Sword in the woods:How plant hosts defend against vascular pathogens

"We shall fight on the beaches, we shall fight on thelanding grounds, we shall fight in the fields and in the streets,we shall fight in the hills". Winston Churchill's famous speech inspiring the will to fight is also applicable to describe the operational mode of the plant defense system, since the foes, microbial pathogens, are not restricted to the initial in-vading sites of plant (e.g., wounds, hydathodes and stomata) and, instead, they may spread and colonize the whole plant in a successful infection. Pathogens also attempt to escape or overcome the host plant immune responses;therefore, unlike pathogens specifically infecting mesophyll cells, a large number of bacterial pathogens evolved to colonize the plant vascular system as their niche, so called"vascular patho-gens". The plant vascular system has two fundamental functions:transporting water and minerals from the roots to the leaves by xylem, and transporting sugars from the leaves to the non-photosynthetic parts of the plant by phloem. Since the xylem consists of dead tracheary elements with a rela-tively low osmotic pressure and the phloem is composed of living cells with a high osmotic pressure, most vascular pathogens prefer to colonize the nutrient-poor xylem vessels (Yadeta and Thomma, 2013). Most Xanthomonas bacteria are plant vascular pathogens, including X. campestris pv. cam-pestris (causative agent of black rot disease of crucifers), X. citri pv. citri (causing citrus canker) and X. oryzae pv. or-yzae (causing bacterial leaf blight of rice (Oryza sativa)). These pathogens proliferate in the xylem and disrupt the normal water and nutrient supply of plants, often leading to severe wilting and death of the entire plant (Kashyap et al., 2021). In contrast, X. oryzae pv. oryzicola is a nonvascular pathogen, which infects mesophyll cells and causes bacterial leaf streak disease on rice. While knowledge on immune responses in mesophyll tissues has extensively accumulated in the last three decades, how plant immunity is structured in the xylem to defend against vascular pathogens is largely unclear.