traG Gene Is Conserved across Mesorhizobium spp. Able to Nodulate the Same Host Plant and Expressed in Response to Root Exudates.

Evidences for an involvement of the bacterial type IV secretion system (T4SS) in the symbiotic relationship between rhizobia and legumes have been pointed out by several recent studies. However, information regarding this secretion system in Mesorhizobium is still very scarce. The aim of the present study was to investigate the phylogeny and expression of the traG gene, which encodes a substrate receptor of the T4SS. In addition, the occurrence and genomic context of this and other T4SS genes, namely, genes from tra/trb and virB/virD4 complexes, were also analyzed in order to unveil the structural and functional organization of T4SS in mesorhizobia. The location of the T4SS genes in the symbiotic region of the analyzed rhizobial genomes, along with the traG phylogeny, suggests that T4SS genes could be horizontally transferred together with the symbiosis genes. Regarding the T4SS structural organization in Mesorhizobium, the virB/virD4 genes were absent in all chickpea (Cicer arietinum L.) microsymbionts and in the Lotus symbiont Mesorhizobium japonicum MAFF303099T. Interestingly, the presence of genes belonging to another secretion system (T3SS) was restricted to these strains lacking the virB/virD4 genes. The traG gene expression was detected in M. mediterraneum Ca36T and M. ciceri LMS-1 strains when exposed to chickpea root exudates and also in the early nodules formed by M. mediterraneum Ca36T, but not in older nodules. This study contributes to a better understanding of the importance of T4SS in mutualistic symbiotic bacteria.

Mesorhizobium ciceri LMS-1 expressing an exogenous 1-aminocyclopropane-1-carboxylate (ACC) deaminase increases its nodulation abilities and chickpea plant resistance to soil constraints.

AIMS: Our goal was to understand the symbiotic behaviour of a Mesorhizobium strain expressing an exogenous 1-aminocyclopropane-1-carboxylate (ACC) deaminase, which was used as an inoculant of chickpea (Cicer arietinum) plants growing in soil. METHODS AND RESULTS: Mesorhizobium ciceri LMS-1 (pRKACC) was tested for its plant growth promotion abilities on two chickpea cultivars (ELMO and CHK3226) growing in nonsterilized soil that displayed biotic and abiotic constraints to plant growth. When compared to its wild-type form, the M. ciceri LMS-1 (pRKACC) strain showed an increased nodulation performance of c. 125 and 180% and increased nodule weight of c. 45 and 147% in chickpea cultivars ELMO and CHK3226, respectively. Mesorhizobium ciceri LMS-1 (pRKACC) was also able to augment the total biomass of both chickpea plant cultivars by c. 45% and to reduce chickpea root rot disease susceptibility. CONCLUSIONS: The results obtained indicate that the production of ACC deaminase under free living conditions by Mesorhizobium strains increases the nodulation, plant growth abilities and biocontrol potential of these strains. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first study regarding the use of a transformed rhizobial strain expressing an exogenous ACC deaminase in different plant cultivars growing in soil. Hence, obtaining Mesorhizobium strains with high ACC deaminase activity is a matter of extreme importance for the development of inoculants for field applications.

Moderately acidophilic mesorhizobia isolated from chickpea.

AIMS: Our aim was to evaluate the effect of acid and alkaline pH on chickpea rhizobia, and on chickpea-rhizobia symbiosis. METHODS AND RESULTS: Forty-seven rhizobia isolates obtained from 12 Portuguese soils were grown at pH 5, 7 and 9. Among these, 26 grew more at pH 5 than at 7, suggesting the existence of acidophiles. All isolates were identified as mesorhizobia by 16S rDNA partial sequence analysis. Molecular phylogeny of isolates based on partial 16S rDNA sequences suggests that pH tolerance might be species related. Further studies were conducted with six isolates, which were able to grow at acid pH. Isolates PT-35 and 64b grow optimally at pH 6-5-7, with a minimal pH range from 5 to 3, and may thus be considered as moderately acidophilic. Both isolates belong to a previously identified putative new Mesorhizobium species, based on 16S rDNA sequence. CONCLUSIONS: Two moderately acidophilic mesorhizobia isolated from chickpea were identified (PT-35 and 64b). A positive correlation was found between the symbiotic effectiveness at low pH and the acid tolerance of rhizobia isolates. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first report on moderately acidophilic mesorhizobia, and is an important contribution for the development of highly effective inoculants for chickpea in acid soils.