Differentiation of Xanthomonas oryzae pv. oryzae in vitro and during rice leaf infection

Xanthomonas oryzae pv. oryzae (Xoo) is classified as a xylem pathogen responsible for bacterial blight of rice causing substantial yield losses in Asia and Africa. Xoo virulence depends on the ability to trigger SWEET sucrose efflux transporters in xylem parenchyma cells (XP) by injection of transcription activation like effectors (TALe) into host cells, possibly to access host-derived sucrose. To establish infection, Xoo must overcome physical barriers, immune responses and the hydraulic xylem flow. Here, we found that Xoo forms filamentous cells in vitro and during infection of rice leaves. In planta, rod-shaped cells predominated at the basipetal infection front and were frequently associated with xylem vessel pits, whereas filamentous Xoo cells were mainly observed at later infection stages and in tissues adjacent to and beyond the vascular bundle. Infection route mapping further revealed a discontinuous, basipetal colonization pattern along the vasculature. Together, these findings show that Xoo exhibits stage- and tissue-associated morphological variation during host colonization and is not strictly confined to xylem vessels at later stages of infection. Morphological plasticity may therefore contribute to bacterial spread within the host tissue. Xoo forms filamentous cells that are transient and generate pleomorphic progeny in vitro. In planta, rod-shaped Xoo cells are predominantly observed at xylem vessel pits at the infection front. Reporter-based mapping revealed a basipetal infection pattern progressing from major to minor veins via transverse veins. At later infection stages, Xoo cells were detected outside xylem vessels in xylem parenchyma, bundle sheath, and mesophyll tissues. Filamentous Xoo cells were frequently observed in tissues adjacent to xylem vessels and beyond the vascular bundle. Xoo exhibited distinct morphological forms at different stages and in different tissues during infection. Overall, our data show that Xoo undergoes stage- and tissue-associated morphological transitions during rice infection, including reversible filamentation and pleomorphic population re-establishment.