ABA-glucose ester hydrolyzing enzyme ATBG1 and PHYB antagonistically regulate stomatal development

The performance of partially substituted composite ester materials with weathered red-bed soil in ecological restoration

Ester materials have become a significant topic in ecological restoration because of their degradability and lack of pollution. However, these artificial materials have issues such as high resource consumption and high cost. Therefore, finding a scientific substitute for ester materials is crucial to reduce costs. This study proposes the use of weathered red-bed soil to partially replace ester materials. Orthogonal coupled compounding and ecological effect tests were performed to analyze the soil improvement mechanism based on the mineral composition, soil structure, and electrical conductivity properties of the weathered red-bed soil. The experimental findings indicated that the soil modified using ester materials exhibited improved strength, water retention, and aeration owing to changes in the soil structure. Plant germination and height increased by 55% and 37 mm, respectively, when using a ratio of 15 g/m2 absorbent ester material, 2.5 g/m2 adhesive ester material, and 5% weathered red-bed soil. Through this approach, the amount of ester material to be used could be further reduced by 75%. The weathered red-bed soil offers improved ecological effects by altering the physical, mechanical, and hydraulic properties of the soil structure. This study presents a theoretical foundation for ecological conservation using weathered red-bed soil as a substitute for certain ester materials.

ABA guides stomatal proliferation and patterning through the EPF-SPCH signaling pathway in Arabidopsis thaliana

Adaptation to dehydration stress requires plants to coordinate environmental and endogenous signals to inhibit stomatal proliferation and modulate their patterning. The stress hormone abscisic acid (ABA) induces stomatal closure and restricts stomatal lineage to promote stress tolerance. Here, we report that mutants with reduced ABA levels, xer-1, xer-2 and aba2-2, developed stomatal clusters. Similarly, the ABA signaling mutant snrk2.2/2.3/2.6, which lacks core ABA signaling kinases, also displayed stomatal clusters. Exposure to ABA or inhibition of ABA catabolism rescued the increased stomatal density and spacing defects observed in xer and aba2-2, suggesting that basal ABA is required for correct stomatal density and spacing. xer-1 and aba2-2 displayed reduced expression of EPF1 and EPF2, and enhanced expression of SPCH and MUTE. Furthermore, ABA suppressed elevated SPCH and MUTE expression in epf2-1 and epf1-1, and partially rescued epf2-1 stomatal index and epf1-1 clustering defects. Genetic analysis demonstrated that XER acts upstream of the EPF2-SPCH pathway to suppress stomatal proliferation, and in parallel with EPF1 to ensure correct stomatal spacing. These results show that basal ABA and functional ABA signaling are required to fine-tune stomatal density and patterning.

Update on stomata development and action under abiotic stress

Stomata, key gatekeepers of plant hydration, have long been known to play a pivotal role in mitigating the impacts of abiotic stressors. However, the complex molecular mechanisms underscoring this role remain unresolved fully and continue to be the subject of research. In the context of water-use efficiency (WUE), a key indicator of a plant's ability to conserve water, this aspect links intrinsically with stomatal behavior. Given the pivotal role of stomata in modulating water loss, it can be argued that the complex mechanisms governing stomatal development and function will significantly influence a plant's WUE under different abiotic stress conditions. Addressing these calls for a concerted effort to strengthen plant adaptability through advanced, targeted research. In this vein, recent studies have illuminated how specific stressors trigger alterations in gene expression, orchestrating changes in stomatal pattern, structure, and opening. This reveals a complex interplay between stress stimuli and regulatory sequences of essential genes implicated in stomatal development, such as MUTE, SPCH, and FAMA. This review synthesizes current discoveries on the molecular foundations of stomatal development and behavior in various stress conditions and their implications for WUE. It highlights the imperative for continued exploration, as understanding and leveraging these mechanisms guarantee enhanced plant resilience amid an ever-changing climatic landscape.

