious'/MM.111.Fruit set of 'Pink Lady™'/M.7 was reduced by application of 20 mg⋅L-1 (+)-8'-acetylene ABA + 100 mg-L ' 6-BA at full bloom or 10 mg⋅L-1 (+)-8'-acetylene ABA + 100 mg⋅L-1 6-BA at the 10-mm fruit diameter stage. Fruit set of 'Morganspur Delicious/MM.111 was reduced by application of 25 mg⋅L-1 (+)-8'-acetylene ABA, either alone or in combination with 75 mg⋅L-1 6-BA, at the 10-mm fruit diameter stage. ABA and (+)-8'-acetylene ABA triggered leaf abscission at rates above 250 mg⋅L-1 and 25 mg⋅L-1, respectively. Fruit set andgs data from the present studies indicate the biological activity of (+)-8'-acetylene ABA is 10-fold higher than ABA. These results suggest that ABA and (+)-8'-acetylene ABA reduced fruit set by causing a stomatal limitation in photosynthesis that resulted in a transient carbohydrate stress. Thinning responses to ABA and (+)-8'-acetylene ABA at the concentrations used in these experiments were reduced compared with standard concentrations of currently available chemical thinning agents. However, increasing the concentration of ABA or (+)-8'-acetylene ABA to levels that would achieve comparable thinning are also likely to result in unacceptable leaf abscission. [ABSTRACT FROM AUTHOR]

Abscisic acid (ABA) hydroxylation is an important pathway for ABA inactivation and homeostasis maintenance. Here, we discover a new downstream catabolite of neophaseic acid (neoPA) in the ABA 9'-hydroxyl pathway and identify it as epi-neodihydrophaseic acid (epi-neoDPA) by comparing its accurate mass, retention time, and MS n spectra with those of our chemically synthesized epi-neoDPA. Analyses of Arabidopsis seed germination and ABA-related gene expression reveal that neoPA rather than epi-neoDPA possesses ABA-like hormonal activity. In vitro enzyme activity tests of prokaryotic recombinant protein reveal that NeoPAR1 (neoPA reductase 1) identified from Arabidopsis converts neoPA into epi-neoDPA. Site-directed mutation at Tyr163 in the conserved motif of NeoPAR1 abolishes the catalytic activity of NeoPAR1. Accelerated seed germination was observed in NeoPAR1 knockdown and knockout mutants, whereas retarded seed germination and the accumulation of epi-neoDPA and ABA were observed in NeoPAR1 overexpression lines, suggesting that NeoPAR1 is involved in seed germination and maintenance of ABA homeostasis., (Copyright © 2022 The Author(s). Published by Elsevier Inc. All rights reserved.)

Background and Aims Abscisic acid (ABA) plays crucial roles in plants' responses to abiotic stresses. ABA 8′-hydroxylation controlled by CYP707A genes has been well studied in Arabidopsis and rice, but not in legumes. The aims of the present study were to identify and functionally analyse the soybean CYP707A gene family, and to explore their expression patterns under dehydration and salt stresses. Methods A complementation experiment was employed to verify the function of soybean CYP707A1a in ABA catabolism. Genomic and cDNA sequences of other soybean CYP707A genes were isolated from the Phytozome database based on soybean CYP707A1a. The structure and phylogenetic relationship of this gene family was further analysed. The expression patterns of soybean CYP707A genes under dehydration and salt stress were analysed via quantitative real-time PCR. Key Results Over-expression of GmCYP707A1a in the atcyp707a2 T-DNA insertion mutant decreased its sensitivity to ABA, indicating that GmCYP707A1a indeed functions as an ABA 8′-hydroxylase in higher plants. The soybean genome contains ten CYP707A genes. Gene structure and phylogenetic analysis showed high conservation of ten GmCYP707A genes to the other CYP707A genes from monocots and dicots. Seed imbibition induced expression of A1a, A1b, A2a, A2b, A2c, A3a and A5 in embryo, and expression of A1a, A1b, A2a and A2b in cotyledon. Dehydration induced expression of A1a, A1b, A2b, A2c, A3a, A3b, A4a, A4b and A5 both in roots and in leaves, whereas rehydration stimulated transcription of A2a, A2b, A3b, A4a and A5 in roots, and only A3b and A5 in leaves. Expression of all soybean CYP707A genes was induced either by short- or by long-term salt stress. Conclusions The first biological evidence is provided that GmCYP7071a encodes an ABA 8′-hydroxylase through transgenic studies. Ten soybean GmCYP707A genes were identified, most of them expressed in multiple soybean tissues, and were induced by imbibition, dehydration and salinity. [ABSTRACT FROM AUTHOR]

