<scp>CIPK9</scp> – <scp>PP2C39</scp> module modulates the trade‐off between abscisic acid‐dependent drought tolerance and plant growth in <i>Brassica napus</i> L.

Drought poses a critical threat to global agriculture, food security, and livestock sustainability. In Brassica napus L. (B. napus), a major oilseed crop, yield losses under water deficit underscore the urgent need for breeding strategies that improve water-use efficiency. Here, this study demonstrates that CIPK9 loss-of-function lines of B. napus displayed enhanced drought tolerance compared with the wild-type (WT). Yeast two-hybrid, split-luciferase complementation (LCI), and bimolecular fluorescence complementation (BiFC) assays showed that BnaCIPK9 physically interacts with PP2C39 both in vivo and in vitro. Transcript analysis and enzymatic assays further revealed that CIPK9 functioned in an abscisic acid (ABA)-dependent pathway regulating ABA biosynthesis and enhanced antioxidant capacity by promoting reactive oxygen species (ROS) scavenging. In Arabidopsis, both pp2c39 and cipk9 mutants exhibited stronger drought tolerance than WT plants. Notably, cipk9 mutants showed greater detoxification capacity, resulting in reduced ROS accumulation, higher stomatal conductance, and increased growth, whereas pp2c39 mutants triggered hyperactive stress signaling, leading to elevated ROS levels, lower stomatal conductance, and growth inhibition. These findings highlight a dual drought-response strategy that balances stress defense with cellular homeostasis to sustain growth. Specifically, PP2C39 mediates ABA-induced stress signaling, while the PP2C39-CIPK9 module mitigates its cytotoxic consequences, and their interaction links two distinct pathways to maintain equilibrium between defense and growth. This work provides mechanistic insights for breeding, suggesting that exploiting functional redundancy can reduce excessive self-regulation to improve stress resilience while avoiding modifications that overactivate stress responses and compromise plant development.