An outer membrane autotransporter, AoaA, of Azorhizobium caulinodans is required for sustaining high N2-fixing activity of stem nodules.
In this study, we investigated the function of a putative high-molecular-weight outer membrane protein, azorhizobial outer membrane autotransporter A (AoaA), of Azorhizobium caulinodans ORS571. Sequence analysis revealed that AoaA was an autotransporter protein belonging to the type V protein secretion system. Azorhizobium caulinodans forms N(2)-fixing nodules on the stems and roots of Sesbania rostrata. The sizes of stem nodules formed by an aoaA mutant having transposon insertion within this ORF were as large as those in the wild-type strain, but the N(2)-fixing activity of the nodules by the aoaA mutant was lower than that of wild-type nodules. cDNA-amplified fragment length polymorphism and reverse transcriptase-PCR analysis revealed that the expressions of several pathogen-related genes of host plants were induced in the aoaA mutant nodules. Furthermore, exopolysaccharide production was defective in the aoaA mutant under free-living conditions. These results indicate that AoaA may have an important role in sustaining the symbiosis by suppressing plant defense responses. The exopolysaccharide production controlled by AoaA might mediate this suppression mechanism.
Biotechnology Research Center, The University of Tokyo, Tokyo, Japan.
Tadahiro
T
Suzuki
Toshihiro
T
Aono
Chi-Te
CT
Liu
Shino
S
Suzuki
Taichiro
T
Iki
Keisuke
K
Yokota
Hiroshi
H
Oyaizu
D000595:Amino Acid Sequence; D020599:Azorhizobium caulinodans / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D009586:Nitrogen Fixation; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D018547:Plant Stems / Q000235:genetics / Q000378:metabolism / Q000382:microbiology*; D021381:Protein Transport; D049931:Sesbania / Q000235:genetics / Q000378:metabolism / Q000382:microbiology*
D001425:Bacterial Outer Membrane Proteins; D010940:Plant Proteins