The stringent response triggers the expression of the type III secretion system in Bradyrhizobium diazoefficiens

COLLA, DELFINA; CASTELLANI, LUCAS G.; QUELAS, J. I.; MONGIARDINI, E.J.; TORRES TEJERIZO, GONZALO A.; PÉREZ GIMÉNEZ, JULIETA

Napoles

Conferencia; 15th European Nitrogen Fixation Conference; 2023

Bradyrhizobium diazoefficiens is a soil bacterium that stablish a symbiosis with soybean plants. Soybean flavonoids induce the expression of the Type 3 Secretion System (T3SS) that injects effector proteins known as Nodulations Outer Proteins (Nops). These Nops are involved in the suppression of the host's defenses and host range expansion [1,2]. In B. diazoefficiens, the T3SS is activated by the flavonoid genistein through the transcriptional regulator TtsI, which also controls the expression of the Nops [3]. In some pathogens, the T3SS induction is affected by the second messenger (p)ppGpp. In response to starvation, bacteria trigger the stringent response (SR) and (p)ppGpp is synthesized. A rsh mutant of B. diazoefficiens, unable to synthesize (p)ppGpp, expresses less ttsI than the wild type strain in presence of genistein. As consequence, this mutant is not able to modulate the plant defense response, so its symbiotic performance is affected [4]. We hypothesize that B. diazoefficiens USDA 110 under C and N starvation, increases (p)ppGpp levels [4] and prepares rhizobia for symbiotic interaction through the regulation of the T3SS, stimulating the symbiotic interaction. We measured the transcript levels of ttsI by qRT-PCR in wild type and rsh mutant strains, in cultures with genistein, without induction and treated 1 or 24 hours with a C/N free mineral solution. We found that ttsI is overexpressed in nutritional stress condition than with genistein treatment in the wild type strain. In contrast, the ttsI expression in the rsh mutant was not affected in any condition. Other preliminary results from the T3SS’s signalling cascade have shown that (p)ppGpp levels affect genes under ttsI regulation.In order to find determinants linking SR and T3SS, we performed comparative proteomics. The analysis of the different metabolic conditions showed that the effects of stress and T3SS induction were achieved.Furthermore, the host range was unaffected since the rsh mutant was capable of establish a symbiotic interaction with both soybean and mung bean plants.