Oral Technical Session: Bacterial-Host Interactions

2-O

Ralstonia solanacearum T3SS activation in planta requires nitric oxide reductase.
B. L. DALSING (1), J. M. Jacobs (1), A. Milling (1), C. Allen (1)
(1) University of Wisconsin-Madison, Madison, WI, U.S.A.

In response to pathogen recognition, plants produce nitric oxide (NO), a key host defense signaling molecule with bactericidal properties. Some pathogens are susceptible to NO toxicity, but Ralstonia solanacearum apparently benefits from the NO in its interactions with plants. Deleting norB, which encodes the catalytic subunit of NO reductase, eliminated the pathogen’s ability to respire on NO, reduced its virulence on a susceptible host, delayed its growth in xylem, and caused a 1000-fold decrease in its competitive fitness in planta. Surprisingly, the norB mutant induced no HR in the normally incompatible tobacco host and even grew to 5x10⁷ CFU/cm² tobacco leaf tissue 24hpi, unlike the WT strain. Additionally, the norB mutant did not produce its type III secretion system (T3SS) as indicated by dramatically reduced expression of hrpB, the T3S transcriptional activator, and popA, a T3S effector. The norB mutant induced significantly lower expression of plant defense genes in tobacco, which normally launches defenses following recognition of a T3-secreted effector. Genetic analyses suggest that the norB mutant makes less NO, and microscopy with the NO-specific dye DAF-FM DA confirmed that less NO was present in norB-infused tobacco leaves. The T3S transcriptional activator HrpB contains a predicted S-nitrosylation site. We hypothesize that HrpB is activated by S-nitrosylation mediated by high NO levels in planta. Ongoing experiments are testing this hypothesis.