Fig. 9: Proposed model for regulation of nodule organogenesis by nitrate.

In low nitrate, NLP signalling contributes to regulation of symbiotic genes, which stimulate cytokinin biosynthesis, ensuring robust nodule maturation and onset of nitrogen fixation. In high nitrate, NLP-dependent nitrate signalling inhibits expression of symbiotic genes and symbiotic cytokinin biosynthesis that is crucial for nodule organogenesis. Nodulation is thus balanced between a permissive low-nitrate state where high cytokinin levels can accumulate, or a restrictive state with low root cytokinin.