Synergistic Enhancement of Soybean Resistance to Cyst Nematodes via Chemical Modulation of SA-GA Crosstalk

Soybean cyst nematode (SCN, <i>Heterodera glycines</i>) causes major yield losses, and <i>Rhg1</i> locus-mediated genetic resistance is becoming less effective. Hormonal signaling pathways, particularly salicylic acid (SA) and gibberellic acid (GA), are increasingly implicated in nematode resistance, but their use in plant protection remains underexplored. Here, we tested exogenous application of chemical modulators of these pathways for effects on SCN resistance and dependence on the presence or absence of <i>Rhg1-b</i>. We found that foliar application of the SA mimic acibenzolar-S-methyl (ASM) or the systemic acquired resistance mediator pipecolic acid (Pip) conferred strong, genotype-independent resistance, comparable to genetic resistance at <i>Rhg1</i>. Combining ASM with the GA biosynthesis inhibitor paclobutrazol (PBZ) synergistically enhanced protection, reducing cyst formation by 39.2% and effectively making plants moderately resistant to SCN. These effects were observed in the susceptible cultivar Williams 82, as well as across near-isogenic lines with varying <i>Rhg1</i> copy numbers. ASM alone boosted resistance in resistant <i>Rhg1-b</i> and susceptible <i>Rhg1-c</i> backgrounds, while PBZ provided additional benefits only in <i>Rhg1-c</i>, revealing genotype-specific interactions between hormonal signaling and host resistance. Targeted modulation of SA and GA pathways thus provides an effective and sustainable strategy to suppress SCN, complementing and extending the durability of genetic resistance.