Plant growth-promoting rhizobacteria and root system functioning

The rhizosphere supports the development and activity of a huge and diversified microbial community, including microorganisms capable to promote plant growth. Among the latter, plant growth-promoting rhizobacteria (PGPR) colonize roots of monocots and dicots, and enhance plant growth by direct and indirect mechanisms. Modification of root system architecture by PGPR implicates the production of phytohormones and other signals that lead, mostly, to enhanced lateral root branching and development of root hairs. PGPR also modify root functioning, improve plant nutrition and influence the physiology of the whole plant. Recent results provided first clues as to how PGPR signals could trigger these plant responses. Whether local and/or systemic, the plant molecular pathways involved remain often unknown. From an ecological point of view, it emerged that PGPR form coherent functional groups, whose rhizosphere ecology is influenced by a myriad of abiotic and biotic factors in natural and agricultural soils, and these factors can in turn modulate PGPR effects on roots. In this paper, we address novel knowledge and gaps on PGPR modes of action and signals, and highlight recent progress on the links between plant morphological and physiological effects induced by PGPR. We also show the importance of taking into account the size, diversity, and gene expression patterns of PGPR assemblages in the rhizosphere to better understand their impact on plant growth and functioning. Integrating mechanistic and ecological knowledge on PGPR populations in soil will be a prerequisite to develop novel management strategies for sustainable agriculture

Azospirillum Genomes Reveal Transition of Bacteria from Aquatic to Terrestrial Environments

Fossil records indicate that life appeared in marine environments ∼3.5 billion years ago (Gyr) and transitioned to terrestrial ecosystems nearly 2.5 Gyr. Sequence analysis suggests that “hydrobacteria” and “terrabacteria” might have diverged as early as 3 Gyr. Bacteria of the genus Azospirillum are associated with roots of terrestrial plants; however, virtually all their close relatives are aquatic. We obtained genome sequences of two Azospirillum species and analyzed their gene origins. While most Azospirillum house-keeping genes have orthologs in its close aquatic relatives, this lineage has obtained nearly half of its genome from terrestrial organisms. The majority of genes encoding functions critical for association with plants are among horizontally transferred genes. Our results show that transition of some aquatic bacteria to terrestrial habitats occurred much later than the suggested initial divergence of hydro- and terrabacterial clades. The birth of the genus Azospirillum approximately coincided with the emergence of vascular plants on land

Experimental Evolution in Plant-Microbe Systems: A Tool for Deciphering the Functioning and Evolution of Plant-Associated Microbial Communities

International audienceIn natural environments, microbial communities must constantly adapt to stressful environmental conditions. The genetic and phenotypic mechanisms underlying the adaptive response of microbial communities to new (and often complex) environments can be tackled with a combination of experimental evolution and next generation sequencing. This combination allows to analyse the real-time evolution of microbial populations in response to imposed environmental factors or during the interaction with a host, by screening for phenotypic and genotypic changes over a multitude of identical experimental cycles. Experimental evolution (EE) coupled with comparative genomics has indeed facilitated the monitoring of bacterial genetic evolution and the understanding of adaptive evolution processes. Basically, EE studies had long been done on single strains, allowing to reveal the dynamics and genetic targets of natural selection and to uncover the correlation between genetic and phenotypic adaptive changes. However, species are always evolving in relation with other species and have to adapt not only to the environment itself but also to the biotic environment dynamically shaped by the other species. Nowadays, there is a growing interest to apply EE on microbial communities evolving under natural environments. In this paper, we provide a non-exhaustive review of microbial EE studies done with systems of increasing complexity (from single species, to synthetic communities and natural communities) and with a particular focus on studies between plants and plant-associated microorganisms. We highlight some of the mechanisms controlling the functioning of microbial species and their adaptive responses to environment changes and emphasize the importance of considering bacterial communities and complex environments in EE studies

Impacts of Microbial Inoculants on the Growth and Yield of Maize Plant

International audienc

Écologie des Escherichia coli producteurs de Shiga-toxines (STEC) dans les effluents d'élevages bovins et le sol

Les Escherichia coli entérohémorragiques (EHEC) sont considérés comme l un des plus importants groupes de pathogènes émergents responsables de toxi-infections alimentaires. Depuis ces dix dernières années, l environnement est de plus en plus incriminé dans les épidémies à EHEC. Dans cette étude, nous nous sommes intéressés à la prévalence des E. coli producteurs de Shiga-toxines (STEC) dans les exploitations laitières ainsi qu à leur survie dans les effluents d élevages et le sol. Au sein des exploitations laitières, une grande diversité de souches STEC, capables de persister sur de nombreux supports (abreuvoirs, murs et sol des enclos, etc), a été observée. Dans les effluents d élevages (fumier et lisier bovins non traités), la survie des STEC non-O157 a été évaluée à plus de 90 jours. Lorsque les tas de fumier sont retournés (pratique d assainissement menée par les exploitants) la survie est de seulement 45 jours ; la température élevée au cœur des andains (>= 65C) est associée au déclin important des STEC. En ce qui concerne la survie dans les sols, nous avons montré qu in vitro les souches STEC O26:H11 persistaient pendant plus d une année dans différents types de sols mélangés à du fumier, même en présence de taux d humidité faibles (= 65C) is associated with the serious decrease of STEC cells number. In vitro, STEC O26 strains were detected in various manure amended-soil types for at least 1 year, even in presence of low moisture levels (i.e. less than 0,08 g H2O g-1 dry soil). The ambient temperature (i.e. 20C versus 4C) is significantly associated (P<0,001) with the highest STEC count decline in all soils tested. In situ, the persistence of STEC and their transfer from naturally contaminated bovine feces to subsoil layers were determined in different pasture units of a high mountain watershed located in North Alps. STEC are able to persist in bovine feces, and to be transferred in subsoil layers at a depth up to 20 cm, over a period of approximately 2 months, until the fecal material had completely decayed. In the rhizosphere, STEC survival may be affected by antibiotic-producing microbial populations. However, using a 2,4-diacetylphloroglucinol (Phl)-producing Pseudomonas strain as a model of biocontrol rhizobacteria, any negative effect of the Pseudomonas production of Phl on E. coli O157:H7 survival in wheat rhizosphere was observed. According to these results, cattle environment constitutes a second significant reservoir of STEC cells, and effective measures to prevent STEC cells entry into environment should be adoptedLYON1-BU.Sciences (692662101) / SudocSudocFranceF