A Model of NIN Action.
(1) NIN expression is induced in epidermal cells in response to NF. Within the epidermis, NIN promotes NPL expression (Xie et al., 2012) and represses ENOD11 expression by competing with ERN1. NIN promotes movement of a signal from the epidermis to the cortex that initiates cytokinin signaling in the cortex. (2) Cytokinin signaling promotes NIN expression in cortical cells. (3) NIN and cytokinin signaling act within a positive feedback loop, with NIN promoting CRE1 expression and cytokinin signaling promoting NIN expression, expanding the degree of nodulation responses within the root cortex. (4) High levels of NIN in the cortex drive nodule organogenesis through promotion of NF-YA1 and NF-YA2 (Soyano et al., 2013). In the cortex, NIN activates the expression of CLE peptides (Soyano et al., 2014) implicated in the negative regulation of nodulation. Cells expressing NIN are shown in blue. Full lines indicate direct targets of transcription factors. Dotted arrows indicate indirect targets.