Maize leaf yellowing gene ZmCAAX modulates growth and drought resistance by regulating abscisic acid contents through interaction with the ABA biosynthetic enzyme ZmNCED3.

In maize (Zea mays L.), leaves are essential for photosynthesis and transpiration and leaf yellowing is regulated by carotenoid metabolism, hormonal signalling and environmental factors. However, the molecular mechanisms linking drought stress and leaf yellowing remain poorly understood. ZmNCED3, a key regulator of carotenoid degradation and drought stress responses, plays a critical role in ABA biosynthesis, but its upstream regulatory mechanisms remain unclear. This study investigates the association between leaf-yellowing mutations and drought stress response in maize. Through map-based cloning and allelism tests, we identified ZmCAAX as the causal gene underlying the yp1 mutant phenotype. ZmCAAX encodes a CAAX amino-terminal protease family protein. Overexpression of ZmCAAX increases drought sensitivity, whereas knockout mutants exhibit enhanced drought resistance. ZmCAAX physically interacts with ZmNCED3 and promotes its degradation. Under drought stress, the expression of ZmCAAX decreases, resulting in increased ZmNCED3 levels, which in turn promotes carotenoid degradation and ABA biosynthesis. Based on these findings, designing ZmCAAX gene knockouts or selecting natural variant alleles of ZmCAAX could significantly enhance drought stress resistance and carotenoid content. This genetic strategy may be applied to maize breeding to improve maize quality and drought stress resistance.