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

Abstract

We have studied nitrogen metabolism during senescence of the ethylene-insensitive 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 consistently high throughout flower development and senescence and was not associated with senescence per se. SAND1 mRNAs started to accumulate in mature flowers just prior to visual signs of senescence. Sucrose feeding of individual flowers delayed tepal senescence and altered amino acid metabolism. The levels 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 had no effect on the initiation of SAND1 transcription at the onset of flower senescence; however, the later decline in SAND1 mRNA abundance was delayed in sucrose-fed flowers. Elimination of GS activity and the subsequent reduction in Gln levels caused by phosphinothricin treat-ment was associated with a delay of SAND1 mRNA accumulation in senescing flowers.

Flower Senescence-Strategies and Some Associated Events

“…In carnation autocatalytic rise in ethylene has been found to precede the symptoms of senescence and has been found to be associated with increased transcription of genes encoding enzymes involved in ethylene biosynthesis, such as ACC synthase and ACC oxidase (Park et al, 1992). Genes for enzymes that are also upregulated include a glutathione S-transferase, an S-adenosyl methionine synthase, β-glucosidase, β-galactosidase, cysteine proteases, glutamine synthetase, asparagine synthetase, aspartic proteases, nucleases, monodehydroascorbate reductase and phosphate transporter PhPT1 (Meyer et al, 1991;Raghothama et al, 1991;Woodson et al, 1992;Woodson, 1994;Jones et al, 1995;Panavas et al, 1999;Eason et al, 2000;Wagstaff et al, 2002;Jones et al, 2005;Narumi et al, 2006;Xu et al, 2007;Farage-Barhom et al, 2008;Chapin & Jones, 2009;Tripathi et al, 2009;Yamada et al, 2009). The expression of most of these genes has been found to be controlled by the application of exogenous ethylene supporting the view that their upregulation during senescence is regulated by ethylene.…”

“…Among all the proteases, cysteine proteases are the most frequent and well characterized (Stephenson & Rubinstein, 1998). Several cysteine proteases have been shown to be upregulated and characterized from petals of carnation, Hemerocallis, Alstroemeria, Narcissus, Sandersonia, Gladiolus and Petunia (Jones et al, 1995;Valpuesta et al, 1995;Guerrero et al, 1998;Eason et al, 2002;Hunter et al, 2002;Wagstaff et al, 2002;Arora & Singh, 2004;.…”

“…Proteolysis also recycles essential amino acids and is important for the recovery of essential nutrients (Clarke, 2005). Proteolysis is thought to play a significant role in the senescence of flowers because expression of protease genes is one of the earliest senescence-related gene changes to be identified (Eason et al, 2002). Upregulated expression of protease genes, raised enzymatic activity and a decline in soluble protein levels have been found to occur consistently during senescence, both in ethylene sensitive and ethyleneinsensitive flowers (Panavas et al, 1999;Eason et al, 2002;Wagstaff et al, 2002;Jones et al, 2005).…”

“…Proteolysis is thought to play a significant role in the senescence of flowers because expression of protease genes is one of the earliest senescence-related gene changes to be identified (Eason et al, 2002). Upregulated expression of protease genes, raised enzymatic activity and a decline in soluble protein levels have been found to occur consistently during senescence, both in ethylene sensitive and ethyleneinsensitive flowers (Panavas et al, 1999;Eason et al, 2002;Wagstaff et al, 2002;Jones et al, 2005). Large losses in the protein levels during senescence have been reported prior to wilting in such flowers as Ipomoea, Hemerocallis and Petunia (Matile & Winkenbach, 1971;Lay-yee et al, 1992;Sultan & Farooq, 1996;Gulzar, 2003).…”

“…Senescence resembles animal cell death called apoptosis in many features but the characteristic signs of apoptosis have not been convincingly demonstrated in plants (Noodén & Guiamet, 1996;Noodén et al, 1997;Yamada et al, 2003;Hoeberichts et al, 2005;Reape & Mc Cabe, 2008;Yamada et al, 2009). Literature on senescence, postharvest physiology and technology has been reviewed by various authors (Halevy & Mayak, 1979, 1981Noodén & Leopold, 1988;Reid, 1989;Salunkhe et al, 1990;Pandey et al, 2000;Voleti et al, 2000;Wagstaff et al, 2002;van Doorn & Stead, 1997;van Doorn, 2001van Doorn, , 2004van Doorn & Woltering, 2005Zhou et al, 2005;Eason, 2006;Tripathi & Tuteja, 2007). The present review intends to give an update on the available literature on various strategies of petal senescence and to gain an insight into some important events associated with it.…”