The Alternative Sigma Factor RpoN Is Required for hrp Activity in Pseudomonas syringae pv. Maculicola and Acts at the Level of hrpL Transcription

Abstract

ABSTRACT β-Glucuronidase ( uidA ) reporter gene fusions were constructed for the hrpZ , hrpL , and hrpS genes from the phytopathogen Pseudomonas syringae pv. maculicola strain ES4326. These reporters, as well as an avrRpt2-uidA fusion, were used to measure transcriptional activity in ES4326 and a ES4326 rpoN mutant. rpoN was required for the expression of avrRpt2 , hrpZ , and hrpL in vitro in minimal media and in vivo when infiltrated into Arabidopsis thaliana leaves. In contrast, the expression of hrpS was essentially the same in wild-type and rpoN mutant strains. Constitutive expression of hrpL in an rpoN mutant restored hrpZ transcription to wild-type levels, restored the hypersensitive response when infiltrated into tobacco ( Nicotiana tobacum ), and partially restored the elicitation of virulence-related symptoms but not growth when infiltrated into Arabidopsis leaves. These data indicate that rpoN -mediated control of hrp gene expression acts at the level of hrpL and that in planta growth of P. syringae is not required for the elicitation of disease symptoms.