A Transcriptional Regulator of the LuxR-UhpA Family, FlcA, Controls Flocculation and Wheat Root Surface Colonization by 'Azospirillum brasilense' Sp7

Genetic complementation of a spontaneous mutant, impaired in flocculation, Congo red binding, and colonization of root surface, led to the identification of a new regulatory gene in 'Azospirillum brasilense' Sp7, designated 'flcA'. The deduced amino acid sequence of 'flcA' shared high similarity with a family of transcriptional activators of the LuxR-UhpA family. The most significant match was with the AgmR protein, an activator for glycerol metabolism in 'Pseudomonas aeruginosa'. Derivatives of Sp7 resulting from site-directed Tn5 mutagenesis in the 'flcA' coding sequence were constructed by marker exchange. Characterization of the resulting mutant strains showed that 'flcA' controls the production of capsular polysaccharides, the flocculation process in culture, and the colonization of the root surface of wheat. This study provides new information on the genetic control of the mechanism of plant root colonization by 'Azospirillum'.