Role and Regulation of ACC Deaminase Gene in Sinorhizobium meliloti: Is It a Symbiotic, Rhizospheric or Endophytic Gene?

Abstract

Plant-associated bacteria exhibit a number of different strategies and specific genes allow bacteria to communicate and metabolically interact with plant tissues. Among the genes found in the genomes of plant-associated bacteria, the gene encoding the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase (acdS) is one of the most diffused. This gene is supposed to be involved in the cleaving of plant-produced ACC, the precursor of the plant stress-hormone ethylene toning down the plant response to infection. However, few reports are present on the actual role in rhizobia, one of the most investigated groups of plant-associated bacteria. In particular, still unclear is the origin and the role of acdS in symbiotic competitiveness and on the selective benefit it may confer to plant symbiotic rhizobia. Here we present a phylogenetic and functional analysis of acdS orthologs in the rhizobium model-species Sinorhizobium meliloti. Results showed that acdS orthologs present in S. meliloti pangenome have polyphyletic origin and likely spread through horizontal gene transfer, mediated by mobile genetic elements. When acdS ortholog from AK83 strain was cloned and assayed in S. meliloti 1021 (lacking acdS), no modulation of plant ethylene levels was detected, as well as no increase in fitness for nodule occupancy was found in the acdS-derivative strain compared to the parental one. Surprisingly, AcdS was shown to confer the ability to utilize formamide and some dipeptides as sole nitrogen source. Finally, acdS was shown to be negatively regulated by a putative leucine-responsive regulator (LrpL) located upstream to acdS sequence (acdR). acdS expression was induced by root exudates of both legumes and non-leguminous plants. We conclude that acdS in S. meliloti is not directly related to symbiotic interaction, but it could likely be involved in the rhizospheric colonization or in the endophytic behavior.

Symbiotic and non-symbiotic members of the genusEnsifer(syn.Sinorhizobium) are separated into two clades based on comparative genomics and high-throughput phenotyping

“…To evaluate bacterial growth when provided root exudates as a nitrogen source, root exudates were produced from Medicago sativa cv. Maraviglia as described elsewhere (Checcucci et al 2017). Single bacterial colonies from TY plates were resuspended in a 0.9% NaCl solution to an OD 600 of 0.5.…”

Deciphering the Symbiotic Plant Microbiome: Translating the Most Recent Discoveries on Rhizobia for the Improvement of Agricultural Practices in Metal-Contaminated and High Saline Lands

“…Recent works have highlighted that microbial species associated with plants (rhizobial and non-rhizobial strains, including mycorrhizal fungi) can positively influence plant tolerance to water deficiency [36]. This is due to their PGP features such as indoleacetic acid, siderophore production, phosphate and zinc solubilization [37], and the synthesis of 1-aminocyclopropane-1-carboxylate (ACC) deaminase [38], which are more evident and easily identified in stressful conditions [39]. Nevertheless, legumes are strongly affected by water deficit.…”

Optimization of Indole-3-Acetic Acid Production by Diazotrophic B. subtilis DR2 (KP455653), Isolated from Rhizosphere of Eragrostis cynosuroides

“…6) ACC deaminase activity DR2 was found to be positive for the production of 1aminocyclopropane-1-carboxylate deaminase (ACCD) and utilized the 1-aminocyclopropane-1-carboxylate (ACC) as a sole source of N in minimal medium (Table I). 2-5% of rhizobacteria are PGPR, which solubilise phosphate, zinc and alleviate the various plant stresses by secreting ACC, thereby increasing plant growth, biomass and yield [30]. 7) Antifungal activities DR2 was able to inhibit the growth of Aspergillus sp.…”