Hydroxycinnamic Acid Degradation, a Broadly Conserved Trait, Protects Ralstonia solanacearum from Chemical Plant Defenses and Contributes to Root Colonization and Virulence

Plants produce hydroxycinnami c acid (HCA) defense com- pounds to combat pathogens, such as the bacterium Ral- stonia solanacearum . We showed that an HCA degradation pathway is genetically and functionally conserved across diverse R. solanacearum strains. Further, a feruloyl-CoA synthetase ( Δ fcs ) mutant that canno t degrade HCA was less virulent on tomato plants. To understand the role of HCA degradation in bacterial wilt disease, we tested the following hypotheses: HCA degradation helps the patho- gen i) grow, as a carbon source; ii) spread, by reducing HCA-derived physical barriers; and iii) survive plant anti- microbial compounds. Although HCA degradation enabled R. solanacearum growth on HCA in vitro, HCA degrada- tion was dispensable for growth in xylem sap and root exu- date, suggesting that HCA are not significant carbon sources in planta. Acetyl-bromide quantification of lignin demonstrated that R. solanacearum infections did not affect the gross quantity or distribution of stem lignin. However, the Δ fcs mutant was significantly more suscepti- ble to inhibition by two HCA, namely, caffeate and p -couma- rate. Finally, plant colonization assays suggested that HCA degradation facilitates early stages of infection and root colonization. Together, these results indicated that ability to degrade HCA contributes to bacterial wilt virulence by facilitating root entry and by protecting the pathogen from HCA toxicity.