Figure 10.

Proposed Model of NIN Function during Initiation of Nodule Primordia.

After perception of the Nod factor, NIN expression is induced in the epidermis. The –5 kb regulatory region of the NIN promoter is sufficient for both tight root hair curling and infection thread formation. By contrast, expression driven by the –2.2 kb region is sufficient only for the tight root hair curling and formation of bacterial colonies inside the curl. A mobile signal is generated in the epidermis in a NIN-dependent manner, and this signal translocates to the pericycle. Whether or not this mobile signal is cytokinin or an unknown signal, it causes cytokinin accumulation in the inner root cell layers. The CRE1 receptor in the pericycle perceives cytokinin and activates the B-type RR1, which further activates NIN expression. NIN directly or indirectly regulates its own expression via a positive feedback loop, and the –5 kb promoter region is sufficient for this feedback regulation. NIN directly activates NF-YA1 expression and stimulates further cell divisions. Later, the NIN-induced response in pericycle contributes to cell division and NIN expression in the endodermis and cortical cells. C4 and C5, cortical cell layers 4 and 5; Ed, endodermis; Ep, epidermis; Pc, pericycle.