Evaluation of the interspecific competition within Agrobacterium spp. in the soil and rhizosphere of tomato and maize

Abstract

Agrobacterium biovar 1 is responsible for crown gall disease of many plant species. However, according to the current species definition, this taxon displays too large a genomic divergence to be a single species. This is why it is considered as a complex of distinct genomic species. Members of this taxon are common inhabitants of soils and rhizospheres, with several species and strains commonly found in the same samples. Understanding factors that shape their populations is very important for ecological and epidemiological issues. In this regard, this study was aimed at assessing whether plant genotypes differentially select agrobacteria genomospecies in their root systems. To test this hypothesis, we followed the evolution of tagged Agrobacterium fabrum C58 strains co-inoculated with members of different genomospecies (G1, G4, G7 and G8), in the soil and the rhizosphere of tomato and maize seedling plants under controlled conditions. The total number of the agrobacterial population was always statistically higher in rhizospheres than in bulk soils, confirming that agrobacteria are rhizospheric bacteria. The plasmid, pTi, was found not to be necessary for either rhizosphere colonization or the multiplication of the C58 strain. The difference between proportions of co-introduced bacteria from different genomic species shows the presence of a competition between them, which can vary with the plant species. While there were no significant differences between the proportion of co-inoculated strains of G8 (J07 and C58Gmr) in either plant species, they were more abundant in tomato than maize rhizosphere.