Functional analysis of regulatory genes involved in Medicago truncatula nodule organogenesis

摘要

The symbiotic interaction between leguminous plants and rhizobia results in the development of a novel organ on the plant roots, the nodule, where bacteria provide fixed nitrogen for the host. Briefly, the microsymbionts attach to and enter the roothair cells while the plant forms a tube-like infection thread through which the bacteria move into the root cortex. Simultaneously, cortical cells are induced to divide and the infection threads invade these dividing cells forming the nodule primordium.This primordium differentiates into a mature nodule where rhizobia are converted into bacteroids and start to fix nitrogen (Cohn et al. 1998). Rhizobia induce nodule morphogenesis on the plant through the production of nodulation signals (Nod factors; Schultze, Kondorosi 1998). These Nod signals trigger the earliest stages of nodule development, including root hair deformation and curling, cortical cell division and the expression of several early nodulin (enod) genes. Under starvation for combined nitrogen, plants of certain Medicago cultivars have the capacity to form root nodules even in the absence of rhizobia (NAR~+ phenotype), the so-called spontaneous nodules indicating that the developmental program of nodule formation preexists in legumes. These nodules show histological features similar to those of bacterium-induced nodules but the central zone cells contain only amyloplasts (Truchet et al. 1989). Hence, it has been suggested that spontaneous nodules might serve as carbon storage organs induced during nitrogen starvation and be the ancestors of the nitrogen-fixing nodules. Thus, Nod factors act as primary morphogenetic signals triggering a nodule developmental program although other plant factors are likely required for the regulation of nodule morphogenesis such as plant hormones (Fang, Hirsch 1998; Heidstra et al. 1997; Penmetsa, Cook 1997), the stele factor, undine, (Smit et al. 1995) and the metabolic status of the plant (high carbon and low nitrogen) (Bauer et al. 1996).