Nitrate inhibits nodule organogenesis through inhibition of cytokinin biosynthesis inLotus japonicus

Abstract

Legumes balance nitrogen acquisition from soil nitrate with symbiotic nitrogen fixation. Nitrogen fixation requires establishment of a new organ, which is a cytokinin dependent developmental process in the root. We found cytokinin biosynthesis is a central integrator, balancing nitrate signalling with symbiotic acquired nitrogen. Low nitrate conditions provide a permissive state for induction of cytokinin by symbiotic signalling and thus nodule development. In contrast, high nitrate is inhibitory to cytokinin accumulation and nodule establishment in the root zone susceptible to nodule formation. This reduction of symbiotic cytokinin accumulation was further exacerbated in cytokinin biosynthesis mutants, which display hypersensitivity to nitrate inhibition of nodule development, maturation and nitrogen fixation. Consistent with this, cytokinin application can rescue nodulation and nitrogen fixation of biosynthesis mutants in a concentration dependent manner. These inhibitory impacts of nitrate on symbiosis occur in a Nlp1 and Nlp4 dependent manner and contrast with the positive influence of nitrate on cytokinin biosynthesis that occurs in non-symbiotic species. Altogether this shows that legumes, as exemplified by Lotus japonicus, have evolved a different cytokinin response to nitrate compared to non-legumes.

Nitrates increase abscisic acid levels to regulate haustoria formation in the parasitic plant Phtheirospermum japonicum

“…Cytokinin levels were strongly induced by Phtheirospermum infection (Fig.4), consistent with previous findings (Spallek et al, 2017), but we found no evidence that nitrate itself induced cytokinin levels or induced cytokinin response in Phtheirospermum . Notably, nitrate treatment of Lotus japonicus inhibited cytokinin biosynthesis, reduced cytokinin levels and reduced nodule formation (Lin et al, 2020). In Phtheirospermum , nitrate treatment also reduced cytokinin levels and cytokinin response compared to water treatment (Fig.4B, Fig.S4).…”