Genome-wide Identification of Adaptive Genes for Kiwifruit Leaf Surface and Apoplast Colonization in Pseudomonas syringae pv. actinidiae

Bacterial canker of kiwifruit, caused by Pseudomonas syringae pv. actinidiae (Psa), poses a serious threat to the global kiwifruit industry. The leaf serves as a critical interface for systemic infection by Psa, comprising two contiguous yet distinct niches: the leaf surface (epiphytic habitat), which is nutrient-poor and often subjected to desiccation stress, and the apoplast (endophytic habitat), which is laden with host immune defenses. However, the comprehensive genetic basis required for Psa to successfully colonize and traverse this interface remains poorly understood. Here, we employed genome-wide transposon insertion sequencing to systematically profile the fitness genes of Psa under leaf surface and apoplastic conditions. We identified 661 growth-essential genes and further revealed distinct genetic strategies employed by Psa to adapt to these contrasting microenvironments: during leaves surface colonization, 224 key genes were primarily involved in scarce nitrogen source assimilation and osmotic homeostasis; whereas in the apoplast, 1,007 conditionally critical genes were predominantly associated with energy metabolism and oxidative stress response. Phenotypic assays validated the critical roles of multiple candidate genes (e.g., FilR, FleQ) in pathogenicity and colonization, demonstrating that Psa employs a coordinated regulation of motility, and niche-specific factors to successfully adapt to host environments. This study presents the first genome-scale mapping of the genetic adaptation landscape of Psa at a key plant–pathogen interface, providing a valuable genetic resource and theoretical foundation for elucidating its pathogenic mechanisms and developing targeted control strategies.