© 2007 The Author(s).
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RESEARCH PAPER |
Sucrose phosphate synthase activity and the co-ordination of carbon partitioning during sucrose and amino acid accumulation in desiccation-tolerant leaf material of the C4 resurrection plant Sporobolus stapfianus during dehydration
1Dipartimento di Scienze Agronomiche e Gestione del Territorio Agroforestale; Università di Firenze, Piazzale delle Cascine 18, I-50144 Firenze, Italy
2Department of Molecular and Cell Biology, University of Cape Town, Private Bag Rondebosch 7700, South Africa
* To whom correspondence should be sent: E-mail: anne.whittaker{at}unifi.it
Both sucrose and amino acids accumulate in desiccation-tolerant leaf material of the C4 resurrection plant, Sporobolus stapfianus Gandoger (Poaceae). The present investigation was aimed at examining sucrose phosphate synthase (SPS) activity and various metabolic checkpoints involved in the co-ordination of carbon partitioning between these competing pathways during dehydration. In the initial phase of dehydration, photosynthesis and starch content declined to immeasurable levels, whilst significant increases in hexose sugars, sucrose, and amino acids were associated with concomitant significant increases in SPS and pyruvate kinase (PK) activities, and maximal activity levels of phosphoenolpyruvate carboxylase (PEPCase), NADP-dependent isocitrate dehydrogenase (NADP-ICDH), and NADH-dependent glutamate synthase (NADH-GOGAT). The next phase of dehydration was characterized by changes in metabolism coinciding with net hexose sugar phosphorylation. This phase was characterized by a further significant increase in sucrose accumulation, with increased rates of net sucrose accumulation and maximum rates of SPS activity measured under both saturating and limiting (inhibitory) conditions. SPS protein was also increased. The stronger competitive edge of SPS for carbon entering glycolysis during hexose phosphorylation was also demonstrated by the further decrease in respiration and the simultaneous, significant decline in both PEPCase and PK activities. A decreased anabolic demand for 2-oxoglutarate (2OG), which remained constant, was shown by the co-ordinated decrease in GOGAT. It is proposed that the further increase in amino acids in this phase of dehydration may be in part attributable to the breakdown of insoluble proteins.
Key words: Amino acids, carbon partitioning, desiccation tolerance, sucrose, SPS
Received 14 May 2007; Revised 27 August 2007 Accepted 30 August 2007
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