Abstract
Rhizobial NodD proteins and appropriate flavonoids induce rhizobial nodulation gene expression. In this study, we show that the nodD1 gene of Sinorhizobium fredii HH103, but not the nodD2 gene, can restore the nodulation capacity of a double nodD1/nodD2 mutant of Rhizobium tropici CIAT 899 in bean plants (Phaseolus vulgaris). S. fredii HH103 only induces pseudonodules in beans. We have also studied whether the mutation of different symbiotic regulatory genes may affect the symbiotic interaction of HH103 with beans: ttsI (the positive regulator of the symbiotic type 3 protein secretion system), and nodD2, nolR and syrM (all of them controlling the level of Nod factor production). Inactivation of either nodD2, nolR or syrM, but not that of ttsI, affected positively the symbiotic behavior of HH103 with beans, leading to the formation of colonized nodules. Acetylene reduction assays showed certain levels of nitrogenase activity that were higher in the case of the nodD2 and nolR mutants. Similar results have been previously obtained by our group with the model legume Lotus japonicus. Hence, the results obtained in the present work confirm that repression of Nod factor production, provided by either NodD2, NolR or SyrM, prevents HH103 to effectively nodulate several putative host plants.
Highlights
Rhizobia are a diverse group of α- and β-proteobacteria able to enter in symbiosis with legumes [1]
We show that the nolR, nodD2 and syrM derivatives of S. fredii HH103 are able to induce the formation of nodule primordia in Phaseolus vulgaris roots, and to establish an effective symbiotic interaction with this legume
We investigated the symbiotic properties of CIAT 899 ∆nodD1D2 carrying either the nodD1 or the nodD2 genes from S. fredii HH103 in three different host plants: P. vulgaris, L. burttii and L. japonicus (Figure 1)
Summary
Rhizobia are a diverse group of α- and β-proteobacteria able to enter in symbiosis with legumes [1]. In this relationship, rhizobia are able to infect legume roots and induce the formation of new organs called nodules. Rhizobia invade nodule cells by endocytosis and differentiate into bacteroids, which express the nitrogenase genes and, can fix N2 into ammonia [2]. The whole process is commonly referred as nodulation. This symbiosis is extremely important since it can eliminate the necessity of the application of nitrogen fertilizers, which are expensive and highly polluting in agriculture practices [3,4]. That is why understanding the signaling mechanisms that operate in this symbiosis may help to improve the yield of legume crops, which could be extremely beneficial for sustainable agriculture [4]
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