Getting to the root of Ralstonia invasion.

Plant diseases caused by soilborne pathogens are a major limiting factor in crop production. Bacterial wilt disease, caused by soilborne bacteria in the Ralstonia solanacearum Species Complex (Ralstonia), results in significant crop loss throughout the world. Ralstonia invades root systems and colonizes plant xylem, changing plant physiology and ultimately causing plant wilting in susceptible varieties. Elucidating how Ralstonia invades and colonizes plants is central to developing strategies for crop protection. Here we review Ralstonia pathogenesis from root detection and attachment, early root colonization, xylem invasion and subsequent wilting. We focus primarily on studies in tomato from the last 5-10 years. Recent work has identified elegant mechanisms Ralstonia uses to adapt to the plant xylem, and has discovered new genes that function in Ralstonia fitness in planta. A picture is emerging of an amazingly versatile pathogen that uses multiple strategies to make its surrounding environment more hospitable and can adapt to new environments.

Whole Root Transcriptomic Analysis Suggests a Role for Auxin Pathways in Resistance to Ralstonia solanacearum in Tomato.

The soilborne pathogen Ralstonia solanacearum is the causal agent of bacterial wilt and causes significant crop loss in the Solanaceae family. The pathogen first infects roots, which are a critical source of resistance in tomato (Solanum lycopersicum L.). Roots of both resistant and susceptible plants are colonized by the pathogen, yet rootstocks can provide significant levels of resistance. Currently, mechanisms of this 'root-mediated resistance' remain largely unknown. To identify the molecular basis of this resistance, we analyzed the genome-wide transcriptional response of roots of resistant 'Hawaii 7996' and susceptible 'West Virginia 700' (WV) tomatoes at multiple timepoints after inoculation with R. solanacearum. We found that defense pathways in roots of the resistant Hawaii 7996 are activated earlier and more strongly than roots of susceptible WV. Further, auxin signaling and transport pathways are suppressed in roots of the resistant variety. Functional analysis of an auxin transport mutant in tomato revealed a role for auxin pathways in bacterial wilt. Together, our results suggest that roots mediate resistance to R. solanacearum through genome-wide transcriptomic changes that result in strong activation of defense genes and alteration of auxin pathways.