A stringent response-defective Bradyrhizobium diazoefficiens does not activate the type-3-secretion system, elicits early plant defense, and circumvents NH 4 NO 3 -induced inhibition of nodulation

ITURRALDE, ESTEBAN T.; KROL, ELIZAVETA; ESTEVEZ, JOSÉ M.; TORRES-TEJERIZO, GONZALO; BORASSI, CECILIA; LODEIRO, ANÍBAL R.; PÉREZ-GIMÉNEZ, JULIETA; QUELAS, JUAN IGNACIO; BECKER, ANKE

APPLIED AND ENVIRONMENTAL MICROBIOLOGY

AMER SOC MICROBIOLOGY

Lugar: Washington; Año: 2021 vol. 87

0099-2240

When subjected to nutritional stress, bacteria modify their amino acidmetabolism and cell division activities by means of the stringent response, whichis controlled by the Rsh protein in alphaproteobacteria. An important group ofalphaproteobacteria are the rhizobia, which fix atmospheric N2 in symbiosis withlegume plants. Although nutritional stress is common for rhizobia while infectinglegume roots, the stringent response has scarcely been studied in this group ofsoil bacteria. In this report, we obtained a mutant with a mutation in the rsh geneof Bradyrhizobium diazoefficiens, the N2-fixing symbiont of soybean. This mutantwas defective for type 3 secretion system induction, plant defense suppression atearly root infection, and nodulation competition. Furthermore, the mutant producedsmaller nodules, although with normal morphology, which led to lowerplant biomass production. Soybean (Glycine max) genes GmRIC1 and GmRIC2,involved in autoregulation of nodulation, were upregulated in plants inoculatedwith the mutant under the N-free condition. In addition, when plants were inoculatedin the presence of 10mM NH4NO3, the mutant produced nodules containingbacteroids, and GmRIC1 and GmRIC2 were downregulated. The rsh mutant releasedmore auxin to the culture supernatant than the wild type, which might in partexplain its symbiotic behavior in the presence of combined N. These results indicatethat B. diazoefficiens stringent response integrates into the plant defense suppressionand regulation of nodulation circuits in soybean, perhaps mediated by thetype 3 secretion system.