Nitrate restricts nodule organogenesis through inhibition of cytokinin biosynthesis in Lotus 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 rescues 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 species that do not form symbiotic root nodules. Altogether this shows that legumes, as exemplified by Lotus japonicus, have evolved a different cytokinin response to nitrate compared to non-legumes.

Cytokinin synthesis and export from symbiotic root nodules coordinates shoot growth with nitrogen fixation

“…Our findings suggest legumes, which can obtain nitrogen both from the soil and from nitrogen fixation, use cytokinin to coordinate shoot growth with fixed nitrogen. How legumes integrate nitrate with cytokinin in the context of long-distance signalling requires further investigation as we have previously observed a negative relationship between nitrate and cytokinin biosynthesis within the nodule (Lin et al, 2021).…”

“…Cytokinin measurements were conducted as previously described (Lin et al, 2021). Approximately 10-20 mg of nodule and shoot tissue was collected from plants 14 days after inoculation, weighed, and snap frozen in liquid nitrogen.…”

“…Reduced cytokinin biosynthetic gene and receptor transcript levels both appear to contribute to this reduction in signalling output. In legumes, where root development must be balanced with nodule development, symbiotic cytokinin signalling is repressed by nitrate (Lin et al, 2021). How cytokinins contribute to coordinating shoot growth in legumes with varied nitrogen supply from mineral and symbiotic sources therefore remains to be determined.…”

Short-Term Vegetation Restoration Enhances the Complexity of Soil Fungal Network and Decreased the Complexity of Bacterial Network

“…Our findings show that OM, TN, TP, AP, AK, AN, NO3 − , and NH4 + at the top of the soil (0-10 cm) were significantly increased in the AF treatment, while only NO3and AK were significantly increased in the SB treatment. This may be due to the fact that legumes can fix nitrogen by means of symbionts, making the fixed nitrogen available for other plants [59,60], and the soil nitrogen could be considered as a determinant of the concentration of the soil's nutrient content and, consequently, regulate the C and P cycles [61]. Thus, compared with SB, the AF could effectively improve the fertility of the soil's surface in a shorter time.…”

Exogenously Applied Cytokinin Altered the Bacterial Release and Subsequent Stages of Nodule Development in Pea Ipd3/Cyclops Mutant

“…Plant hormones play key roles in the interactions between legume plants and nitrogen-fixing bacteria rhizobia [ 1 , 2 , 3 ]. The perception of bacterial lipochitooligosaccharide signals, the Nod factors, initiates a signaling cascade that induces rhizobial infection in the epidermis and activation of cortical cell division that leads to the nodule formation.…”