Comparative transcriptomic and physiological analyses of weedy rice and cultivated rice to identify vital differentially expressed genes and pathways regulating the ABA response

Weedy rice is a valuable germplasm resource characterized by its high tolerance to both abiotic and biotic stresses. Abscisic acid (ABA) serves as a regulatory signal in plant cells as part of their adaptive response to stress. However, a global understanding of the response of weedy rice to ABA remains to be elucidated. In the present study, the sensitivity to ABA of weedy rice (WR04-6) was compared with that of temperate japonica Shennong9816 (SN9816) in terms of seed germination and post-germination growth via the application of exogenous ABA and diniconazole, an inhibitor of ABA catabolism. Physiological analysis and a transcriptomic comparison allowed elucidation of the molecular and physiological mechanisms associated with continuous ABA and diniconazole treatment. WR04-6 was found to display higher ABA sensitivity than SN9816, resulting in the rapid promotion of antioxidant enzyme activity. Comparative transcriptomic analyses indicated that the number of differentially expressed genes (DEGs) in WR04-6 seedlings treated with 2 μM ABA or 10 μM diniconazole was greater than that in SN9816 seedlings. Genes involved in stress defense, hormone signal transduction, and glycolytic and citrate cycle pathways were highly expressed in WR04-6 in response to ABA and diniconazole. These findings provide new insight into key processes mediating the ABA response between weedy and cultivated rice.

Stomatal responses to carbon dioxide and light require abscisic acid catabolism in Arabidopsis

In plants, stomata control water loss and CO 2 uptake. The aperture and density of stomatal pores, and hence the exchange of gases between the plant and the atmosphere, are controlled by internal factors such as the plant hormone abscisic acid (ABA) and external signals including light and CO 2 . In this study, we examine the importance of ABA catabolism in the stomatal responses to CO 2 and light. By using the ABA 8′-hydroxylase-deficient Arabidopsis thaliana double mutant cyp707a1 cyp707a3 , which is unable to break down and instead accumulates high levels of ABA, we reveal the importance of the control of ABA concentration in mediating stomatal responses to CO 2 and light. Intriguingly, our experiments suggest that endogenously produced ABA is unable to close stomata in the absence of CO 2 . Furthermore, we show that when plants are grown in short day conditions ABA breakdown is required for the modulation of both elevated [CO 2 ]-induced stomatal closure and elevated [CO 2 ]-induced reductions in leaf stomatal density. ABA catabolism is also required for the stomatal density response to light intensity, and for the full range of light-induced stomatal opening, suggesting that ABA catabolism is critical for the integration of stomatal responses to a range of environmental stimuli.

Temporal physiological response of pine to Fusarium circinatum infection is dependent on host susceptibility level: the role of ABA catabolism

Pine pitch canker (PPC), caused by Fusarium circinatum Nirenberg & O’Donnell, represents an important threat to conifer forests worldwide, being associated with significant economic losses. Although essential to develop disease mitigation strategies, little research focused on host susceptibility/resistance mechanisms has been conducted. We aimed to explore the response of a highly susceptible (Pinus radiata) and a relatively resistant (Pinus pinea) species to F. circinatum infection at different stages of infection. Morpho-physiological, hormonal and oxidative stress-related changes were assessed for each pine species and sampling point. Most of the changes found occurred in symptomatic P. radiata, for which an increased susceptibility to photoinhibition was detected together with decreased superoxide dismutase (SOD) activity. Abscisic acid (ABA) catabolism was activated by F. circinatum inoculation in both pine species, leading to the accumulation of the inactive dihydrophaseic acid (DPA) in P. radiata and of the less-active phaseic acid (PA) in P. pinea. Hormones’ confocal analysis revealed that this strategy may be of particular importance at 6 dpi in P. pinea, which together with photosynthesis maintenance to fuel defence mechanism, could in part explain the species resistance to PPC. These results are of great interest for the development of hormone-based breeding strategies or to the use of hormone application as inducers of resistance to F. circinatum infection.

Mechanism of delayed seed germination caused by high temperature during grain filling in rice (Oryza sativa L.)

High temperature during grain filling considerably reduces yield and quality in rice (Oryza sativa L.); however, how high temperature affects seed germination of the next generation is not yet well understood. Here, we report that seeds from plants exposed to high temperature during the grain filling stage germinated significantly later than seeds from unstressed plants. This delay remained even after dormancy release treatments, suggesting that it was not due to primary seed dormancy determined during grain filling. In imbibed embryos of heat-stressed seeds, expression of abscisic acid (ABA) biosynthesis genes (OsNCEDs) was higher than in those of control seeds, whereas that of ABA catabolism genes (OsABA8′OHs) was lower. In the aleurone layer, despite no change in GA signaling as evidenced by no effect of heat stress on OsGAMYB gene expression, the transcripts of α-amylase genes OsAmy1C, OsAmy3B, and OsAmy3E were significantly down-regulated in heat-stressed seeds in comparison with controls. Changes in promoter methylation levels were consistent with transcriptional changes of ABA catabolism-related and α-amylase genes. These data suggest that high temperature during grain filling results in DNA methylation of ABA catabolism-related and α-amylase gene promoters, delaying germination of heat-stressed seeds.

