Factors affecting 'Hass' avocado fruit size: Carbohydrate, abscisic acid and isoprenoid metabolism in normal and phenotypically small fruit

Carbohydrate and abscisic acid (ABA) metabolism were investigated in normal and phenotypically small 'Hass' avocado (Persea americana Mill.) fruit in an attempt to link alterations in sugar and ABA content with changes in 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR, EC 1.1.1.34) activity and fruit size. The small-fruit phenotype was characterized by reduced seed HMGR activity, increased seed insoluble acid invertase (beta-D-fructofuranosidase, EC 3.2.1,26), decreased sucrose synthase (SS; UDP-D-glucose:D-fructose-2-alpha-glucosyltransferase, EC 2.4.1.13) activity, decreased sucrose content, and increased glucose as a proportion of the total soluble sugar. Sucrose phosphate synthase (SPS; UDP-D-glucose:D-fructose 6-phosphate 2-alpha-D-glucosyltransferase, EC 2.4.1.14) activity was unaffected in seed but reduced in mesocarp of the small fruit. In addition, the small-fruit variant displayed enhanced respiration and both seed and mesocarp tissue showed increased ABA metabolism. Applied ABA caused an increase in insoluble acid invertase activity in seed tissue of normal fruit while mevastatin reduced HMGR activity in this tissue, caused sucrose depletion and increased the proportion of glucose from 5 to 57% of total soluble sugars. Exogenous glucose suppressed HMGR activity in seed tissue whereas in mesocarp tissue, HMGR activity was reduced to 38% of the control after 6 h but enhanced by 46% by 48 h, Glucose increased ABA biosynthesis and turnover in competent tissues. These results suggest that ABA turnover is mediated, in part, by carbohydrate content and composition which also affects HMGR activity. It is proposed that sugar and ABA signals act in concert to modulate expression and/or activity of HMGR in the control of 'Hass' avocado fruit growth and final fruit size.