Wheat NAM genes regulate the majority of early monocarpic senescence transcriptional changes including nitrogen remobilisation genes

Senescence enables the remobilisation of nitrogen and micronutrients from vegetative tissues of wheat (Triticum aestivum L.) into the grain. Understanding the molecular players in this process will enable the breeding of wheat lines with tailored grain nutrient content. The NAC transcription factor NAM-B1 is associated with earlier senescence and higher levels of grain protein, iron, and zinc content due to increased nutrient remobilisation. To investigate how related NAM genes control nitrogen remobilization at the molecular level, we carried out a comparative transcriptomic study at seven time points (3, 7, 10, 13, 19 and 26 days after anthesis) in wild type and NAM RNA interference (RNAi) lines with reduced NAM gene expression. Approximately 2.5 times more genes were differentially expressed in WT than NAM RNAi during this early senescence time course (6,508 vs 2,605 genes). In both genotypes, differentially expressed genes were enriched for GO terms related to photosynthesis, hormones, amino acid transport and nitrogen metabolism. However, nitrogen metabolism genes including glutamine synthetase (GS1 and GS2), glutamate decarboxylase (GAD), glutamate dehydrogenase (GDH) and asparagine synthetase (ASN1) showed stronger or earlier differential expression in WT than in NAM RNAi plants, consistent with higher nitrogen remobilisation. The use of time course data identified the dynamics of NAM-regulated and NAM-independent gene expression changes during senescence, and provides an entry point to functionally characterise the pathways regulating senescence and nutrient remobilisation in wheat.