Regulation of NAC transcription factor gene expression in Camelina sativa leaves during early seed filling: influence of nitrogen nutrition and leaf age

Abstract

Camelina [Camelina sativa (L.) Crantz] is an oilseed crop primarily used for biofuels and industrial lubricants. Nitrogen is the most quantitatively important plant mineral nutrient, with photosynthetic machinery representing the major nitrogen sink in leaf mesophyll cells. While camelina is less nitrogen intensive than other field crops, low nitrogen deficiency slows growth and accelerates senescence of older leaves, reducing plant biomass and yield. Increasing yield under reduced nitrogen inputs constitutes an important goal for modern agriculture, justifying research aimed at understanding physiological processes limiting nitrogen use efficiency. While numerous regulatory, metabolic, and transport processes influence plant nitrogen use, research presented here is focused on the role of NAC transcription factors in plant responses to nitrogen deficiency. Camelina (var. 'Suneson') plants were grown under two nitrogen regimes termed 'low' (LN) and 'high' (HN). To identify genes and physiological processes governing plant responses to LN, a group of main stem leaves was analyzed during early seed filling using biochemical and transcriptomic approaches. Differential gene expression and gene enrichment analysis between treatments (LN vs. HN), and between mature-green and senescing leaves, identified numerous processes associated with photosynthesis (downregulated under LN, and with advancing senescence), autophagy, lipid, pigment, and nitrogen metabolism (upregulated with advancing senescence), characterizing leaf physiological state with respect to treatment and age. Based on both RNA-Seq and RT-qPCR, camelina orthologs of two well-known Arabidopsis transcription factors, NAC029/AtNAP and NAC092/ORESARA1, exhibited significant upregulation both under LN and with advancing leaf senescence. Overlaying camelina NAC gene expression data on a phylogenetic tree of Arabidopsis NAC proteins identified several clusters containing camelina NAC genes exhibiting higher expression under LN than HN on day 0, suggesting a role in the plant's response to LN. These data suggest two possibilities: senescence was already more advanced in day 0 leaves from LN-grown plants, despite the SAG12 expression data; or there is substantial functional overlap between NAC transcription factors involved in nitrogen deficiency and those regulating leaf senescence. A combination of biochemical approaches and genetic studies will be required to test these interpretations, which are not mutually exclusive.