Divergent N Deficiency-Dependent Senescence and Transcriptome Response in Developmentally Old and Young Brassica napus Leaves

Abstract

In the spring oilseed rape (OSR) cultivar ‘Mozart’ grown under optimal N supply (NO) or mild N deficiency (NL) the transcriptome changes associated with progressing age until early senescence in developmentally old lower canopy leaves (leaf #4) and younger higher canopy leaves (leaf #8) were investigated. Twelve weeks old NO and NL plants appeared phenotypically and transcriptomically identical, but thereafter distinct nutrition-dependent differences in gene expression patterns in lower and upper canopy leaves emerged. In NO leaves #4 of 14-week-old compared to 13-week-old plants, ∼600 genes were up- or downregulated, whereas in NL leaves #4 ∼3000 genes were up- or downregulated. In contrast, in 15-week-old compared to 13-week-old upper canopy leaves #8 more genes were up- or downregulated in optimally N-supplied plants (∼2000 genes) than in N-depleted plants (∼750 genes). This opposing effect of N depletion on gene regulation was even more prominent among photosynthesis-related genes (PSGs). Between week 13 and 14 in leaves #4, 99 of 110 PSGs were downregulated in NL plants, but none in NO plants. In contrast, from weeks 13 to 16 in leaves #8 of NL plants only 11 PSGs were downregulated in comparison to 66 PSGs in NO plants. Different effects of N depletion in lower versus upper canopy leaves were also apparent in upregulation of autophagy genes and NAC transcription factors. More than half of the regulated NAC and WRKY transcription factor, autophagy and protease genes were specifically regulated in NL leaves #4 or NO leaves #8 and thus may contribute to differences in senescence and nutrient mobilization in these leaves. We suggest that in N-deficient plants the upper leaves retain their N resources longer than in amply fertilized plants and remobilize them only after shedding of the lower leaves.

Arabidopsis WRKY53, a Node of Multi-Layer Regulation in the Network of Senescence

“…OsWRKYs Rice Oryza sativa [21] 17 OsWRKYs (including OsWRKY40, OsWRKY53) Rice Oryza sativa [22] 161 ZmWRKYs Maize Zea mays [23] 11 BnWRKYs Oilseed rape Brassica napus [24] 7 TpWRKYs Clover leaf Trifolium pratense [25] 13 TaWRKYs (including TaWRKY7, TaWRKY6, TaWRKY45)…”

Nitrogen Supply Drives Senescence-Related Seed Storage Protein Expression in Rapeseed Leaves

“…To investigate the process of senescence induction and N-remobilization in OSR in more detail, microarray data made available by Safavi-Rizi et al [ 19 ] were screened for genes related to nitrogen metabolism and nitrogen storage differentially expressed under optimal (N O ) and low (N L ) nitrogen. Among the genes that we identified to be related to nitrogen metabolism, we found a substantial number of genes encoding SSPs or proteins related to them, which were initially described to be exclusively expressed in seed tissue.…”

“…The plant material of Brassica napus cv. Mozart, which was used for transcriptome analyses, the design of the B. napus custom microarray, the identification of Arabidopsis thaliana homologues of the B. napus unigenes, and the basic data analysis workflow are described in Safavi-Rizi et al [ 19 ]. In this experiment, oilseed rape plants were grown under two different nitrogen regimes, optimal (N O ) and low (N L ) nitrogen.…”

“…Transcriptome analyses of leaves in two canopy levels over development of plants grown under two N-fertilization regimes also revealed opposite effects of N-depletion on senescence in lower versus upper canopy leaves. Several transcriptional regulators and protein degradation genes were identified to be differentially expressed in N-depleted lower leaf positions [ 19 ]. Morphology and performance analyses of exactly the same plants used in this study have been previously reported by Franzaring et al [ 20 ] who analyzed the effect of todays and future CO 2 and its interaction with nitrogen supply.…”

“…In order to understand the nitrogen remobilization processes in OSR under different nitrogen regimes more precisely, we analyzed the transcriptome data, which have been produced by Safavi-Rizi and co-workers [ 19 ] for genes involved in nitrogen remobilization and metabolism. Surprisingly, we realized that a remarkable amount of SSPs or proteins related to SSP expression were also expressed in leaf tissue, which we followed up in more detail to get some clues about their function in leaves.…”