Of these, only SMc00135 is expressed at approximately
the same level by bacteria within the nodule and by free-living bacteria ( Additional file 4 and Additional file 5 show images of the free-living expression of GUS fusions of all the ORFs tested). Go6983 ic50 However, none of the other ORFs that are expressed in the nodule are expressed as strongly as SMc00911 (Figure 3 and Figure 4). Two of the ORFs, SMa0044 and SMb20431, are expressed at a very low level in the nodule, and no nodule expression was detected for SMc01986 and SMa1334 (Figure 4). Sma0044 has an unusual expression pattern in that it is expressed strongly by free-living bacteria (Additional file 5A), but its expression appears to be much reduced in the nodule (Figure 4N–O). Because of the strong expression of SMc00911 by bacteria in the nodule, the SMc00911 mutant strains were chosen for further study in competition experiments (see below). An insertion mutant of SMc00911 out-competes the S. meliloti 1021 wild type for nodule occupancy Many S. meliloti mutant strains that are able to form a successful symbiosis when singly inoculated on host plants are deficient in the ability to successfully compete for nodule occupancy against the wild type strain in a mixed infection [42, 51]. Competitive
nodulation experiments are likely to be a better approximation of the situation that rhizobial bacteria encounter in the soil, where they may be competing against several different rhizobial strains for host AZD6738 mouse plant invasion and nodule occupancy. The SMc00911 insertion mutant strains
were chosen for competition analysis because this ORF is strongly expressed in the nodule and these strains might be expected to be at a competitive Selleckchem AZD4547 disadvantage in the absence of the full-length SMc00911 protein. selleck products However, in contrast to expectations, the SMc00911 insertion mutant strains strongly out-compete the S. meliloti 1021 wild type strain for nodule occupancy in a mixed 1:1 infection (Table 6). Of the nodules tested from plants inoculated with a 1:1 mixture of 1021 wild type and an SMc00911 insertion mutant, all of the nodules were colonized by either the SMc00911 insertion mutant alone or by a mixture of the mutant and the wild type (Table 6). Less than 22% of the mixed-inoculum nodules were colonized by 1021 wild type alone. Also, all of the mixed nodules contained a larger proportion of SMc00911 insertion mutant bacteria than 1021 wild type bacteria (Table 6). The recovered bacteria from one of the 8 nodules that had been inoculated with the SMc00911.Xsd1 strain alone included a small number of neomycin-sensitive colonies (Table 6, line 3). This suggests that the gene disruption plasmid inserted in the SMc00911 ORF is lost by bacteria in the nodule at a very low rate. Taken together, these competition results suggest that disruption of the SMc00911 ORF actually confers a competitive advantage to S. meliloti in the symbiosis with host plants.