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Melilotus albus , which contains abundant pharmacologically active coumarins, is usually used as a rotation crop and green manure worldwide. Abscisic acid (ABA) is a crucial plant hormone that plays an important role in plant stress responses. There is a paucity of information about the ABA signaling pathway and its regulatory network in M. albus . Here, we performed a comparative physiological and transcriptomic analysis to assess the response of M. albus to exogenous ABA. Physiological analysis revealed that proline (Pro), soluble protein and H 2 O 2 content after ABA treatment 3 h significantly increased by 14.0%, 12.0% and 32.4% compared with 0 h in M. albus . A total of 19,855 differentially expressed genes (DEGs) were identified under ABA treatment, including 13,392 in shoots and 15,471 in roots. We obtained two modules that were significantly correlated with the ABA treatment (the darkorange module was positively correlated at 24 h in the shoot, brown2 module positively correlated at 3 h in the root) by weighted correlation network analysis (WGCNA). KEGG enrichment analysis showed that genes within two modules were primarily enriched in protein synthesis and metabolism, secondary metabolites, purine and pyrimidine metabolism, and phenylalanine, tyrosine and tryptophan biosynthesis. GO enrichment analysis indicated that genes within two modules were primarily enriched in energy substance metabolism. These pathways were mainly associated with abiothic stress, which indicated that exogenous application of ABA activated the stress resistance system of M. albus . The hub gene 4CL1 (4-Coumarate: CoA ligase 1) was translated and expressed in yeast, resulting in enhanced salt and ABA tolerance in the transgenic yeast. Overexpression of Ma4CL1 in M. albus improved the salt resistance of the transgenic plants. Profiling ABA-responsive genes offers valuable insights into the molecular functions of regulatory genes and will facilitate future molecular breeding efforts in M. albus .

This study comprehensively investigates the physiological and molecular regulatory mechanisms of Camellia oleifera seedlings under drought stress with a soil moisture content of about 30%, where exogenous abscisic acid (ABA) was applied via foliar spraying at concentrations of 50 µg/L, 100 µg/L, and 200 µg/L. The results demonstrated that appropriate concentrations of ABA treatment can regulate the physiological state of the seedlings through multiple pathways, including photosynthesis, oxidative stress response, and osmotic balance, thereby aiding in the restructuring of their drought response strategy. ABA treatment effectively activated the antioxidant system by reducing stomatal conductance and moderately inhibiting the photosynthetic rate, thus alleviating oxidative damage caused by drought stress. Additionally, ABA treatment promoted the synthesis of osmotic regulators such as proline, maintaining cellular turgor stability and enhancing the plant's drought adaptability. The real-time quantitative PCR results of related genes indicated that ABA treatment enhanced the plant's response to the ABA signaling pathway and improved disease resistance by regulating the expression of related genes, while also enhancing membrane lipid stability. A comprehensive evaluation using a membership function approach suggested that 50 µg/L ABA treatment may be the most-effective in mitigating drought effects in practical applications, followed by 100 µg/L ABA. The application of 50 µg/L ABA for 7 h induced significant changes in various biochemical parameters, compared to a foliar water spray. Notably, superoxide dismutase activity increased by 17.94%, peroxidase activity by 30.27%, glutathione content by 12.41%, and proline levels by 25.76%. The content of soluble sugars and soluble proteins rose by 14.79% and 87.95%, respectively. Additionally, there was a significant decrease of 31.15% in the malondialdehyde levels.

