Redzich, L.; Ma, Z.; Eescobar, A. R.; Baeumers, M.; Schepler-Luu, V.; Loo, E. P. I.; Frommer, W. B.

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 the xylem parenchyma (XP) by injection of transcription activation like effectors (TALe) into host cells, likely to access host-derived sucrose. To establish infection, Xoo must overcome physical barriers, immune responses and the hydraulic xylem flow. To gain insights into the colonization process, we used translational SWEET11a-GUS reporter lines, scanning electron microscopy, and confocal laser scanning microscopy of Xoo tagged with a fluorescent protein. We found that Xoo can differentiate in vitro into filamentous forms. We mapped the infection route of Xoo along the vasculature, identified distinct spatiotemporal phases of Xoo colonization marked by rod-shaped and, notably, filamentous Xoo cells. Rod-shaped Xoo were found to attach to xylem pits during basipetal progression of the infection. Notably, we found that at later infection stages, Xoo could enter the XP. Strikingly, subsequently Xoo forms filament that can traverse bundle sheath cells and enter into the mesophyll. Chlorosis and necrosis of leaves is thus likely not just due to blockage of xylem flow, but to direct tissue damage. Filamentation had been reported as important for virulence of human pathogens e.g. Yersinia pestis, uropathogenic E. coli and Shigella and had been associated to sugar utilization in Bacillus subtilis. We thus hypothesize that Xoo differentiation during host colonization is critical for virulence.