H2O2 is known as a signal molecule in plant cells, but its role in the regulation of aqbscisic acid (ABA) and gibberellic acid (GA) metabolism and hormonal balance is not yet clear. In this study it was found that H2O2 affected the regulation of ABA catabolism and GA biosynthesis during seed imbibition and thus exerted control over seed dormancy and germination. As seen by quantitative RT-PCR (QRT-PCR), H2O2 up-regulated ABA catabolism genes (e.g. CYP707A genes), resulting in a decreased ABA content during imbibition. This action required the participation of nitric oxide (NO), another signal molecule. At the same time, H2O2 also up-regulated GA biosynthesis, as shown by QRT-PCR. When an ABA catabolism mutant, cyp707a2, and an overexpressing plant, CYP707A2-OE, were tested, ABA content was negatively correlated with GA biosynthesis. Exogenously applied GA was able to over-ride the inhibition of germination at low concentrations of ABA, but had no obvious effect when ABA concentrations were high. It is concluded that H2O2 mediates the up-regulation of ABA catabolism, probably through an NO signal, and also promotes GA biosynthesis. High concentrations of ABA inhibit GA biosynthesis but a balance of these two hormones can jointly control the dormancy and germination of Arabidopsis seeds. [ABSTRACT FROM PUBLISHER]

ABA, acting as a stress signal, plays crucial roles in plant resistance to water stress. Because ABA signal production is based on ABA biosynthesis, the regulation of NCED, a key enzyme in the ABA biosynthesis pathway, is normally thought of as the sole factor controlling ABA signal production. Here we demonstrate that ABA catabolism in combination with a synergistic regulation of ABA biosynthesis plays a crucial role in governing ABA signal production. Water stress induced a significant accumulation of ABA, which exhibited different patterns in detached and attached leaves. ABA catabolism followed a temporal trend of exponential decay for both basic and stress ABA, and there was little difference in the catabolic half-lives of basic ABA and stress ABA. Thus, the absolute rate of ABA catabolism, i.e. the amount of ABA catabolized per unit time, increases with increased ABA accumulation. From the dynamic processes of ABA biosynthesis and catabolism, it can be inferred that stress ABA accumulation may be governed by a synergistic regulation of all the steps in the ABA biosynthesis pathway. Moreover, to maintain an elevated level of stress ABA sustained activation of NCED3 should be required. This inference was supported by further findings that the genes encoding major enzymes in the ABA biosynthesis pathway, e.g. NCED3, AAO3 and ABA3 were all activated by water stress, and with ABA accumulation progressing, the expressions of NCED3, AAO3 and ABA3 remained activated. Data on ABA catabolism and gene expression jointly indicate that ABA signal production is controlled by a sustained activation of NCED3 and the synergistic regulation of ABA biosynthesis and catabolism. [ABSTRACT FROM AUTHOR]

Abscisic acid (ABA) regulates many important processes in normal growth and development as well as in adaptive responses to environmental stresses. For correct and accurate actions, physiologically active ABA level is controlled through fine-tuning of de novo biosynthesis and catabolism. Hydroxylation at the 8′-position of ABA is the key step in the oxidative catabolism of ABA, and this reaction is catalyzed by ABA 8′-hydroxylase, a cytochrome P450 (P450). Recently, the CYP707A family of Arabidopsis has been identified as ABA 8′-hydroxylase through genomic and biochemical approaches. The CYP707A family is present in a wide range of plant kingdom and functions in ABA catabolism in plants. CYP707A is the pivotal enzyme controlling the endogenous ABA levels by its transcriptional regulation and plays a key role in ABA-mediated physiological processes such as seed dormancy and stress response. Specific inhibitors of ABA catabolism can manipulate ABA homeostasis in plants and are potentially very useful tools for cellular and molecular investigations in the field of plant physiology as well as for potential agricultural chemicals. Identification of the ABA catabolic genes gives us new insight into the development of chemical inhibitors specific to ABA 8′-hydroxylase. [ABSTRACT FROM AUTHOR]

