Symbiotic Performance of Lacking ppGpp Depends on the Host Species.

Host specificity in the root-nodule symbiosis between legumes and rhizobia is crucial for the establishment of a successful interaction and ammonia provision to the plant. The specificity is mediated by plant-bacterial signal exchange during early stages of interaction. We observed that a mutant ∆, which is deficient in initiating the bacterial stringent response, fails to nodulate (alfalfa) but successfully infects . We used biochemical, histological, transcriptomic, and imaging approaches to compare the behavior of the mutant and wild type (WT) on the two plant hosts. ∆ performed almost WT-like on , except for reduced nitrogen-fixation capacity and a disorganized positioning of bacteroids within nodule cells. In contrast, ∆ showed impaired root colonization on alfalfa and failed to infect nodule primordia. Global transcriptome analyses of ∆ cells treated with the alfalfa flavonoid luteolin and of mature nodules induced by the mutant on revealed normal gene expression but overexpression of exopolysaccharide biosynthesis genes and a slight suppression of plant defense-like reactions. Many RelA-dependent transcripts overlap with the hypo-osmolarity-related FeuP regulon or are characteristic of stress responses. Based on our findings, we suggest that RelA is not essential until the late stages of symbiosis with , in which it may be involved in processes that optimize nitrogen fixation.