Role of the K-Antigen subgroup of capsular polysaccharides in the early recognition process between Rhizobium meliloti and alfalfa leaves

We used a model system to investigate the induction of plant genes by bacterial surface compounds, We have infiltrated alfalfa leaves with wild-type Rhizobium meliloti strain Rm41 and mutant derivatives, which are deficient in the production of exopolysaccharides (EPS), capsular K-polysaccharides (KPS or K-Antigens), or both. We have shown that wild-type R. meliloti was able to induce transcript accumulation of genes encoding enzymes of the iso-flavonoid biosynthetic pathway: chalcone synthase (CHS), chalcone reductase (CHR), and isoflavone reductase (IFR), Kinetics of these transcript accumulations were biphasic, occurring 0.75 to 1.5 h and 6 to 30 h after treatment. The exoB derivative of Bm41 (strain Ag631), which is unable to produce either EPS I or EPS II, was still able to induce very rapid (45-min posttreatment) transcript accumulation of CNS and CHR genes. These results suggested that EPS were not involved in this induction. We then tested three R. meliloti fix-23 mutants (strains PP699, PP711, and PP671), which were deficient in KPS production as shown by PAGE and immunoblot analyses, In control to the results obtained with the mutant strains, infiltration of alfalfa leaves with purified KPS, isolated from strain AK631, led to a very rapid induction of the CHS and CHR genes. Therefore, we hypothesize that KPS may play a role in the early recognition of rhizobia by the leaf cells.