High levels of cadmium (Cd) in soil can cause crop yield reduction or death. Cadmium accumulation in crops affects human and animal health as it passes through the food chain. Therefore, a strategy is needed to enhance the tolerance of crops to this heavy metal or reduce its accumulation in crops. Abscisic acid (ABA) plays an active role in plants' response to abiotic stress. The application of exogenous ABA can reduce Cd accumulation in shoots of some plants and enhance the tolerance of plants to Cd; therefore, ABA may have good application prospects. In this paper, we reviewed the synthesis and decomposition of ABA, ABA-mediated signal transduction, and ABA-mediated regulation of Cd-responsive genes in plants. We also introduced physiological mechanism underlying Cd tolerance because of ABA. Specifically, ABA affects metal ion uptake and transport by influencing transpiration and antioxidant systems, as well as by affecting the expression of metal transporter and metal chelator protein genes. This study may provide a reference for further research on the physiological mechanism of heavy metal tolerance in plants., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Cleistocalyx operculatus and Syzygium cumini possess a certain waterlogging tolerance. However, the comparable and adaptable strategies to waterlogging stress between these two species on the basis of waterlogging adventitious root (AR) regulation were still unclear. In this study, the plant performance in response to AR regulation based on AR removal (AR-R) and exogenous hormone application was investigated in terms of plant morphology, physiology, photosynthesis and AR traits. Results showed that C. operculatus possesses stronger waterlogging tolerance than S. cumini based on waterlogging tolerance coefficient, which is mainly due to the higher root biomass, root porosity and length, and activity of ARs, and shorter emergence time of ARs in C. operculatus than in S. cumini. The AR-R treatment increased activity and porosity of primary root, and induced a large amount of up-vertical ARs from the primary root systems in C. operculatus, while similar adaptive morphological changes in roots did not occur in AR-R-treated S. cumini. Exogenous abscisic acid (ABA) application had better effects on alleviating waterlogging damages than exogenous auxin (IAA) in balancing endogenous hormones (ABA and zeatin riboside), promoting AR development (porosity and activity, and the ratio of cortex area to stele area), improving the photosynthesis process and the antioxidant system (soluble protein, free proline and peroxidase). Moreover, under waterlogging conditions, exogenous ABA application induced greater increases in net photosynthesis rate, stomatal conductance, chlorophyll b and carotenoid in S. cumini than in C. operculatus, which suggested that S. cumini responded more positively and efficiently to exogenous ABA application than C. operculatus under waterlogging conditions. Thus, the findings provided new insights into the waterlogging adaptable strategies in waterlogging tolerant woody species on the basis of ARs and could provide scientific guidance for the application of these two species during revegetation activities in wetlands. Cleistocalyx operculatus could alternatively form a majority of up-vertical adventitious roots (ARs) from the primary roots after removing the normal ARs, but Syzygium cumini could not.Cleistocalyx operculatus possessed positive strategies to waterlogging stress, while S. cumini used traditional passive strategies.Exogenous abscisic acid (ABA) application had better effects on alleviating waterlogging damages in both species than exogenous auxin application.Syzygium cumini could more positively and efficiently respond to exogenous ABA application than C. operculatus.Waterlogging tolerance coefficient was significantly controlled by the chlorophyll contents and AR factors in C. operculatus and the AR factors and O2- in S. cumini.The best development of the AR number (ARN) and AR length (ARL) in exogenous ABA-treated C. operculatus may be closely related with positive zeatin riboside accumulation.The development of ARN and ARL was more important to waterlogging tolerance than that of AR porosity under waterlogging conditions., (© The Author(s) 2021. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Plants have evolved multiple strategies to survive and adapt when confronting the changing climate, including elevated CO 2 concentration ( e [CO 2 ]) and intensified drought stress. To explore the role of abscisic acid (ABA) in modulating the response of plant water relation characteristics to progressive drought under ambient ( a [CO 2 ], 400 ppm) and e [CO 2 ] (800 ppm) growth environments, two tomato ( Solanum lycopersicum ) genotypes, Ailsa Craig (AC) and its ABA-deficient mutant ( flacca ), were grown in pots, treated with or without exogenous ABA, and exposed to progressive soil drying until all plant available water in the pot was depleted. The results showed that exogenous ABA application improved leaf water potential, osmotic potential, and leaf turgor and increased leaf ABA concentrations ([ABA] leaf ) in AC and flacca . In both genotypes, exogenous ABA application decreased stomatal pore aperture and stomatal conductance ( g s ), though these effects were less pronounced in e [CO 2 ]-grown AC and g s of ABA-treated flacca was gradually increased until a soil water threshold after which g s started to decline. In addition, ABA-treated flacca showed a partly restored stomatal drought response even when the accumulation of [ABA] leaf was vanished, implying [ABA] leaf might be not directly responsible for the decreased g s . During soil drying, [ABA] leaf remained higher in e [CO 2 ]-grown plants compared with those under a [CO 2 ], and a high xylem sap ABA concentration was also noticed in the ABA-treated flacca especially under e [CO 2 ], suggesting that e [CO 2 ] might exert an effect on ABA degradation and/or redistribution. Collectively, a fine-tune ABA homeostasis under combined e [CO 2 ] and drought stress allowed plants to optimize leaf gas exchange and plant water relations, yet more detailed research regarding ABA metabolism is still needed to fully explore the role of ABA in mediating plant physiological response to future drier and CO 2 -enriched climate., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Li and Liu.)

