The molecular basis of the colonization of the plant xylem by pathogenic bacteria

• Applicant: Professor Dr. Wolf-Bernd Frommer
• Subject Area: Plant Breeding and Plant Pathology ; Organismic Interactions, Chemical Ecology and Microbiomes of Plant Systems ; Cell Biology
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 567816879

Several genera of bacteria infect the water transporting xylem of plants, causing substantial yield losses across a wide range of crops. The xylem stands out as a unique habitat due to its high sap flow rates. The sap is attractive for pathogens since it contains many nutrients, however, particularly low are sources for ATP production such as sugars. This niche poses unique challenges for bacteria. They must enter hydathodes. Processes that occur there are elusive. Subsequently, they invade xylem vessels and swim against the stream; a challenge given that bacterial swim velocities are ～10x lower relative to sap flow rates. Furthermore, the flow dilutes signaling molecules, e.g. quorum sensing factors. Bacteria then anchor via adhesins on the xylem cell walls. They require well-timed production of secretion systems to manipulate host cells to access host resources for ATP production and to undermine host immunity. Limited data have been generated for certain aspects in Ralstonia and Xylella, but a systematic analysis of the molecular processes in bacteria and host is lacking. Subject of this project is the infection of rice xylem by Xanthomonas oryzae pv. oryzae (Xoo), which Xoo causes bacterial blight (BB; established as model in my lab). The primary objective of this project is to comprehensively characterize the key processes involved in bacterial colonization of both bacteria and host at cellular and subcellular resolutions (deep within rice leaves). This will be achieved through imaging approaches, systematic uncovering of gene regulatory networks, and interference with critical functions. We provide relevant preliminary data and novel tools, including in planta labeling and imaging technology, single-cell transcriptomics, combined with novel transcription factor mapping technology (MOAseq) for Xoo and host genes, and genome editing of factors essential for successful disease establishment. We intend to analyze with high spatial and temporal resolution how Xoo achieves: (i) initial entry and infection of hydathodes; (ii) planktonic migration (colonization patterns indicate movement at night or during cavitation); (iii) anchoring, T3S induction and effector injection, quorum sensing (adhesins and T3S contact host cell surfaces, despite strengthened cell walls and hydrophobic surface coating; thinned host cell walls at pits might allow T3S to dock; pits as likely sites for initial anchoring, effector injection, sucrose release, and host defense); (iv) subsequent colonization, production of extracellular polysaccharides (EPS; reduced dilution of quorum sensing factor DSF in biofilm), Xoo cell elongation; (v) host cell penetration and spread (unknown mechanisms for cell entry, cell elongation, filamentation & septation). The comprehensive analyses are expected to aid in the development of new genetic strategies to protect against BB, as well as other xylem-diseases.

• DFG Programme: Reinhart Koselleck Projects