Adaptation of Rhizobium leguminosarum to pea, alfalfa and sugar beet rhizospheres investigated by comparative transcriptomics.
BACKGROUND
The rhizosphere is the microbe-rich zone around plant roots and is a key determinant of the biosphere's productivity. Comparative transcriptomics was used to investigate general and plant-specific adaptations during rhizosphere colonization. Rhizobium leguminosarum biovar viciae was grown in the rhizospheres of pea (its legume nodulation host), alfalfa (a non-host legume) and sugar beet (non-legume). Gene expression data were compared to metabolic and transportome maps to understand adaptation to the rhizosphere.


RESULTS
Carbon metabolism was dominated by organic acids, with a strong bias towards aromatic amino acids, C1 and C2 compounds. This was confirmed by induction of the glyoxylate cycle required for C2 metabolism and gluconeogenesis in all rhizospheres. Gluconeogenesis is repressed in R. leguminosarum by sugars, suggesting that although numerous sugar and putative complex carbohydrate transport systems are induced in the rhizosphere, they are less important carbon sources than organic acids. A common core of rhizosphere-induced genes was identified, of which 66% are of unknown function. Many genes were induced in the rhizosphere of the legumes, but not sugar beet, and several were plant specific. The plasmid pRL8 can be considered pea rhizosphere specific, enabling adaptation of R. leguminosarum to its host. Mutation of many of the up-regulated genes reduced competitiveness for pea rhizosphere colonization, while two genes specifically up-regulated in the pea rhizosphere reduced colonization of the pea but not alfalfa rhizosphere.


CONCLUSIONS
Comparative transcriptome analysis has enabled differentiation between factors conserved across plants for rhizosphere colonization as well as identification of exquisite specific adaptation to host plants.
School of Biological Sciences, University of Reading, Reading, RG6 6AJ, UK.
Vinoy K
VK
Ramachandran
Alison K
AK
East
Ramakrishnan
R
Karunakaran
J Allan
JA
Downie
Philip S
PS
Poole
D000220:Adaptation, Biological; D024322:Amino Acids, Aromatic / Q000378:metabolism; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D027461:Beta vulgaris / Q000382:microbiology*; D002244:Carbon / Q000378:metabolism; D020869:Gene Expression Profiling; D015964:Gene Expression Regulation, Bacterial; D005798:Genes, Bacterial; D005943:Gluconeogenesis; D006038:Glyoxylates / Q000378:metabolism; D000455:Medicago sativa / Q000382:microbiology*; D009154:Mutation; D018532:Pisum sativum / Q000382:microbiology*; D010957:Plasmids / Q000235:genetics / Q000378:metabolism; D016961:Rhizobium leguminosarum / Q000235:genetics* / Q000254:growth & development* / Q000378:metabolism; D058441:Rhizosphere; D013045:Species Specificity; D013997:Time Factors; D015854:Up-Regulation
D024322:Amino Acids, Aromatic; D001426:Bacterial Proteins; D006038:Glyoxylates; D002244:Carbon