Alterations in root colonization and nodC gene induction in the peanut–rhizobia interaction under acidic conditions

Soil bacteria collectively called rhizobia invade and establish symbiosis with host plants such as peanut (Arachis hypogaea L.). Initial root colonization by rhizobia is one of the early steps preceding root invasion. Here we demonstrated that this process was affected when both acid-tolerant or acid-sensitive (pH 5.0) peanut rhizobia were grown at low pH (5.5). This effect seems to be a result of alterations produced by the acidity on the microsymbiont and not on its host. Different flavonoids as well as peanut root exudates were tested for their ability to induce bacterial nod genes. We determined that, at pH 7, all the flavonoids assayed were able to induce the nodC gene expression but at different levels. At acid pH, very low induction was observed in acid-sensitive isolates while a change in the flavonoidsʹ inducer effectiveness was determined in acid-tolerant isolates. Exudates released by peanut roots growing in an acid medium seem not to be altered in their ability to induce nodC gene activity.