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.

The aims of this study were to isolate, identify and characterize culturable endophytic bacteria from chickpea (Cicer arietinum L.) roots grown in different soils. In addition, the effects of rhizobial inoculation, soil and stress on the functionality of those culturable endophytic bacterial communities were also investigated. Phylogenetic analysis based on partial 16S rRNA gene sequences revealed that the endophytic bacteria isolated in this work belong to the phyla Proteobacteria, Firmicutes and Actinobacteria, with Enterobacter and Pseudomonas being the most frequently observed genera. Production of indoleacetic acid and ammonia were the most widespread plant growth-promoting features, while antifungal activity was relatively rare among the isolates. Despite the fact that the majority of bacterial endophytes were salt- and Mn-tolerant, the isolates obtained from soil with Mn toxicity were generally more Mn-tolerant than those obtained from the same soil amended with dolomitic limestone. Several associations between an isolate’s genus and specific plant growth-promoting mechanisms were observed. The data suggest that soil strongly impacts the Mn tolerance of endophytic bacterial communities present in chickpea roots while rhizobial inoculation induces significant changes in terms of isolates’ plant growth-promoting abilities. In addition, this study also revealed chickpea-associated endophytic bacteria that could be exploited as sources with potential application in agriculture.

Symbiotic rhizobia-legumes associations are extremely important in terms of sustainable agricultural practices. This symbiosis involves a complex interaction between both partners, plant and bacterium, for bacterial infection and the formation of symbiotic N-fixing nodules. In this regard, the phytohormone ethylene plays a significant role in nodule formation, acting as an inhibitor of the nodulation process. Ethylene not only regulates nodule development but also regulates many other plant developmental cues, including various stress responses that inhibit overall plant growth. Some rhizobia produce the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, thus, being able to decrease ACC and, consequently, decrease deleterious ethylene levels that affect the nodulation process. This occurs because ACC is the immediate precursor of ethylene in all higher plants. Hence, rhizobia that express this enzyme have an increased symbiotic potential. In addition to the direct role that ACC deaminase plays in the nodulation processper se, in a limited number of instances, ACC deaminase can also modulate nodule persistence. This review focuses on the important role of rhizobial ACC deaminase during the nodulation process, emphasizing its significance to legume growth promotion.