Integration of light and ABA signaling pathways to combat drought stress in plants

Drought is one of the most critical stresses, which causes an enormous reduction in crop yield. Plants develop various strategies like drought escape, drought avoidance, and drought tolerance to cope with the reduced availability of water during drought. Plants adopt several morphological and biochemical modifications to fine-tune their water-use efficiency to alleviate drought stress. ABA accumulation and signaling plays a crucial role in the response of plants towards drought. Here, we discuss how drought-induced ABA regulates the modifications in stomatal dynamics, root system architecture, and the timing of senescence to counter drought stress. These physiological responses are also regulated by light, indicating the possibility of convergence of light- and drought-induced ABA signaling pathways. In this review, we provide an overview of investigations reporting light-ABA signaling cross talk in Arabidopsis as well as other crop species. We have also tried to describe the potential role of different light components and their respective photoreceptors and downstream factors like HY5, PIFs, BBXs, and COP1 in modulating drought stress responses. Finally, we highlight the possibilities of enhancing the plant drought resilience by fine-tuning light environment or its signaling components in the future.

Carbon Starved Anther modulates sugar and ABA metabolism to protect rice seed germination and seedling fitness

Seed germination is critical for plant survival and agricultural production, which is affected by both internal seed factors and external environmental conditions. However, the genetic basis and underlying molecular mechanisms of early seed germination in crops remain largely unclear. Here, we report that R2R3 MYB transcription factor Carbon Starved Anther (CSA) is expressed specifically in Oryza sativa embryo and aleurone in response to seed imbibition, peaking at 3-6 h and undetectable by 24-h post-imbibition. CSA seeds germinated more quickly than wild-type rice seeds and had higher levels of amylase activity, glucose, and inactive abscisic acid-glucose ester (ABA-GE), but lower levels of ABA. Through analyzing the CSA-associated transcriptome and CSA binding to downstream target genes, we identified two glycolytic genes as direct CSA targets. CSA inhibits Amylase 3A expression to limit glucose production from starch and activates Os3BGlu6 expression to promote de-conjugation of ABA-GE to ABA; these functions serve to slow germination and improve seedling resilience to abiotic stress in the first 3 weeks of growth. Therefore, this study unveils a protection mechanism conferred by CSA during early seed germination by balancing glucose and ABA metabolism to optimize seed germination and stress response fitness.

Role of Basal ABA in Plant Growth and Development

Abscisic acid (ABA) regulates various aspects of plant physiology, including promoting seed dormancy and adaptive responses to abiotic and biotic stresses. In addition, ABA plays an im-portant role in growth and development under non-stressed conditions. This review summarizes phenotypes of ABA biosynthesis and signaling mutants to clarify the roles of basal ABA in growth and development. The promotive and inhibitive actions of ABA in growth are characterized by stunted and enhanced growth of ABA-deficient and insensitive mutants, respectively. Growth regulation by ABA is both promotive and inhibitive, depending on the context, such as concentrations, tissues, and environmental conditions. Basal ABA regulates local growth including hyponastic growth, skotomorphogenesis and lateral root growth. At the cellular level, basal ABA is essential for proper chloroplast biogenesis, central metabolism, and expression of cell-cycle genes. Basal ABA also regulates epidermis development in the shoot, by inhibiting stomatal development, and deposition of hydrophobic polymers like a cuticular wax layer covering the leaf surface. In the root, basal ABA is involved in xylem differentiation and suberization of the endodermis. Hormone crosstalk plays key roles in growth and developmental processes regulated by ABA. Phenotypes of ABA-deficient and insensitive mutants indicate prominent functions of basal ABA in plant growth and development.

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

In plants, stomata control water loss and CO₂ 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 CO₂. In this study, we examine the importance of ABA catabolism in the stomatal responses to CO₂ 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 CO₂ and light. Intriguingly, our experiments suggest that endogenously produced ABA is unable to close stomata in the absence of CO₂. Furthermore, we show that when plants are grown in short day conditions ABA breakdown is required for the modulation of both elevated [CO₂]-induced stomatal closure and elevated [CO₂]-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.

