Distinct Hormone Regulation of Determinate and Indeterminate Nodule Development in Legumes

Symbiotic root nodules arise from a well-coordinated interaction between leguminous plants and diazotropic soil bacterial collectively termed rhizobia. Legume roots release specific flavonoid compounds that are recognized by compatible rhizobia species. The rhizobia respond by producing lipochitooligosaccharide “nod factors” that are in turn recognized by LysM domain receptor-like kinases of compatible legume hosts. Recognition of root hair colonization by compatible rhizobia initiates a number of cellular and developmental responses in the plant host. These events can be broadly classified into (i) root hair and epidermal responses that occur early and (ii) cortical responses that follow in a coordinated manner. The earliest response is the initiation of calcium oscillations in the perinuclear region of epidermal and root hair cells. Decoding of these oscillations by a symbiotic calcium-calmodulin-dependent protein kinase subsequently results in the induction of several transcription factors in these cell types including those that potential mediate hormone responses (e.g. ERN1). Constitutive activation of the symbiotic CCaMK is sufficient to produce spontaneous nodules. Therefore, perception of nod factors at the root surface appears to generate and yet to be identified diffusible signal that initiates nodule organogenesis in the root cortex. A number of different transcription factors (e.g. NIN, NSP1, NSP2) and hormone signaling elements (see below) are activated in specific cortex cells that divide to form the nodule primordium. Simultaneously, infected root hairs form invaginations that transport rhizobia towards the inner root cell layers. The formation and elongation of these specialized “infection threads” are also strictly regulated. The infection threads ultimately reach the developing nodules in the cortex where the bacteria are released as membrane-enclosed structures. Cells surrounding the infection zone form the nodule parenchyma where distinct cell differentiation to form the nodule vasculature, cortex and endodermis occur. A number of genes potentially associated with all these different processes have been identified through genetic and functional genomic studies [1,2] (Figure 1).