Abstract
Bacteria of the genus Azospirillum colonize roots of important cereals and grasses, and promote plant growth by several mechanisms, notably phytohormone synthesis. The genomes of several Azospirillum strains belonging to different species, isolated from various host plants and locations, were recently sequenced and published. In this study, an additional genome of an A. brasilense strain, isolated from maize grown on an alkaline soil in the northeast of Mexico, strain CBG497, was obtained. Comparative genomic analyses were performed on this new genome and three other genomes (A. brasilense Sp245, A. lipoferum 4B and Azospirillum sp. B510). The Azospirillum core genome was established and consists of 2,328 proteins, representing between 30% to 38% of the total encoded proteins within a genome. It is mainly chromosomally-encoded and contains 74% of genes of ancestral origin shared with some aquatic relatives. The non-ancestral part of the core genome is enriched in genes involved in signal transduction, in transport and in metabolism of carbohydrates and amino-acids, and in surface properties features linked to adaptation in fluctuating environments, such as soil and rhizosphere. Many genes involved in colonization of plant roots, plant-growth promotion (such as those involved in phytohormone biosynthesis), and properties involved in rhizosphere adaptation (such as catabolism of phenolic compounds, uptake of iron) are restricted to a particular strain and/or species, strongly suggesting niche-specific adaptation.
Highlights
Bacteria of the genus Azospirillum colonize roots of important cereals and grasses, and promote plant growth by several mechanisms, notably phytohormone synthesis [1,2]
The genome sequence of another member of this genus, A. brasilense CBG497, a strain isolated from maize grown on an alkaline soil in the northeast of Mexico, was first obtained
The whole genome sequence of A. brasilense CBG497 was obtained by the 454 pyrosequencing technology and after assembly a total of 156 contigs was obtained and clustered into six scaffolds corresponding to the six expected replicons
Summary
Bacteria of the genus Azospirillum colonize roots of important cereals and grasses, and promote plant growth by several mechanisms, notably phytohormone synthesis [1,2]. Besides their potential as biofertilizer, some strains can benefit plant health through biological control of phytoparasitic plants [3] or bacterial pathogens [4], or by inducing disease resistance [5]. The genomes of four strains belonging to different species, isolated from various host plants and locations, were sequenced and published. The following questions were addressed: Does the core genome contain mainly genes of ancestral origin shared with aquatic relatives? Does the core genome contain genes putatively involved in rhizosphere adaptation and interaction with plants? Which functions are specific to a strain or to a species?
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