The non-flavonoid inducible nodA3 and the flavonoid regulated nodA1 genes of Rhizobium tropici CIAT 899 guarantee nod factor production and nodulation of different host legumes

Aims The symbiosis between rhizobia and their host legumes is initiated by a complex molecular dialogue in which the activation of bacterial NodD proteins by appropriate plant flavonoids triggers the expression of the bacterial nodulation (nod) genes. These genes are involved in the synthesis and export of the Nodulation factors (NF), which are signal molecules that, when recognized by plant receptors, launch the symbiotic process. The core of NF is synthesized by proteins encoded by the nodABC genes, whereas the rest of the nod genes protein products are involved in the decoration of NF with different chemical substituents and their export to the environment. Rhizobium tropici CIAT 899, the microsymbiont of common bean (Phaseolus vulgaris), is characterized for tolerating multiple physical stresses and for synthesizing a large variety of NF not only in the presence of inducing flavonoids but also when high concentrations of salt are present. In addition, another interesting feature of the R. tropici CIAT 899 genome is the presence of three different nodA genes on the symbiotic plasmid, although their exact roles remain to be elucidated. In this work, we characterize the role of the three NodA proteins of R. tropici CIAT 899 in symbiosis.