Akhtar Ali

Abscisic acid (ABA) is a fundamental regulator of plant development, growth, and drought adaptation, and it modulates the transition to flowering. However, the molecular mechanisms by which ABA delays flowering remain unclear. Recently, ABA INSENSITIVE 2 (ABI2), a core component of the ABA signaling pathway, was reported to positively regulate floral transition in Arabidopsis. The expression of floral repressors such as FLC and CDF1 was strongly upregulated, whereas flowering promoting genes including FT, SOC1 and CO were downregulated in the abi2-2 mutant. Moreover, ABI5 protein levels and phosphorylation status were enhanced in abi2-2, suggesting that ABI2 reduces ABI5 activity and suppresses the activation of its target genes, such as FLC. Genetic analyses revealed that ABI2 functions upstream of ABI5. In conclusion, these findings indicate that ABI2 is a major switch that fine-tunes the crosstalk between ABA signaling and floral transition in Arabidopsis.

Background: Abiotic stresses, such as drought, salinity, temperature fluctuations, waterlogging, and heavy metal contamination, have a detrimental impact on plants, leading to reduced global agricultural productivity. The accumulation of cadmium (Cd) and arsenic (As) in agricultural soil, resulting from both natural and anthropogenic activities, poses significant threats to crop production and food safety. Dehydrins, also known as Group II Late Embryogenesis Abundant (LEA) proteins, are intrinsically disordered proteins that play crucial roles in protecting cellular structures during abiotic stress conditions. These proteins are considered promising candidates for enhancing plant tolerance to environmental stresses through their membrane-stabilizing and protective functions. Methods: This study evaluated the tolerance of Arabidopsis transgenic lines expressing a bacterial dehydrin gene (BG757) to Cd and As stresses using various physiological and biochemical parameters. Results: Compared with the wild-type (WT) control, the transgenic line (35S::BG757-1/Col-0) displayed significant increases in root and shoot growth upon exposure to Cd and As. Furthermore, transgenic plants exposed to heavy metal stress exhibited higher concentrations of chlorophyll, total protein, free proline, total flavonoid, and total phenolic content compared to WT plants. Likewise, transgenic plants showed higher 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity and retained higher relative water content under stress conditions. Conclusions: Taken together, these findings suggest that bacterial dehydrins confer enhanced tolerance to heavy metal stress in transgenic Arabidopsis plants, highlighting their potential application in developing stress-resilient crops for contaminated environments.

Key message This review highlights the central role of the HOS15–PWR–HDA9 complex in integrating chromatin remodeling with plant development and stress adaptation, offering insights for improving crop resilience. Abstract Being sessile by nature, plants have signaling pathways that perceive and transmit environmental signals, inducing epigenetic modifications that are instrumental in plant development and stress responses. Acetylation and methylation are key features of chromatin remodeling and are crucial in transcriptional reprogramming for tissue-specific gene expression and silencing. Recently, different classes of chromatin remodelers have been studied in plants. Among them, the multiprotein complex HOS15–PWR–HDA9 has been well documented to regulate the acetylation status of histone 3 at the chromatin of target genes during plant development and in response to various stresses, thereby facilitating gene expression and/or repression. In this review, we discuss the critical role of the HOS15–PWR–HDA9 complex in chromatin remodeling during plant development and stress responses.

Summary: HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENE 15 (HOS15) acts as a substrate receptor of E3 ligase complex, which plays a negative role in drought stress tolerance. However, whether and how HOS15 participates in controlling important transcriptional regulators remains largely unknown. Here, we report that HOS15 physically interacts with and tightly regulates DROUGHT‐INDUCED LIKE 19 (DIL9) protein stability. Moreover, application of exogenous abscisic acid (ABA) stabilizes the interaction between DIL9 and HOS15, leading to ABA‐induced proteasomal degradation of DIL9 by HOS15. Genetic analysis revealed that DIL9 functions downstream to HOS15 and that the drought tolerance of hos15‐2 plants was impaired in dil9/hos15 double mutants. Notably, DIL9 is directly associated with the promoter regions of ABF transcription factors and facilitates their expression, which is pivotal in enhancing ABA‐dependent drought tolerance. Collectively, these findings demonstrate that HOS15 consistently degrades DIL9 under normal condition, while stress (drought/ABA) promotes the DIL9 activity for binding to the promoter regions of ABFs and positively regulates their expression in response to dehydration.

