Affiliation:
1. Biology Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Abstract
ABSTRACT
Rhizobium meliloti
Rm1021 must be able to synthesize succinoglycan in order to invade successfully the nodules which it elicits on alfalfa and to establish an effective nitrogen-fixing symbiosis. Using
R. meliloti
cells that express green fluorescent protein (GFP), we have examined the nature of the symbiotic deficiency of
exo
mutants that are defective or altered in succinoglycan production. Our observations indicate that an
exoY
mutant, which does not produce succinoglycan, is symbiotically defective because it cannot initiate the formation of infection threads. An
exoZ
mutant, which produces succinoglycan without the acetyl modification, forms nitrogen-fixing nodules on plants, but it exhibits a reduced efficiency in the initiation and elongation of infection threads. An
exoH
mutant, which produces symbiotically nonfunctional high-molecular-weight succinoglycan that lacks the succinyl modification, cannot form extended infection threads. Infection threads initiate at a reduced rate and then abort before they reach the base of the root hairs. Overproduction of succinoglycan by the
exoS96
::Tn
5
mutant does not reduce the efficiency of infection thread initiation and elongation, but it does significantly reduce the ability of this mutant to colonize the curled root hairs, which is the first step of the invasion process. The
exoR95
::Tn
5
mutant, which overproduces succinoglycan to an even greater extent than the
exoS96
::Tn
5
mutant, has completely lost its ability to colonize the curled root hairs. These new observations lead us to propose that succinoglycan is required for both the initiation and elongation of infection threads during nodule invasion and that excess production of succinoglycan interferes with the ability of the rhizobia to colonize curled root hairs.
Publisher
American Society for Microbiology
Subject
Molecular Biology,Microbiology
Cited by
425 articles.
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