Rhizobial Factors Required for Stem Nodule Maturation and Maintenance in Sesbania rostrata-Azorhizobium caulinodans ORS571 Symbiosis

Abstract

ABSTRACT The molecular and physiological mechanisms behind the maturation and maintenance of N 2 -fixing nodules during development of symbiosis between rhizobia and legumes still remain unclear, although the early events of symbiosis are relatively well understood. Azorhizobium caulinodans ORS571 is a microsymbiont of the tropical legume Sesbania rostrata , forming N 2 -fixing nodules not only on the roots but also on the stems. In this study, 10,080 transposon-inserted mutants of A. caulinodans ORS571 were individually inoculated onto the stems of S. rostrata , and those mutants that induced ineffective stem nodules, as displayed by halted development at various stages, were selected. From repeated observations on stem nodulation, 108 Tn 5 mutants were selected and categorized into seven nodulation types based on size and N 2 fixation activity. Tn 5 insertions of some mutants were found in the well-known nodulation, nitrogen fixation, and symbiosis-related genes, such as nod, nif , and fix , respectively, lipopolysaccharide synthesis-related genes, C 4 metabolism-related genes, and so on. However, other genes have not been reported to have roles in legume-rhizobium symbiosis. The list of newly identified symbiosis-related genes will present clues to aid in understanding the maturation and maintenance mechanisms of nodules.