Abstract
Glutamine synthetase and asparagine synthetase are two master enzymes involved in ammonium assimilation in plants. Their roles in nitrogen remobilization and nitrogen use efficiency have been proposed. In this report, the genes coding for the cytosolic glutamine synthetases (HvGS1) and asparagine synthetases (HvASN) in barley were identified. In addition to the three HvGS1 and two HvASN sequences previously reported, two prokaryotic-like HvGS1 and three HvASN cDNA sequences were identified. Gene structures were then characterized, obtaining full genomic sequences. The response of the five HvGS1 and five HvASN genes to leaf senescence was then studied. Developmental senescence was studied using primary and flag leaves. Dark-exposure or low-nitrate conditions were also used to trigger stress-induced senescence. Well-known senescence markers such as the chlorophyll and Rubisco contents were monitored in order to characterize senescence levels in the different leaves. The three eukaryotic-like HvGS1_1, HvGS1_2, and HvGS1_3 sequences showed the typical senescence-induced reduction in gene expression described in many plant species. By contrast, the two prokaryotic-like HvGS1_4 and HvGS1_5 sequences were repressed by leaf senescence, similar to the HvGS2 gene, which encodes the chloroplast glutamine synthetase isoenzyme. There was a greater contrast in the responses of the five HvASN and this suggested that these genes are needed for N remobilization in senescing leaves only when plants are well fertilized with nitrate. Responses of the HvASN sequences to dark-induced senescence showed that there are two categories of asparagine synthetases, one induced in the dark and the other repressed by the same conditions.
Highlights
Barley (Hordeum vulgare L.) is a major grain cereal grown widely and used as animal fodder and for fermentation to make beer or whisky
The induction of genes by stress-induced senescence via nitrate limitation or dark treatment was investigated on plants at the vegetative stage
In plants grown under low (LN) or high (HN) nitrate conditions, chlorophyll content decreased with ageing in all leaf ranks (Fig. 1A, B)
Summary
Barley (Hordeum vulgare L.) is a major grain cereal grown widely and used as animal fodder and for fermentation to make beer or whisky. Cereals are of primary importance to ensure food security and nitrogen use efficiency is a key target for improvement. Barley is cultivated across substantially less area than maize, rice or wheat. The fact that a whole-genome shotgun assembly and an integrated physical map are available (Mayer et al, 2012), makes barley a useful model system for the study of temperate cereal crops, especially wheat whose genome is much more complex. Senescence is the last developmental stage before leaves die and a very important physiological process for the plant. The numerous molecular and biological processes that contribute to senescence are essential for the recycling and remobilization of mineral nutrients and nitrogen-containing
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