Vascular network-mediated systemic spread of Pseudomonas syringae pv. actinidiae causes the bacterial canker of kiwifruit

Pseudomonas syringae pv. actinidiae (Psa) causes destructive kiwifruit bacterial canker by invading vascular tissues across multiple plant organs. However, the cellular mechanism underlying its systemic transmission and cell-to-cell movement within these specialized vascular conduits remains unclear. In this study, a Psa-GFP strain and various microscopic techniques were used to investigate the interaction between kiwifruit and Psa. Our results reveal that Psa strategically exploits host vascular conduits for systemic movement, with the xylem vessel being the predominant avenue. In the phloem, Psa exhibits adaptive alteration in bacterial shape to traverse sieve pores, facilitating its systemic spread along sieve tubes and inducing phloem necrosis. Within the xylem, Psa breaches pit membranes to migrate between adjacent vessels. Furthermore, phloem fibers act as an initial barrier at the early stages of infection, delaying Psa's entry into vascular tissues during its journey to the xylem. Additionally, at the junctions of stem–stem or stem-leaf, branch trace or leaf trace mediates the bacterial organ-to-organ translocation, thus facilitating the systemic progression of disease. In conclusion, our findings shed light on the cellular mechanism employed by Psa to exploit the woody plant's vascular network for infection, thereby enhancing a better understanding of the biology of this poorly defined bacterium. These insights carry implications for the pathogenesis of Psa and other vascular pathogens, offering theoretical guidance for effective control strategies.