Physiological and transcriptome analysis of citrus under drought stress

Background Drought stress is one of the major environmental factors limiting citrus growth and productivity. In this study, we systematically analyzed physiological responses and transcriptomic dynamics of Citrus reticulata 'Hong Mei Ren' leaves under varying drought conditions—control group (T0), mild (T1), moderate (T2), and severe (T3) drought conditions. Results Under drought stress, the levels of SOD, CAT, GSH-Px, MDA, soluble proteins and amino acids in the leaves of pomelo increased overall, while POD and soluble sugars decreased; in T2, the increments of SOD, CAT, carotenoids and soluble proteins were the largest, in T3, MDA and amino acids were the highest, and the decrease in soluble sugars was the greatest, and total chlorophyll in T2 increased while in T3 it decreased. Our results demonstrated that citrus leaves employ an integrated adaptive strategy to cope with drought stress through coordinated regulation of antioxidant defense systems, osmotic adjustment metabolism, photosynthetic pathways and plant hormone signal transduction. Gene expression analysis indicated that the plant response to drought exhibits distinct phase-dependent characteristics: under mild to moderate drought, photoprotection genes (e.g., PsbS) were upregulated, whereas genes associated with photosynthetic capacity are downregulated; core components of the abscisic acid (ABA) signaling pathway—including PYR/PYL, SnRK2, and ABF—were significantly induced and act synergistically to suppress growth-related signaling pathways (e.g., AUX1, GID1); concurrently, lipid signaling molecules (such as PLD and PIP5K) and membrane trafficking genes (e.g., Rab11) were upregulated to facilitate plasma membrane remodeling. However, under severe drought stress, the photosynthetic apparatus, hormone signaling networks were broadly suppressed. Conclusions This study provided a theoretical foundation for elucidating the drought tolerance mechanisms in citrus and identifies potential candidate genes for use in drought-resilient breeding programs.