Rm41 induced root hair curling and nodule primordium formation but failed to infect the roots of F83005.5 S. meliloti Rm41 is a wild-type strain originally isolated from alfalfa nodules in Hungary [37]. Specifically, this strain contains a strain-specific K antigen (also known as capsular polysaccharides or KPS) which is able to com pensate for the lack of exopolysaccharides (EPS) produc- tion that is generally required for successful invasion o indeterminate nodules on the alfalfa roots [12,38,39] The Rm41 genome has recently been sequenced, consists ing of a 3.68-Mb chromosome, two symbiotic plasmids (1.56-Mb pSymA and 1.66-Mb pSymB), and a 246-kb nonsymbiotic plasmid pRme4la I401 noteworthy that, similar to many other sequenced S meliloti strains, the genome of Rm41 does not possess genes encoding a type III secretion system (T3ss) that delivers effector pro- teins into the host cell [40] To examine the infection process, we used an Rm41 strain that constitutively expresses the green fluorescent