Soil-to-fruit nitrogen flux mediates the onset of fruit-nitrogen remobilization and color change in citrus

In non-climacteric fruit tree species, color change is primarily controlled by the interplay between environmental conditions, nutritional factors (nitrogen and sugars) and hormones, mainly abscisic acid and gibberellins (GA), through a complex mechanism which is not completely understood. Nitrogen has a strong impact on color change, influenced by environmental changes, either locally or at the whole tree level. We use Citrus trees, as a non-climacteric model species, to understand the long-distance nitrogen signaling mechanism delaying the chloroplast-to-chromoplast transition at the molecular level. It is unknown whether nitrogen regulates the branch point of geranylgeranyl diphosphate (GGPP), the shared precursor for gibberellin, chlorophyll and carotenoid synthesis. We used 15N to trace the root-to-fruit nitrogen flux in trees grown under soils differing in nitrogen content and temperature, and treated with NH4NO3 and GA. Key genes involved in nitrogen signaling, transport and metabolism, and those from the GGPP branch point were analyzed in the fruit. Results explain how soil temperature modifies 15N transport to play a key role in signaling citrus color change, and show that fruit-nitrogen remobilization is required for triggering degreening. Nitrogen content in the fruit modulates the onset of glutamate deamination, asparagine synthesis, nitrite assimilation and GA1 depletion. Expression of the nitrate transporter NRT1.2, glutamate dehydrogenase and asparagine synthetase genes was high right before the start of degreening, together with NH4+ concentration. Nitrogen delayed carotenoid synthesis (phytoene synthase gene expression) without modifying gibberellin synthesis (ent-kaurene synthase and oxidase) at the chloroplast level.