Amino acid metabolism in senescing Sandersonia aurantiaca flowers: cloningand characterization of asparagine synthetase and glutamine synthetase cDNAs

We have studied nitrogen metabolism during senescence of the ethylene-insen sitive flower Sandersonia aurantiaca Hook. We report here on the isolation and characterization of two cDNAs from senescing sandersonia tepals. SAND1 encodes asparagine synthetase (AS; EC 6.3.5.4) and SAND3 encodes glutamine synthetase (GS; EC 6.3.1.2). The accumulation of SAND3 mRNAs was consistent ly high throughout flower development and senescence and was not associated with senescence per se. SAND1 mRNAs started to accumulate in mature flower s just prior to visual signs of senescence. Sucrose feeding of individual f lowers delayed tepal senescence and altered amino acid metabolism. The leve ls of Asn, Gln and Glu were higher and Asp levels were lower in the sucrose -fed flowers. The senescence-associated peak in Gln was delayed in sucrose- fed flowers compared to controls that were held in water. Sucrose feeding h ad no effect on the initiation of SAND1 transcription at the onset of flowe r senescence; however, the later decline in SAND1 mRNA abundance was delaye d in sucrose-fed flowers. Elimination of GS activity and the subsequent red uction in Gln levels caused by phosphinothricin treatment was associated wi th a delay of SAND1 mRNA accumulation in senescing flowers.