Role of phytohormones in legumes infected intercellularly by rhizobia without infection threads formation

Abstract

Rhizobia grow in the soil as free-living organisms but they also can live as nitrogen-fixing symbionts inside root nodules of legumes. Two modes of rhizobial entry into roots are a) intracellularly, through root hairs, inside infection threads, and b) intercellularly, between epidermal cells, never involving root hair or transcellular infection thread formation. Intracellular entry is the best-known infection process, especially in the model legumes Medicago truncatula and Lotus japonicus. Intercellular entry of rhizobia never involving infection thread formation is found in some of the dalbergioid and genistoid legumes, such as Aeschynomene spp. and Arachis hypogaea. This infection mechanism has been scarcely investigated and little is known about molecular players involved in this infection mechanism. Here we focus our interest in comparing the role of phytohormones between legumes infected intracellular and intercellularly. To reach this aim we selected the legumes A. hypogaea and Aeschynomene spp., and the model legumes M. truncatula and L. japonicus.