SUCCINOGLYCAN IS REQUIRED FOR INITIATION AND ELONGATION OF INFECTION THREADS DURING NODULATION OF ALFALFA BY RHIZOBIUM-MELILOTI

Rhizobium meliloti Rm1021 must be able to synthesize succinoglycan in order to invade successfully the nodules which it elicits on alfalfa a nd to establish an effective nitrogen-fixing symbiosis, Using R. rneli loti cells that express green fluorescent protein (GFP), we have exami ned the nature of the symbiotic deficiency of exo mutants that are def ective or altered in succinoglycan production. Our observations indica te that an exoY mutant, which does not produce succinoglycan, is symbi otically defective because it cannot initiate the formation of infecti on threads. An exoZ mutant, which produces succinoglycan without the a cetyl modification, forms nitrogen-fixing nodules on plants, but it ex hibits a reduced efficiency in the initiation and elongation of infect ion threads. An exoH mutant, which produces symbiotically nonfunctiona l high-molecular-weight succinoglycan that lacks the succinyl modifica tion, cannot form extended infection threads. Infection threads initia te at a reduced rate and then abort before they reach the base of the root hairs. Overproduction of succinoglycan by the exoS96::Tn5 mutant does not reduce the efficiency of infection thread initiation and elon gation, but it does significantly reduce the ability of this mutant to colonize the curled root hairs, which is the first step of the invasi on process. The exoR95::Tn5 mutant, which overproduces succinoglycan t o an even greater extent than the exoS96::Tn5 mutant, has completely l ost its ability to colonize the curled root hairs. These new observati ons lead us to propose that succinoglycan is required for both the ini tiation and elongation of infection threads during nodule invasion and that excess production of succinoglycan interferes with the ability o f the rhizobia to colonize curled root hairs.