Integrative physiological and transcriptomic analysis reveals drought response mechanisms in Abrus mollis Hance

BACKGROUND: Drought stress represents a major environmental constraint that severely limits plant growth and agricultural productivity worldwide. Abrus mollis Hance is a valuable medicinal and edible legume in China, yet its drought adaptation mechanisms remain poorly understood, particularly regarding the integrated regulation of chlorophyll and flavonoid metabolism. This study employed an integrative physiological and transcriptomic approach to elucidate the drought response strategies of A. mollis under progressive drought and rehydration. RESULTS: Under progressive drought, soil water content declined to 16.1% (moderate stress) and 7.0% (severe stress), while leaf relative water content decreased by 58.2% and 76.5%, respectively. These changes were accompanied by a sharp reduction in stomatal conductance (over 85.6%) and net photosynthetic rate (declining by 85.2%). Transcriptome analysis identified 9,038 differentially expressed genes under severe drought, enriched in pathways related to photosynthesis, chlorophyll metabolism, plant hormone signal transduction, and phenylpropanoid/flavonoid biosynthesis. The ABA signalling pathway and NAC transcription factors (TFs), particularly NAC002, were central to coordinating chlorophyll degradation, with chlorophyll content reduced by 42.5% under severe drought. Furthermore, total flavonoid content increased by up to 43.0% under moderate drought, with co‑expression network analysis highlighting CIPK4, DOF5, and ABR17-like as key candidate regulators of drought‑induced flavonoid biosynthesis. CONCLUSIONS: This study demonstrates that A. mollis combats drought via integrated physiological and transcriptional adaptations. Photosynthetic inhibition resulted from stomatal and non-stomatal limitations, with the latter involving ABA- and NAC-mediated repression of chlorophyll biosynthesis and induction of degradation. Drought also promoted the accumulation of protective flavonoids, potentially regulated by key candidate gene identified through co‑expression analysis. These findings elucidate stress-responsive networks in this non-model legume and provide candidate targets for improving drought tolerance.