Abstract
The transcriptomes of senescing flag leaves collected from barley field plots with standard or high nitrogen supply were compared to identify genes specifically associated with nitrogen remobilization during leaf senescence under agronomically relevant conditions. In flag leaves collected in field plots with high nitrogen supply, the decline in chlorophyll content was delayed. By comparing changes in gene expression for the two nitrogen levels, it was possible to discriminate genes related to nitrogen remobilization during senescence and genes involved in other processes associated with the late development of leaves under field conditions. Predominant genes that were more strongly upregulated during senescence of flag leaves from plants with standard nitrogen supply included genes encoding the transcription factor HvNAC026, serine type protease SCPL51, and the autophagy factors APG7 and ATG18F. Elevated expression of these genes in senescing leaves from plants with standard nitrogen supply indicates important roles of the corresponding proteins in nitrogen remobilization. In comparison, the genes upregulated in both flag leaf samples might have roles in general senescence processes associated with late leaf development. Among these genes were the transcription factor genes HvNAC001, HvNAC005, HvNAC013, HvWRKY12 and MYB, genes encoding the papain-like cysteine peptidases HvPAP14 and HvPAP20, as well as a subtilase gene.
Highlights
Leaf senescence is a highly regulated complex developmental process terminated by death of the organ
No significant differences were observed between the dry weight of standard nitrogen plots (SN) ears and high nitrogen supply (HN) ears indicating that the standard nitrogen supply did not limit the filling of the kernels
The high nitrogen supply in HN plots was in excess and could diminish/suppress the development of senescence symptoms such as chlorophyll degradation
Summary
Leaf senescence is a highly regulated complex developmental process terminated by death of the organ. The purpose of senescence is to remobilize nutrients, in particular nitrogen, from the senescing organs to still growing plant parts, and to the developing seeds. In crop plants such as cereals, senescence is a slow process, whereas in other plants, in particular Arabidopsis, senescence might occur quickly, and has been discussed as a cell-death process (Jansson and Thomas, 2008). It has been estimated that during seed filling of most annual crop plants more than 70% of the leaf nitrogen is exported from the senescing leaves (Peoples and Dalling, 1988). Up to 70% of the chloroplast nitrogen is assembled into ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Another major fraction is contained in the Abbreviations: HN, high nitrogen; qRT-PCR, quantitative real-time PCR; Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase; SD, standard deviation; SN, standard nitrogen; SP2 splicing factor 2
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