Premium
Population density‐dependent regulation of the Bradyrhizobium japonicum nodulation genes
Author(s) -
Loh John T.,
YuenTsai Joyce P.Y.,
Stacey Minviluz G.,
Lohar Dasharath,
Welborn April,
Stacey Gary
Publication year - 2001
Publication title -
molecular microbiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.857
H-Index - 247
eISSN - 1365-2958
pISSN - 0950-382X
DOI - 10.1046/j.1365-2958.2001.02625.x
Subject(s) - biology , bradyrhizobium japonicum , bradyrhizobium , gene , population , genetics , symbiosis , rhizobiaceae , computational biology , bacteria , demography , sociology
The nodulation genes of Bradyrhizobium japonicum are essential for infection and establishment of a nitrogen‐fixing symbiosis. Here, we demonstrate that plant‐produced isoflavones induce nodulation gene expression in a population density‐dependent fashion. Nodulation gene induction is highest at a low population density and significantly reduced in more dense cultures. A quorum signal molecule in the conditioned medium of B. japonicum cultures mediates this repression. Repression in response to the quorum signal results from the induction of NolA which, in turn, induces NodD2 leading to inhibition of nod gene expression. Consistent with this, nolA–lacZ and nodD2–lacZ expression increased with increasing population density. Unlike the wild type, the ability to induce nodY–lacZ expression did not decline with population density in a NolA mutant. Normally, nod gene expression is repressed in planta (i.e. within nodules). However, expression of a nodY–GUS fusion was not repressed in a NolA mutant, suggesting that quorum‐sensing control may mediate in planta repression of the nod genes. Addition of conditioned medium to cultures significantly reduced nod gene expression. Treatment of inoculant cultures with conditioned medium also reduced the ability of B. japonicum to nodulate soybean plants.