Aims The main goal of the study reported herein was to assess the nodulation performance of a Mesorhizobium strain transformed with an exogenous ACC deaminase gene (acdS), and its subsequent ability to increase chickpea plant growth under normal and waterlogged conditions. Methods The Mesorhizobium ciceri strain LMS-1 was transformed with the acdS gene of Pseudomonas putida UW4 by triparental conjugation using plasmid pRKACC. A plant growth assay was conducted to verify the plant growth promotion ability of the LMS-1 (pRKACC) transformed strain under normal and waterlogging conditions. Bacterial ACC deaminase and nitrogenase activity was measured.Results By expressing the exogenous acdS gene, the transformed strain LMS-1 showed a 127% increased ability to nodulate chickpea and a 125% promotion of the growth of chickpea compared to the wild-type strain, under normal conditions. Plants inoculated with the LMS-1 wild-type strain showed a higher nodule number under waterlogging stress than under control conditions, suggesting that waterlogging increases nodulation in chickpea. No significant relationship was found between ACC deaminase and nitrogenase activity. Conclusions The results obtained in this study show that the use of rhizobial strains with improved ACC deaminase activity might be very important for developing microbial inocula for agricultural purposes.

As functional diversity influences the benefits conferred on host plants by arbuscular mycorrhizal fungi (AMF) and large scale commercial inoculation is currently impracticable, strategies are required to manage communities of indigenous AMF associated with different hosts within agricultural cropping systems. In a non-sterilized soil, using 454 pyrosequencing of the LSU-D2 rDNA gene, host plant AMF diversity was assessed following successions of different plant species, grown with or without prior soil disturbance. Diversity present in the

Several Mesorhizobium species are able to induce effective nodules in chickpea, one of the most important legumes worldwide. Our aims were to examine the biogeography of chickpea rhizobia, to search for a predominant species, and to identify the most efficient microsymbiont, considering Portugal as a case study. One hundred and ten isolates were obtained from continental Portugal and Madeira Island. The 16S ribosomal RNA gene phylogeny revealed that isolates are highly diverse, grouping with most Mesorhizobium type strains, in four main clusters (A-D). Interestingly, only 33% of the isolates grouped with Mesorhizobium ciceri (cluster B) or Mesorhizobium mediterraneum (cluster D), the formerly described specific chickpea microsymbionts. Most isolates belong to cluster A, showing higher sequence similarity with Mesorhizobium huakuii and Mesorhizobium amorphae. The association found between the province of origin and species cluster of the isolates suggests biogeography patterns: most isolates from the north, center, and south belong to clusters B, A, and D, respectively. Most of the highly efficient isolates (symbiotic effectiveness > 75%) belong to cluster B. A correlation was found between species cluster and origin soil pH of the isolates, suggesting that pH is a key environmental factor, which influences the species geographic distribution. To our knowledge, this is one of the few surveys on chickpea rhizobia and the first systematic assessment of indigenous rhizobia in Portugal. Document Type: ArticleLanguage: English

Rhizobia strains expressing the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase have been reported to display an augmented symbiotic performance as a consequence of lowering the plant ethylene levels that inhibit the nodulation process. Genes encoding ACC deaminase (acdS) have been studied in Rhizobium spp.; however, not much is known about the presence of acdS genes in Mesorhizobium spp. The aim of this study was to assess the prevalence and phylogeny of acdS genes in Mesorhizobium strains including a collection of chickpea-nodulating mesorhizobia from Portugal. ACC deaminase genes were detected in 10 of 12 mesorhizobia type strains as well as in 18 of 18 chickpea Mesorhizobium isolates studied in this work. No ACC deaminase activity was detected in any Mesorhizobium strain tested under free-living conditions. Despite the lack of ACC deaminase activity, it was possible to demonstrate that in Mesorhizobium ciceri UPM-Ca7(T) , the acdS gene is transcribed under symbiotic conditions. Phylogenetic analysis indicates that strains belonging to different species of Mesorhizobium, but nodulating the same host plant, have similar acdS genes, suggesting that acdS genes are horizontally acquired by transfer of the symbiosis island. This data, together with analysis of the symbiosis islands from completely sequenced Mesorhizobium genomes, suggest the presence of the acdS gene in a Mesorhizobium common ancestor that possessed this gene in a unique symbiosis island.

The symbiosis established between arbuscular mycorrhizal fungi (AMF) and roots of most land plants plays a key role in plant nutrient acquisition and alleviation of environmental stresses. Despite the ubiquity of the symbiosis, AMF and host species display significant specificity in their interactions. To clarify preferential associations between wheat (Triticum aestivum) and AMF, we characterized root AMF communities in the transition from two first host species, ryegrass (Lolium rigidum) and yellow-serradella (Ornithopus compressus), grown separately or together, to a second host (wheat), by sequencing the large subunit ribosomal DNA (LSU rDNA) gene. The response of AMF communities in wheat to prior soil disturbance – and consequently of the mycelial network [intact extraradical mycelium (ERM) vs. disrupted mycelium] established with either of the first hosts – was also investigated. Since the outcome of a specific host–symbiont interaction depends on the molecular responses of the host plant upon microbial colonization, we studied the expression of six key symbiosis-related genes in wheat roots. AMF communities on L. rigidum and O. compressus roots were clearly distinct. Within an undisturbed ERM, wheat AMF communities were similar to that of previous host, and O. compressus-wheat-AMF interactions supported a greater growth of wheat than L. rigidum-wheat-AMF interactions. This effect declined when ERM was disrupted, but generated a greater activation of symbiotic genes in wheat, indicating that plant symbiotic program depends on some extent on the colonizing symbiont propagule type. When a mixture of L. rigidum and O. compressus was planted, the wheat colonization pattern resembled that of O. compressus, although this was not reflected in a greater growth. These results show a lasting effect of previous hosts in shaping wheat AMF communities through an efficient use of the established ERM, although not completely obliterating host–symbiont specificity.