Rhizobial Nod factors are required for nodule primordia development in the crack entry infection process in peanut

IBAÑEZ, F., REINOSO, H., FABRA, A.

Carlos Paz, Córdoba, Argentina

Congreso; VI Congreso Argentino de Microbiología General; 2009

Sociedad Argentina de Microbiología General

Plant growth is often limited by the availability of nitrogen. Plants of the Fabaceae family (legumes) have developed a nitrogen-fixing symbiotic association with soil bacteria collectively known as rhizobia to overcome this nutrient limitation. Arachis hypogaea L. (peanut) is a widespread leguminous plant of great agricultural and economic significance that can be used as food crop or as source of several food products. Peanut exhibits a particular mode of rhizobial invasion, known as crack entry. In this mechanism, infection threads are never formed, not for epidermal or cortical invasion neither for nodular dissemination. The development of the symbiotic association involves a complex molecular signal exchange between the symbiotic partners. Nod factors are the best characterized of these signals, and their role in the symbiosis with legumes where rhizobial entry involves infection thread formation is well known. Recently, our group reported the Nod factor structure from a native bradyrhizobial isolate. However, information regarding the role of Nod factors in peanut-rhizobia symbiosis is scarce, and the question whether these molecules are required for crack entry infection in peanut remains unanswered. In this work, a bradyrhizobial mutant strain unable to produce Nod factors was obtained. Further research revealed that it is not affected in the colonization of peanut roots. In addition, the mutant strain is not able to reinitiate the meristematic activity in cortical cells and, therefore, it is incapable to nodulate peanut. Taken together, the results obtained in this work indicate that rhizobial Nod factors are required for nodulation of peanut, and that they are involved in the induction of the cortical cell division in order to form the nodule primordia. Although this role for Nod factors has been proposed for the root invasion that involves infection thread formation, this work constitutes the first evidence of the link existing between these molecules and the cortical cell division for nodule primordia formation in peanut.