High levels of asparagine synthetase in hypocotyls of pine seedlings suggest a role of the enzyme in re-allocation of seed-stored nitrogen

摘要

A pine asparagine synthetase gene expressed in developing seedlings has been identified by cloning its cDNA (PsAS1) from Scots pine (Pinus sylvestris L.). Genomic DNA analysis with PsAS1 probes and a sequence-based phylogenetic tree are consistent with the possibility of more than one gene encoding asparagine synthetase in pine. However, the parallel patterns of free asparagine content and PsAS1 products indicate that the protein encoded by this gene is mainly responsible for the accumulation of this amino acid during germination and early seedling development. The temporal and spatial patterns of PsAS1 expression together with the spatial distribution of asparagine content suggest that, early after germination, part of the nitrogen mobilized from the megagametophyte is diverted toward the hypocotyl to produce high levels of asparagine as a reservoir of nitrogen to meet later specific demands of development. Furthermore, the transcript and protein analyses in seedlings germinated and growth for extended periods under continuous light or dark suggest that the spatial expression pattern of PsAS1 is largely determined by a developmental program. Therefore, our results suggest that the spatial and temporal control of PsAS1 expression determines the re-allocation of an important amount of seed-stored nitrogen during pine germination.