Histocytological analysis reveals the biocontrol activity of a rhizospheric bacterium Pseudomonas rhizophila Z98 against kiwifruit bacterial canker

Abstract

Kiwifruit bacterial canker (KBC), caused by Pseudomonas syringae pv. actinidiae (Psa), poses a significant threat to the global kiwifruit industry. Currently, there is a scarcity of highly efficient biocontrol agents for the prevention and control of KBC, which limits the comprehensive management of the disease. This study investigates the biocontrol potential of P. rhizophila Z98, isolated from kiwifruit rhizosphere, which exhibits significant inhibitory effects on Psa. The in vitro leaf disc and vein assays demonstrated Z98's potent preventive effect, achieving a 98.89 % reduction in KBC and its ability to limit Psa's vascular spread. Microscopic analysis showed that Psa cells exposed to Z98 underwent significant morphological changes, including cell wall depressions, wrinkling, tumorous protrusions, and intracellular disruptions like cytoplasmic disintegration and vacuolization, culminating in cell death. These effects were were mirrored with Z98's fermentation broth crude extract, suggesting that Z98 combats Psa through the secretion of bioactive substances. Additionally, Z98 successfully colonizes kiwifruit tissues, achieving a biomass of 3.78 × 10⁵ CFU·g⁻¹ without compromising tissue integrity. Moreover, Z98 induces the upregulation of defense-related genes and callose deposition in kiwifruit, thereby activating plant immune responses. These findings elucidate the cellular mechanisms underlying the biocontrol effects of rhizosphere bacteria and offer a novel biological resource for managing bacterial canker in woody plants.