Disturbed gene expression and bacteroid development in Trifolium pratense root nodules induced by a Tn5 mutant of Rhizobium leguminosarum bv. trifolii defective in exopolysaccharide synthesis

Abstract

A newly isolated mutant of the symbiotic bacterium Rhizobium leguminosarum bv. trifolii, Rt79, is deficient in exopolysaccharide synthesis. It induced nodules defective in nitrogen fixation on clover (Trifolium pratense L.). The expression of plant (ENOD2, leghemoglobin) and bacterial (ropA, nifH) genes was studied by in-situ hybridization. The absence of nifH transcripts was noticed. Mutant-induced nodules showed changes in nodule development, such as precocious degeneration of bacteroids and deviating bacteroid ultrastructure, from the moment that bacteria were released from infection threads, and reduction of the number of infected plant cells. Abnormal bacteroids still invaded plant cells but in the interzone between the pre-fixation zone and the nitrogen fixing zone the maturation process required for nitrogen fixation was eventually blocked. These observations as well as the gene-expression data point to a failure in bacteroid development from the onset of the infection process rather than to a sudden block in the infection process.