Control Of Sucrose Synthesis Research Articles

Tansley Review No. 105.

Sucrose has a central role in the metabolism of all higher plants. Here we discuss the location of sucrose synthesis in the leaves of C4 plants, the control of flux through the pathway and its response to environmental conditions, and finally some of the prospects for the genetic manipulation of sucrose metabolism in C4 plants. Much of our knowledge about sucrose synthesis in plants comes from C3 species; it is evident, from those studies of C4 species that are available, that there are many similarities between sucrose synthesis in C3 and C4 plants. However, some of the basic regulatory mechanisms common to both C3 and C4 plants seem to have been adapted for the specialized photosynthetic metabolism of C4 plants. There are also some other important differences: for example, some C4 species possess photosynthetic cells with little or no capacity for sucrose synthesis. For these reasons it is not always appropriate to extrapolate directly from C3 to C4 plants. Even where data are available from one C4 species, usually maize, it should not be assumed that all C4 plants are necessarily the same in every respect. Although C4 plants constitute a small fraction of the plant kingdom as a whole, they include a disproportionate number of agriculturally important species. C4 plants also have some advantages over C3 plants for studies of the regulation of cell‐specific gene expression and the control of metabolism. For these reasons we suggest that they are worthy of more attention by researchers. contents Summary 221 I. Introduction 221 II. Pathway of sucrose synthesis in C4 plants 223 III. Control of sucrose synthesis in C4 plants 225 IV. Future prospects 233 Acknowledgements 234

Short-term effect of nitrate on carbon metabolism of two sugar cane cultivars differing in their biomass production

The effect of nitrate (NO 3 −) on net sucrose synthesis and phosphoenolpyruvate activity was studied using two cultivars of the C 4-plant Saccharinum officinarum, Jaronu 60-5 (Ja 60-5) and Barbados 63–118 (B 63–118), high- and low-biomass producers respectively. Leaf segments were treated with a N-free solution, or 40–80 mM NO 3 − in the light for 90 min. Rates of NO 3 − uptake and assimilation were lower in Ja 60-5 than in B 63–118. In Ja 60-5 net sucrose synthesis was high and drastically reduced by NO 3 −, while in B 63–118 it was lower and stimulated by NO 3 − assimilation. In both cultivars, the main effect of NO 3 − on phosphoenolpyruvate carboxylase was to reduce its sensitivity to malate. It is suggested that the relative capacities for NO 3 − assimilation and sucrose synthesis may be a factor involved in the control of sucrose synthesis by NO 3 − and in the rapidity in which C metabolism is adjusted to N assimilation.

Photorespiratory Amino Donors, Sucrose Synthesis and the Induction of CO2Fixation in Barley Deficient in Glutamine Synthetase and/or Glutamate Synthase

Murray, A. J. S., Blackwell, R. D., Lea, P. J. and Joy, K. W. 1988. Photorespiratory amino donors, sucrose synthesis and the induction of C02 fixation in barley deficient in glutamine synthetase and/or glutamate synthase.—J. exp. Bot. 39: 845-858. A number of mutants of barley have been produced which lack both chloroplastic glutamine synthetase and ferredoxin-dependent glutamate synthase activities. The plants accumulated ammonia to the same extent as mutants deficient in only glutamine synthetase but shared the gas-exchange characteristics of the glutamate synthase deficient parent. These mutants have been used to demonstrate directly the ability of alanine to ameliorate the dramatic drop in fixation rate normally exhibited by glutamate synthase deficient mutants on transfer to photorespiratory conditions. Immediately after transfer to air, the mutants deficient in glutamate synthase activity demonstrated a reduced ability to incorporate 14C derived from 14C02 into sucrose. This effect was, however, dependent on the previous induction of C02 fixation. Use of 14C02 revealed that the induction phase of C02 fixation was altered in all three mutants. Neither of the parents nor the double mutant accumulated sucrose in air under conditions which promote sucrose accumulation by the wild type. The implications of these results for photosynthesis and the control of sucrose synthesis are discussed.

The purification and properties of sucrose-phosphate synthetase from spinach leaves: The involvement of this enzyme and fructose bisphosphatase in the regulation of sucrose biosynthesis

The properties of spinach leaf sucrose-phosphate synthetase (EC 2.4.1.14) and cytosolic fructose-1,6-bisphosphatase (EC 3.1.3.11) have been studied. These two enzymes have been considered to be important in the control of sucrose synthesis. Sucrose-phosphate synthetase from leaf tissue has not been studied in detail previously and we report a technique for purifying this enzyme 50-fold by chromatography on AH-Sepharose 4B. This method frees the enzyme from contaminants which interfere with assay procedures with little or no loss of activity. The partially purified enzyme has a K m for UDP-glucose of 7.1 m m and for fructose 6-phosphate of 0.8 m m. Fructose 1,6-bisphosphate, inorganic phosphate and UDP are strong inhibitors. The inhibition patterns of these suggest that the enzyme operates either by an ordered bi-bi or a Theorell-Chance mechanism. Partially purified cytosolic fructose-1,6-bisphosphatase is not only inhibited by AMP as previously reported, but is also inhibited by fructose 6-phosphate and UDP. From our observations, we conclude that sucrose biosynthesis is indeed controlled through these two enzymes and it appears that the rate of sucrose synthesis is largely dependent upon the supply of triose phosphate and ATP from the chloroplast.