Abstract: Since their early migration from aquatic environments to the land, plants have had to cope with periodic and unpredictable environmental stresses during growth and development. Surviving such stresses over a long evolutionary scale led plants to acquire mechanisms by which they can sensitively perceive incoming stresses and regulate their physiology accordingly. The plant hormone abscisic acid (ABA) plays a major role in plant responses to stress. Although rapid production of ABA in response to drought and salt stresses is essential to define ABA as a stress hormone, an equally rapid catabolism of ABA when such stresses are relieved is also essential in that role. Since ABA mediates so many stress responses, the initial perception of dehydration and the subsequent changes in gene expression that lead to rapid ABA biosynthesis constitute the most important stress signal transduction pathway among all the plant responses to stresses. Identification of the genes involved and understanding their roles during stress perception and physiological regulation has become an important and exciting research field in recent years. This review covers mainly our understanding of this aspect. ABA-induced changes in gene expression and their roles in physiological regulation are dealt with in less detail. [Copyright &y& Elsevier]

Chilling stress is an important constraint for maize seedling establishment in the field. To examine the role of salicylic acid (SA) and hydrogen peroxide (H 2 O 2 ) in response to chilling stress, we investigated the effects of seed priming with SA, H 2 O 2 , and SA+H 2 O 2 combination on maize resistance under chilling stress (13°C). Priming with SA, H 2 O 2 , and especially SA+H 2 O 2 shortened seed germination time and enhanced seed vigor and seedling growth as compared with hydropriming and non-priming treatments under low temperature. Meanwhile, SA+H 2 O 2 priming notably increased the endogenous H 2 O 2 and SA content, antioxidant enzymes activities and their corresponding genes ZmPAL, ZmSOD4, ZmAPX2, ZmCAT2 , and ZmGR expression levels. The α-amylase activity was enhanced to mobilize starch to supply metabolites such as soluble sugar and energy for seed germination under chilling stress. In addition, the SA+H 2 O 2 combination positively up-regulated expressions of gibberellic acid (GA) biosynthesis genes ZmGA20ox1 and ZmGA3ox2 , and down-regulated GA catabolism gene ZmGA2ox1 expression; while it promoted GA signaling transduction genes expressions of ZmGID1 and ZmGID2 and decreased the level of seed germination inhibitor gene ZmRGL2 . The abscisic acid (ABA) catabolism gene ZmCYP707A2 and the expressions of ZmCPK11 and ZmSnRK2.1 encoding response receptors in ABA signaling pathway were all up-regulated. These results strongly suggested that priming with SA and H 2 O 2 synergistically promoted hormones metabolism and signal transduction, and enhanced energy supply and antioxidant enzymes activities under chilling stress, which were closely relevant with chilling injury alleviation and chilling-tolerance improvement in maize seed. Highlights: Seed germination and seedling growth were significantly improved under chilling stress by priming with SA+H 2 O 2 combination, which was closely relevant with the change of reactive oxygen species, metabolites and energy supply, hormones metabolism and regulation.

The phytohormone abscisic acid (ABA) plays crucial roles in numerous physiological processes during plant growth and abiotic stress responses. The endogenous ABA level is controlled by complex regulatory mechanisms involving biosynthesis, catabolism, transport and signal transduction pathways. This complex regulatory network may target multiple levels, including transcription, translation and post-translational regulation of genes involved in ABA responses. Most of the genes involved in ABA biosynthesis, catabolism and transport have been characterized. The local ABA concentration is critical for initiating ABA-mediated signalling during plant development and in response to environmental changes. In this chapter we discuss the mechanisms that regulate ABA biosynthesis, catabolism, transport and homoeostasis. We also present the findings of recent research on ABA perception by cellular receptors, and ABA signalling in response to cellular and environmental conditions., (© 2015 Authors; published by Portland Press Limited.)

The phytohormone abscisic acid (ABA) plays a crucial role in numerous aspects of plant growth and environmental stress responses. Endogenous ABA levels are regulated by a balance between its biosynthetic and catabolic activities. This balance may occur at multiple levels and includes the expression of genes involved in these processes. ABA UDP-glucosyltransferase (UGT), the major player in the ABA conjugation pathway, has been shown to have a marginal effect on free ABA levels. However, recent studies provide new insight into the importance of the ABA conjugation pathway in contributing to the control of ABA homeostasis. Gain-of-function and loss-of-function mutant analyses have revealed that UGT71B6, an ABA UGT, and its 2 closely related homologs, UGT71B7 and UGT71B8, play a crucial role in ABA homeostasis and in adaptation to various abiotic stresses.