Exploring the plant-associated bacterial communities in Medicago sativa L

Francesco Pini,Luisa Santopolo,Arcangela Frascella,Carla Scotti,Alessio Mengoni,Marco Bazzicalupo,Emanuele G Biondi

doi:10.1186/1471-2180-12-78

Abstract

BackgroundPlant-associated bacterial communities caught the attention of several investigators which study the relationships between plants and soil and the potential application of selected bacterial species in crop improvement and protection. Medicago sativa L. is a legume crop of high economic importance as forage in temperate areas and one of the most popular model plants for investigations on the symbiosis with nitrogen fixing rhizobia (mainly belonging to the alphaproteobacterial species Sinorhizobium meliloti). However, despite its importance, no studies have been carried out looking at the total bacterial community associated with the plant. In this work we explored for the first time the total bacterial community associated with M. sativa plants grown in mesocosms conditions, looking at a wide taxonomic spectrum, from the class to the single species (S. meliloti) level.ResultsResults, obtained by using Terminal-Restriction Fragment Length Polymorphism (T-RFLP) analysis, quantitative PCR and sequencing of 16 S rRNA gene libraries, showed a high taxonomic diversity as well as a dominance by members of the class Alphaproteobacteria in plant tissues. Within Alphaproteobacteria the families Sphingomonadaceae and Methylobacteriaceae were abundant inside plant tissues, while soil Alphaproteobacteria were represented by the families of Hyphomicrobiaceae, Methylocystaceae, Bradyirhizobiaceae and Caulobacteraceae. At the single species level, we were able to detect the presence of S. meliloti populations in aerial tissues, nodules and soil. An analysis of population diversity on nodules and soil showed a relatively low sharing of haplotypes (30-40%) between the two environments and between replicate mesocosms, suggesting drift as main force shaping S. meliloti population at least in this system.ConclusionsIn this work we shed some light on the bacterial communities associated with M. sativa plants, showing that Alphaproteobacteria may constitute an important part of biodiversity in this system, which includes also the well known symbiont S. meliloti. Interestingly, this last species was also found in plant aerial part, by applying cultivation-independent protocols, and a genetic diversity analysis suggested that population structure could be strongly influenced by random drift.

Highlights

Plant-associated bacterial communities caught the attention of several investigators which study the relationships between plants and soil and the potential application of selected bacterial species in crop improvement and protection
Medicago sativa L. is one of the most important legume crop in temperate areas throughout the world, commonly used as forage or in crop rotation practices to contribute organic nitrogen to the soil via its symbiosis with the nitrogen fixing bacteria [15]. It is important for bioenergy production [16] and is one of the most suited plant species for land restoration [17]. This species, and the diploid relative M. truncatula Gaertn., are among the most studied model species regarding the molecular aspects of plant-bacteria symbiosis, in relation with the alphaproteobacterium Sinorhizobium
In this work we investigated the bacterial communities associated with the legume M. sativa, focusing on both the total bacterial community composition and on the presence and populations structure of the symbiotic partner S. meliloti in soil and plant tissues

Summary

Introduction

Plant-associated bacterial communities caught the attention of several investigators which study the relationships between plants and soil and the potential application of selected bacterial species in crop improvement and protection. (alfalfa) is one of the most important legume crop in temperate areas throughout the world, commonly used as forage or in crop rotation practices to contribute organic nitrogen to the soil via its symbiosis with the nitrogen fixing bacteria [15]. It is important for bioenergy production [16] and is one of the most suited plant species for land restoration [17]. In the last years S. meliloti has been found able to endophytically colonize the aerial part of other plant species, as rice [22], suggesting the presence of several ecological niches for this species (soil, nodule, other plant tissues)

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