Abscisic acid and nitric oxide confer poplar rust resistance through host defense activation and cross-kingdom regulation of effector genes.

Leaf rust disease caused by Melampsora larici-populina severely impacts the photosynthesis and growth of Populus cathayana, causing substantial productivity losses in forest trees. Abscisic acid (ABA) and nitric oxide (NO) are known to play key roles in plant defense. However, their synergistic interplay in woody plant rust resistance remains unresolved. Here, we investigated the roles of ABA and NO in P. cathayana defense against the biotrophic rust fungus M. larici-populina using physiological, transcriptomic, and transgenic approaches. Exogenous ABA and the NO donor sodium nitroprusside synergistically enhanced rust resistance by inducing stomatal closure, modulating reactive oxygen species homeostasis, promoting lignin deposition, and increasing chitinase activity. Transcriptomic profiling revealed that ABA modulated phenylpropanoid and amino sugar/nucleotide sugar metabolism pathways, with DEGs in these pathways tightly correlating with lignin biosynthesis and chitinase activity. Among these, the chitinase gene PcCHIB1 was identified and functionally validated as a pivotal positive regulator of rust resistance, as its overexpression significantly reduced fungal biomass, whereas knockout compromised resistance. Notably, ABA exerted cross-kingdom regulatory effects on M. larici-populina, downregulating genes involved in the spliceosome pathway and two small secreted protein effector genes (MLP105684 and MLP124543) that are critical for fungal virulence. Collectively, these findings establish a bipartite defense paradigm in which ABA and NO synergistically reinforce host basal defense while ABA suppresses fungal virulence, providing a mechanistic foundation for engineering rust resistance in poplar.