Morphological and Transcriptomic Analyses Provide New Insights into Linseed (Linum usitatissimum L.) Seedling Roots Response to Nitrogen Stress

Nitrogen (N) is the most important macro-nutrient for plant growth and development, which not only results in the highest cost in crop production but may also lead to environmental pollution. Hence, there is a need to develop N and use efficient genotypes, a prerequisite for which is a better understanding of N stress adaptation. Here, responses of two contrasting linseed accessions at the seedling stage were assessed for N stress-induced changes in twelve phenotypic traits and for gene expression profiling in the roots. The results showed that nine out of twelve phenotypic traits were affected under N stress conditions, and include total root length (TRL), root tips (RT), shoot dry weight (SDW), root dry weight (RDW), root-to-shoot ratio (R/S), plant nitrogen content (PNC), shoot nitrogen content (SNC), root nitrogen content (RNC), and nitrogen use efficiency (NUE). For example, under N stress, the TRL, RDW, SDW, PNC, SNC, and RNC showed reductions of 7.1, 7.6, 16.0, 43.7, 43.3, and 38.7%, respectively. The N-efficient (NE) genotype outperformed the N-inefficient (NI) genotype for all root and shoot traits and NUE under N stress and N normal conditions. Transcriptome analysis identified 1034 differentially expressed genes (DEGs) under the contrasting N conditions and uncovered the opposite responses of the two linseed genotypes to N starvation at the gene expression level. DEGs included 153 transcription factors distributed in 27 families, among which ERF, MYB, NAC, and WRKY were the most represented. In addition, DEGs involved in N absorption and transport, root development, amino acid transport, and antioxidant activity were found to be differentially expressed. The candidate genes identified in the current study are purported for their roles in N metabolism in other crops and might also play a pivotal role in N stress adaptation in linseed, and therefore could be useful for further detailed research on N stress response in linseed, paving the way toward developing N-efficient linseed cultivars with improved root system architecture.