N-glycosylation is involved in stomatal development by modulating the release of active abscisic acid and auxin in Arabidopsis

Asparagine-linked glycosylation (N-glycosylation) is one of the most important protein modifications in eukaryotes, affecting the folding, transport, and function of a wide range of proteins. However, little is known about the roles of N-glycosylation in the development of stomata in plants. In the present study, we provide evidence that the Arabidopsis stt3a-2 mutant, defective in oligosaccharyltransferase catalytic subunit STT3, has a greater transpirational water loss and weaker drought avoidance, accompanied by aberrant stomatal distribution. Through physiological, biochemical, and genetic analyses, we found that the abnormal stomatal density of stt3a-2 was partially attributed to low endogenous abscisic acid (ABA) and auxin (IAA) content. Exogenous application of ABA or IAA could partially rescue the mutant's salt-sensitive and abnormal stomatal phenotype. Further analyses revealed that the decrease of IAA or ABA in stt3a-2 seedlings was associated with the underglycosylation of β-glucosidase (AtBG1), catalysing the conversion of conjugated ABA/IAA to active hormone. Our results provide strong evidence that N-glycosylation is involved in stomatal development and participates in abiotic stress tolerance by modulating the release of active plant hormones.

The GIGANTEA-ENHANCED EM LEVEL Complex Enhances Drought Tolerance via Regulation of Abscisic Acid Synthesis

Drought is one of the most critical environmental stresses limiting plant growth and crop productivity. The synthesis and signaling of abscisic acid (ABA), a key phytohormone in the drought stress response, is under photoperiodic control. GIGANTEA (GI), a key regulator of photoperiod-dependent flowering and the circadian rhythm, is also involved in the signaling pathways for various abiotic stresses. In this study, we isolated ENHANCED EM LEVEL (EEL)/basic Leu zipper 12, a transcription factor involved in ABA signal responses, as a GI interactor in Arabidopsis (Arabidopsis thaliana). The diurnal expression of 9-CIS-EPOXYCAROTENOID DIOXYGENASE 3 (NCED3), a rate-limiting ABA biosynthetic enzyme, was reduced in the eel, gi-1, and eel gi-1 mutants under normal growth conditions. Chromatin immunoprecipitation and electrophoretic mobility shift assays revealed that EEL and GI bind directly to the ABA-responsive element motif in the NCED3 promoter. Furthermore, the eel, gi-1, and eel gi-1 mutants were hypersensitive to drought stress due to uncontrolled water loss. The transcript of NCED3, endogenous ABA levels, and stomatal closure were all reduced in the eel, gi-1, and eel gi-1 mutants under drought stress. Our results suggest that the EEL-GI complex positively regulates diurnal ABA synthesis by affecting the expression of NCED3, and contributes to the drought tolerance of Arabidopsis.

Hydrolysis of abscisic acid glucose ester occurs locally and quickly in response to dehydration

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Han Y, Watanabe S, Shimada H, Sakamoto A. 2020. Dynamics of the leaf endoplasmic reticulum modulate β-glucosidase-mediated stress-activated ABA production from its glucosyl ester. Journal of Experimental Botany 71, 2058–2071.

Blue Light Improves Stomatal Function and Dark-Induced Closure of Rose Leaves (Rosa x hybrida) Developed at High Air Humidity

Plants developed under constant high (>85%) relative air humidity (RH) have larger stomata that are unable to close completely in response to closing stimuli. Roses (Rosa x hybrida) developed in high RH have previously been shown to have high water loss during leaf dehydration and reduced dark-induced closure resulting in a shorter postharvest life. In this study, the effect of B-light on stomatal function under high RH conditions was investigated. The ability of rose leaves developed under continuous high (90%) or moderate (60%) RH to close their stomata in response to darkness and leaf dehydration assay was studied. Moreover, the level and regulation of ABA in light and darkness in relation to B-light was measured. Our results show that increased B-light proportion improved stomatal function and dark-induced stomatal closure under high RH conditions and that was associated with increased [ABA] in general and a dynamic ABA peak during darkness. Furthermore, increased B-light during the day was associated with the presence of high β-glucosidase activity during night. This indicates that B-light is important as a signal to activate the β-glucosidase enzyme and release ABA during night. Altogether, the improved stomatal function and reduced transpiration in combination with increased [ABA] indicate that preharvest B-light plays an important role in governing stomatal functionality and ABA homeostasis under high RH and can be a useful method to improve postharvest water balance of roses.