Low nitrogen availability inhibits the phosphorus starvation response in maize (Zea maysssp.maysL.)

Abstract

Background: Nitrogen (N) and phosphorus (P) are macronutrients essential for crop growth and productivity. In cultivated fields, N and P levels are rarely sufficient, contributing to the yield gap between realized and potential production. Fertilizer application increases nutrient availability, but not all farmers have access to such additions, nor are current rates of application sustainable or environmentally desirable. Transcriptomic studies of cereal crops have revealed dramatic responses to either low N or low P single stress treatments. In the field, however, levels of both N and P may be suboptimal. The interaction between N and P starvation responses remains to be fully characterized. Results: We characterized the root and leaf transcriptomes of young maize plants grown under control, low N, low P or combined low NP conditions. We identified 1, 555 genes that responded to at least one of the treatments, in at least one of the tissues. Transcriptional responses to low N and low P were distinct, with very few genes responding in a similar way to the two single stress treatments. Furthermore, the regulation of a large group of genes was antagonistic between the two conditions. In combined NP stress, the low N response dominated, the classical P starvation response being largely suppressed. An additional experiment over a range of N availability indicated that even a mild reduction in N levels was sufficient to repress the low P induction of well-characterized P starvation genes. Although expression of P transporter genes was repressed under low N or low NP, we confirmed earlier reports of P hyper accumulation under low N. To lay the foundations for further study of the mechanistic basis of NP signaling in maize, we annotated the complete set of maize Spx genes, and found evidence of N and P regulation across the family. Conclusions: A mild reduction in N availability is sufficient to repress the P starvation response in young maize plants. The Spx gene family are candidates for integrating low N and low P responses.