To control net sodium (Na ⁺ ) uptake, Arabidopsis plants utilize the plasma membrane (PM) Na ⁺ /H ⁺ antiporter SOS1 to achieve Na ⁺ efflux at the root and Na ⁺ loading into the xylem, and the channel-like HKT1;1 protein that mediates the reverse flux of Na ⁺ unloading off the xylem. Together, these opposing transport systems govern the partition of Na ⁺ within the plant yet they must be finely co-regulated to prevent a futile cycle of xylem loading and unloading. Here, we show that the Arabidopsis SOS3 protein acts as the molecular switch governing these Na ⁺ fluxes by favoring the recruitment of SOS1 to the PM and its subsequent activation by the SOS2/SOS3 kinase complex under salt stress, while commanding HKT1;1 protein degradation upon acute sodic stress. SOS3 achieves this role by direct and SOS2-independent binding to previously unrecognized functional domains of SOS1 and HKT1;1. These results indicate that roots first retain moderate amounts of salts to facilitate osmoregulation, yet when sodicity exceeds a set point, SOS3-dependent HKT1;1 degradation switches the balance toward Na ⁺ export out of the root. Thus, SOS3 functionally links and co-regulates the two major Na ⁺ transport systems operating in vascular plants controlling plant tolerance to salinity.

The plant hormone abscisic acid (ABA) is an important regulator of plant growth and development and plays a crucial role in both biotic and abiotic stress responses. ABA modulates flowering time, but the precise molecular mechanism remains poorly understood. Here we report that ABA INSENSITIVE 2 (ABI2) is the only phosphatase from the ABA-signaling core that positively regulates the transition to flowering in Arabidopsis. Loss-of-function abi2-2 mutant shows significantly delayed flowering both under long day and short day conditions. Expression of floral repressor genes such as FLOWERING LOCUS C (FLC) and CYCLING DOF FACTOR 1 (CDF1) was significantly up-regulated in abi2-2 plants while expression of the flowering promoting genes FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) was down-regulated. Through genetic interactions we further found that ost1-3 and abi5-1 mutations are epistatic to abi2-2, as both of them individually rescued the late flowering phenotype of abi2-2. Interestingly, phosphorylation and protein stability of ABA INSENSITIVE 5 (ABI5) were enhanced in abi2-2 plants suggesting that ABI2 dephosphorylates ABI5, thereby reducing protein stability and the capacity to induce FLC expression. Our findings uncovered the unexpected role of ABI2 in promoting flowering by inhibiting ABI5-mediated FLC expression in Arabidopsis.

The plant hormone, abscisic acid (ABA), is not only important for promoting abiotic stress responses but also plays a versatile and crucial role in plant immunity. The pathogen infection-induced dynamic accumulation of ABA mediates the degradation of non-expresser of PR genes 1 (NPR1) through the CUL3^NPR3NPR4 proteasome pathway. However, the functional significance of NPR1 degradation by other E3 ligases in response to ABA remains unclear. Here, we report that NPR1 is induced transcriptionally by ABA and that npr1-1 mutation results in ABA insensitivity during seed germination and seedling growth. Mutants lacking NPR1 downregulate the expression of ABA-responsive transcription factors ABA INSENSITIVE4 (ABI4) and ABA INSENSITIVE5 (ABI5), and that of their downstream targets EM6, RAB18, RD26, and RD29B. The npr1-1 mutation also affects the transcriptional activity of WRKY18, which activates WRKY60 in the presence of ABA. Furthermore, NPR1 directly interacts with and is degraded by HOS15, a substrate receptor for the DDB1-CUL4 ubiquitin E3 ligase complex. Collectively, our findings demonstrate that NPR1 acts as a positive regulator of ABA-responsive genes, whereas HOS15 promotes NPR1 degradation in a proteasome-dependent manner.

Among all the major environmental challenges, drought stress causes considerable damage to plant growth and agricultural productivity. Drought stress directly promotes the accumulation of abscisic acid (ABA) via the activation of genes that encode enzymes involved in ABA biosynthesis, which protect the plant against water-limiting conditions. At the same time, the expression of genes that encode ABA-hydroxylases that inactivate the newly synthesized ABA, is repressed by drought stress. These phenomena occur through epigenetic modifications via the reversible processes of histone acetylation and deacetylation, also known as chromatin remodeling, which is an important regulatory mechanism that responds to various environmental stresses. Recently, we had reported that the chromatin remodeling complex HDA9-PWR-ABI4 promotes the development of drought tolerance through the deacetylation of CYP707A1/2 genes that encode the major enzymes involved in ABA catabolism. Here, we discuss the role of HDA9 and PWR in regulating drought stress by modulating the acetylation status of the CYP707A genes.