Participation of ABA Metabolism and ROS Generation in Sugar Starvation-Induced Senescence of Rice Flag Leaves

Background: Both sucrose and abscisic acid (ABA) play pivotal role in the regulation of plant leaf senescence. However, the exact mechanism by which sugar starvation , ABA, and reactive oxygen species (ROS) interact with each other during leaf senescence remains largely unknown. In this study, the genotype-dependent alteration in temporal patterns of sugar concentration during leaf senescence and its relation to ABA metabolism and ROS generation were investigated by using the premature senescence of flag leaf ( psf ) mutant and its wild type. Results: Results showed that sugar starvation-induced leaf senescence was closely associated with the endogenous ABA concentration and ROS level in senescent leaves. Sugar starvation accelerated leaf senescence, concomitantly with the marked increase in ABA concentration and malonaldehyde (MDA) accumulation in detached leaves. Conversely, exogenous sugar treatment significantly suppressed the ABA concentration ad ROS level in detached leaves, thus leaf senescence was delayed by exogenous sugar supply. Pharmacological tests revealed that ABA biosynthesis inhibitor (NDGA) delayed the sugar starvation-induced leaf senescence, while ABA catabolism inhibitor (DNCZ) accelerated leaf senescence and significantly increased the endogenous ABA content in senescent leaves. For the expression patterns of ABA synthesis and catabolism related genes induced by sugar starvation, exogenous sucrose supply, NDGA and DNCZ. sugar starvation up-regulated the OsABA8ox1 transcript, while exogenous sucrose and NDGA down-regulated the transciptional expressions of OsNCED1 , OsNCED4 and OsNCED5 and OsABA8ox2 and OsABA8ox3 e by sugar starvation and DNCZ, while the transcript of was increased. Conclusion: Together, our results demonstrated that the rise in endogenous ABA content during sugar starvation-induced leaf senescence is mostly caused by the suppression of ABA catabolism, rather than the enhancement of ABA biosynthesis, and the expression of ABA metabolic genes determines the equilibrium between ABA biosynthesis and catabolism that eventually influence cross-talk between endogenous factors. The breaking for the equilibrium between ABA biosynthesis and catabolism was strongly responsible for sugar starvation-induced leaf senescence, which was resulted from the suppression of ABA catabolism, rather than the enhancement of ABA biosynthesis .

An abscisic acid (ABA) homeostasis regulated by its production, catabolism and transport in peanut leaves in response to drought stress

ABA is an important messenger that acts as the signaling mediator for regulating the adaptive response of plants to drought stress. Two production pathways,de novobiosynthesis and hydrolysis of glucose-conjugated ABA by β-glucosidase (BG), increase cellular ABA levels in plants. ABA catabolism via hydroxylation by 8’-hydroxylase (CYP707A), or conjugation by uridine diphosphate glucosyltransferase (UGT), decreases cellular ABA levels. The transport of ABA through ATP-binding cassette (ABC)-containing transporter proteins, members of ABC transporter G family (ABCG), across plasma membrane (PM) is another important pathway to regulate cellular ABA levels. In this study, based on our previously constructed transcriptome of peanut leaves in response to drought stress, fourteen candidate genes involved in ABA production (includingAhZEP,AhNCED1andAhNCED3,AhABA2,AhAAO1andAhAAO2,AhABA3,AhBG11andAhBG24), catabolism (includingAhCYP707A3,AhUGT71K1andAhUGT73B4) and transport (includingAhABCG22-1andAhABCG22-2), were identified homologously and phylogenetically, and further analyzed at the transcriptional level by real-time RT-PCR, simultaneously determining ABA levels in peanut leaves in response to drought. The high sequence identity and very similar subcellular localization of the proteins deduced from 14 identified genes involved in ABA production, catabolism and transport with the reported corresponding enzymes in databases suggest their similar roles in regulating cellular ABA levels. In response to drought stress, ABA accumulation levels in peanut leaves agree very well with the up-regulated expressions of ABA-producing genes (AhZEP,AhNCED1,AhAAO2,AhABA3,AhBG11andAhBG24) and PM-localized ABA importer genes (AhABCG22-1andAhABCG22-2), although the expression of ABA catabolic genes (AhCYP707A3andAhUGT71K1) was also up-regulated. It is likely that drought-responsive induction of catabolic genes helps not only to maintain ABA levels within a permissible range, but also to prepare the plant for degradation of ABA after removal of the stress. These results suggest that ABA homeostasis in peanut leaves in response to drought may be coordinated by a master regulatory circuit that involves production, catabolism, and as well as transport.

Interlinked regulatory loops of ABA catabolism and biosynthesis coordinate fruit growth and ripening in woodland strawberry

Fruit growth and ripening are controlled by multiple phytohormones. How these hormones coordinate and interact with each other to control these processes at the molecular level is unclear. We found in the early stages of Fragaria vesca (woodland strawberry) fruit development, auxin increases both widths and lengths of fruits, while gibberellin [gibberellic acid (GA)] mainly promotes their longitudinal elongation. Auxin promoted GA biosynthesis and signaling by activating GA biosynthetic and signaling genes, suggesting auxin function is partially dependent on GA function. To prevent the repressive effect of abscisic acid (ABA) on fruit growth, auxin and GA suppressed ABA accumulation during early fruit development by activating the expression of FveCYP707A4a encoding cytochrome P450 monooxygenase that catalyzes ABA catabolism. At the onset of fruit ripening, both auxin and GA levels decreased, leading to a steep increase in the endogenous level of ABA that drives fruit ripening. ABA repressed the expression of FveCYP707A4a but promoted that of FveNCED, a rate-limiting step in ABA biosynthesis. Accordingly, altering FveCYP707A4a expression changed the endogenous ABA levels and affected FveNCED expression. Hence, ABA catabolism and biosynthesis are tightly linked by feedback and feedforward loops to limit ABA contents for fruit growth and to quickly increase ABA contents for the onset of fruit ripening. These results indicate that FveCYP707A4a not only regulates ABA accumulation but also provides a hub to coordinate fruit size and ripening times by relaying auxin, GA, and ABA signals.