The alternative sigma factor RpoN is required for hrp activity in Pseudomonas syringae pv. maculicola and acts at the level of hrpL transcription

beta-Glucuronidase (uidA) reporter gene fusions were constructed for the hr pZ hrpL, and hrpS genes from the phytopathogen Pseudomonas syringae pv. mac ulicola 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 thalia na leaves. In contrasts the expression of hrpS was essentially the same in wild-type and rpoN mutant strains. Constitutive expression of hrpL in an rp oN mutant restored hrpZ transcription to wild-type levels, restored the hyp ersensitive response when infiltrated into tobacco (Nicotiaita tobacum), an d 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.