Two Homologues of the Global Regulator Csr/Rsm Redundantly Control Phaseolotoxin Biosynthesis and Virulence in the Plant Pathogen pv. phaseolicola 1448A.

The widely conserved Csr/Rsm (carbon storage regulator/repressor of stationary-phase metabolites) post-transcriptional regulatory system controls diverse phenotypes involved in bacterial pathogenicity and virulence. Here we show that pv. phaseolicola 1448A contains seven genes, four of which are chromosomal. In RNAseq analyses, only was thermoregulated, with increased expression at 18 °C, whereas the antagonistic sRNAs , , and showed increased levels at 28 °C. Only double - mutants showed significantly altered phenotypes in functional analyses, being impaired for symptom elicitation in bean, including in planta growth, and for induction of the hypersensitive response in tobacco. Double mutants were also non-motile and were compromised for the utilization of different carbon sources. These phenotypes were accompanied by reduced mRNA levels of the type III secretion system regulatory genes and , and the flagellin gene, . Biosynthesis of the phytotoxin phaseolotoxin by mutants in and was delayed, occurring only in older cultures, indicating that these homologues act as inductors of toxin synthesis. Therefore, genes and act redundantly, although with a degree of specialization, to positively regulate diverse phenotypes involved in niche colonization. Additionally, our results suggest the existence of a regulatory molecule different from the Rsm proteins and dependent on the GacS/GacA (global activator of antibiotic and cyanide production) system, which causes the repression of phaseolotoxin biosynthesis at high temperatures.