Thellungiella salsuginea is highly tolerant to abiotic stress, while its a close relative Arabidopsis thaliana is sensitive to stress. This characteristic makes T. salsuginea an excellent model for uncovering the mechanisms of abiotic stress tolerance. Abscisic acid (ABA) plays essential roles in plant abiotic and biotic stress tolerance. To test the changes in gene expression of T. salsuginea under ABA treatment, in this study, the transcriptomes of T. salsuginea roots and leaves were compared in response to exogenously application of ABA. The results showed that ABA treatment caused different expression of 2,200 and 3,305 genes in leaves and roots, respectively, compared with the untreated control. In particular, genes encoding transcription factors such as WRKY, MYB, NAC, GATA, ethylene-responsive factors (ERFs), heat stress transcription factors, basic helix-loop-helix, PLATZ and B3 domain-containing family members were enriched. In addition, 49 and 114 differentially expressed genes were identified as ABA-regulated genes, separately in leaves and roots, respectively, which were related to biotic and abiotic stresses. The expression levels of some genes were validated by qRT-PCR. Different responses of genes to ABA treatment were discovered in T. salsuginea and A. thaliana. This transcriptome analysis expands our understanding of the role of ABA in stress tolerance in T. salsuginea. Our study provides a wealth of information for improving stress tolerance in crop plants., (© 2019 German Society for Plant Sciences and The Royal Botanical Society of the Netherlands.)

Main Conclusion: Application of proper ABA can improve acid tolerance of rice roots by balancing endogenous hormones and promoting nutrient uptake. Abscisic acid (ABA) has an important signaling role in enhancing plant tolerance to environmental stress. To alleviate the inhibition on plant growth and productivity caused by acid rain, it is crucial to clarify the regulating mechanism of ABA on adaptation of plants to acid rain. Here, we studied the effects of exogenously applied ABA on nutrients uptake of rice roots under simulated acid rain (SAR) stress from physiological, biochemical and molecular aspects. Compared to the single SAR treatment (pH 4.5 or 3.5), exogenous 10 μM ABA alleviated the SAR-induced inhibition of root growth by balancing endogenous hormones (abscisic acid, indole-3-acetic acid, gibberellic acid and zeatin), promoting nutrient uptake (nitrate, P, K and Mg) in rice roots, and increasing the activity of the plasma membrane H + -ATPase by up-regulating expression levels of genes (OSA2, OSA4, OSA9 and OSA10). However, exogenous 100 μM ABA exacerbated the SAR-caused inhibition of root growth by disrupting the balance of endogenous hormones, and inhibiting nutrient uptake (nitrate, P, K, Ca and Mg) through decreasing the activity of the plasma membrane H + -ATPase. These results indicate that proper concentration of exogenous ABA could enhance tolerance of rice roots to SAR stress by promoting nutrients uptake and balancing endogenous hormones.