Rhizobia that nodulate lupins do so efficiently if they have a secretory nanosyringe

In this work we have studied the importance of a bacterial protein secretion system called T6SS presented by a rhizobium capable of nodulating lupins. T6SSs are similar to a nanosyringe and can inject proteins called effectors into other bacteria and eukaryotic cells. To gain insight into the importance of the T6SS of our rhizobia, a mutation that prevents the formation of the nanosyringe was generated and it was observed that the mutant strain induced fewer nodules and smaller plants than when the wild type strain was used. Analysis of nodule structure revealed that the level of colonization was significantly reduced in the mutants relative to the wild type. Furthermore when plants were co-inoculated with the two strains together it was found that the mutant strain was less competitive.

The genes coding for the T6SS of our rhizobia are organized in a 26 kb DNA region into two divergent gene clusters of nine genes each. One of these genes encodes a protein (Tsb1) of unknown function but containing a methyltransferase domain. A mutant in this tsb1 gene showed an intermediate symbiotic phenotype relative to the nanosyringe mutant.

Studies of the T6SS genes indicate expression in nodules but not in free-living cells grown in different media and conditions.

Our main interest now is to know what this T6SS secretes!