Transcriptome analysis of Azospirillum brasilense vegetative and cyst states reveals large-scale alterations in metabolic and replicative gene expression.

Elizabeth A. Malinich,Carl E. Bauer

doi:10.1099/mgen.0.000200

Elizabeth A. Malinich, Carl E. Bauer

Open Access

https://doi.org/10.1099/mgen.0.000200

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Abstract

Several Gram-negative soil bacteria have the ability to differentiate into dormant cysts when faced with harsh environmental conditions. For example, when challenged with nutrient deprivation or desiccation, the plant-growth-promoting bacterium Azospirillum brasilense differentiates from a replicative and motile rod-shaped vegetative cell into a non-motile dormant spherical cyst. Currently, little is known about either the metabolic differences that exist between vegetative and cyst cell types, or about aspects of cyst physiology that allow dormant cells to survive harsh conditions. Here we compared transcriptomic profiles of vegetative and encysted A. brasilense. We observed that approximately one fifth of the A. brasilense transcriptome undergoes changes in expression between replicative vegetative cells and non-replicative cysts. A dramatic alteration in expression of genes involved in cell wall or cell membrane biogenesis was observed, which is congruent with changes in exopolysaccharide and lipid composition that occur between these cell types. Encysted cells also exhibited repressed mRNA abundance of genes involved in amino acid biosynthesis, ribosomal biogenesis and translation. We further observed that cysts create an anaerobic/micro-aerobic environment, as evidenced by repressed expression of oxidative phosphorylation genes coupled with increased expression of nitrate/nitrite reduction and nitrogen fixation genes.

Highlights

Many bacteria can differentiate into specialized morphotypes in response to changing environmental conditions
RNA sequencing (RNA-seq) transcriptome data from A. brasilense vegetative and cyst cells was generated in order to understand transcriptional differences between the two cell states
The remaining 685 functionally annotated genes were manually sorted into 21 clusters of orthologous groups (COGs) based on the A. brasilense Sp7 Kyoto Encylopedia of Genes and Genomes orthologue and UniProt GO Biological process (Fig. 1)

Summary

Introduction

Many bacteria can differentiate into specialized morphotypes in response to changing environmental conditions. Such life-style switches allow survival in a diverse array of environments. Gram-positive spores are known to persist for decades and be resistant to such adverse environmental conditions as UV radiation, heat, starvation and dehydration. A lesser-studied dormant cell state involves the development of cysts by various Gram-negative bacteria such as members of the genera Azotobacter, Azospirillum, Ramlibacter and Rhodospirillum [4,5,6,7]. Cysts are metabolically dormant cells that exhibit resistance to desiccation, starvation and oxidative stress. Gram-negative cysts do not form internally and instead develop by formation of a thick extracellular polysaccharide exine layer that protects cysts from desiccation [8, 9]. Cysts store large quantities of poly-b-hydroxybutyrate granules for use as an energy source [10]

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