Recently we have found, using more sensitive assays based on competition, that CFNX289 occupies only 10% of the nodules formed in P. vulgaris cv. Negro Jamapa roots when co-inoculated with CFN42. The reduced competitive ability of CFNX289 strain was partially restored (31% occupancy) upon introduction of a wild-type nolL gene. These results suggest the existence of subtle effects on the infection process, undetectable by measurements of nodulation kinetics. Transmission electron microscopy analysis of the nodules formed on P. vulgaris cv. Negro Jamapa roots revealed that a nolL::Km mutation also affects bacteroid development. Strain CFNX289 infects plant cells later than CFN42; additionally, the bacteroids formed have a reduced accumulation of PHB, and there are a larger number of bacteroids per symbiosome in cells infected by CFNX289 than in cells infected by CFN42. These alterations provoke differences in symbiotic effectiveness; P. vulgaris cv. Negro Jamapa plants inoculated with CFNX289 displayed reduced nodule dry weight and total nitrogenase activity (evaluated by ARA) at 30 days after inoculation, compared with plants inoculated with the wild type strain. The alterations found here are different from those previously described for Bradyrhizobium-Lupinus interaction. Our results indicate that the nolL gene participates not only in the structuring of the Nod factor, but also in bacteroid development.
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