Metabolic control of avocado fruit growth: 3-hydroxy-3-methylglutaryl coenzyme a reductase, active oxygen species and the role of C7 sugars

The presence of the seven carbon (7C) sugar D-manno-heptulose (D-manno-2-ketoheptose) and its corresponding sugar alcohol perseitol (D-glycero-D-galacto-heptitol) as the predominant sugars in avocado fruits is of considerable interest for two reasons. First, D-manno-heptulose is a potent inhibitor of plant and animal hexokinase and second, nothing is known about the role of 7C sugars in avocado fruit growth and development. In this paper the contribution of avocado 7C sugars was explored. Results show that D-manno-heptulose and perseitol are major soluble sugars in phloem sap, leaf petiole exudates, and in seed and mesocarp of avocado. Whereas levels of perseitol remained constant throughout fruit growth, D-manno-heptulose levels increased linearly until maturity. D-manno-heptulose does not appear to be a precursor of perseitol in fruit tissue. However, exogenous D-manno-heptulose increased the endogenous pool of perseitol in avocado seed and enhanced activity of microsomal 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR, EC 1.1.1.34). Although perseitol did not affect seed HMGR when applied to whole fruit, this 7C sugar effectively negated the deleterious effects of tert-butylhydroperoxide and hydrogen peroxide in in vitro assays. It is concluded that D-manno-heptulose and perseitol fulfill several important functions during avocado fruit growth. These are: modulation of carbon flux, and as a protectant against reactive oxidative damage of key enzymes such as HMGR required for growth and development of the fruit.