LOW-MOLECULAR-WEIGHT EPS-II OF RHIZOBIUM-MELILOTI ALLOWS NODULE INVASION IN MEDICAGO-SATIVA

Effective invasion of alfalfa by Rhizobium meliloti Rm1021 normally re quires the presence of succinoglycan, an exopolysaccharide (EPS) produ ced by the bacterium. However, Rm1021 has the ability to produce a sec ond EPS (EPS II) that can suppress the symbiotic defects of succinogly can-deficient strains. EPS II is a polymer of modified glucose-(beta-1 ,3)-galactose subunits and is produced by Rm1021 derivatives carrying either an expR101 or mucR mutation. If the ability to synthesize succi noglycan is blocked genetically, expR101 derivatives of Rm1021 are nod ulation-proficient, whereas mucR derivatives of Rm1021 are not. The di fference in nodulation proficiency between these two classes of EPS II -producing strains is due to the specific production of a low molecula r weight form of EPS II by expR101 strains. A low molecular weight EPS II fraction consisting of 15-20 EPS II disaccharide subunits efficien tly allows nodule invasion by noninfective strains when present in amo unts as low as 7 pmol per plant, suggesting that low molecular weight EPS II may act as a symbiotic signal during infection.