Inorganic nitrogen sources and light cooperatively stimulate gene regulation for non-photosynthetic...

Phosphoenolpyruvate carboxylase (PEPC) is a bicarbonate fixation enzyme that is associated with nitrogen metabolism. We previously observed that NO₃^- affected the expression levels of PEPC genes differently in detached wheat leaves. Three types of PEPC isogenes, including the orthologous isogene group (Tappc1b) for C₄-photosynthetic PEPC, exhibited distinct transcriptionally up-regulated patterns from one another. However, it remains unclear which nitrogen compounds exactly trigger the gene expression responses and their light dependency. Here, we investigated the cause of isogene-specific expression responses of PEPCs to different forms of nitrogen supplementation. Detached leaves from wheat seedlings cultivated under a nitrogen-deficient condition were incubated with NO₃^-, NH₄⁺, and urea under both light and dark conditions. Notably, the genes analyzed exhibited markedly different expression patterns across the nitrogen forms under the light condition. Tappc1a, which did non-respond to NO₃^-, was up-regulated by NH₄⁺ exclusively under light. Tappc2 was also up-regulated by NO₃^- and NH₄⁺ only under light, and these up-regulations were suppressed under the dark condition. The candidate transcriptional regulator DOF1 for Tappc1b in wheat was also up-regulated by NO₃^- only under light condition, but earlier than Tappc1b, implicating a DOF1-mediated transcriptional response of Tappc1b to NO₃^-. Comparison of Tappc1b promoter sequences with that of NO₃^--responsive maize C₄PEPC revealed conserved light-responsive elements near the Dof1 binding site. These findings suggest that distinct transcriptional regulatory cascades would exist for non-photosynthetic PEPCs in response to different inorganic nitrogen forms.