The effect of isoprenoid growth regulators on avocado (Persea americana Mill. cv Hass) fruit growth and mesocarp 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMCR) activity was investigated during the course of fruit ontogeny. Both normal and small-fruit phenotypes were used t o probe the interaction between the end products of isoprenoid biosynthesis and the activity of HMCR in the metabolic control of avocado fruit growth. Kinetic analysis of the changes in both cell number and size revealed that growth was limited by cell number in phenotypically small fruit. I n small fruit a 70% reduction i n microsomal HMCR activity was associated with an increased mesocarp abscisic acid (ABA) concentration. Application of mevastatin, a competitive inhibitor of HMCR, reduced the growth of normal fruit and increased mesocarp ABA concentration. These effects were reversed by co-treatment of fruit with mevalonic acid lactone, isopentenyladenine, or N-(2-chloro-4-pyridyI)-N-phenylurea, but were not significantly affected by either gibberellic acid or stigmasterol. However, stigmasterol appeared t o partially restore fruit growth when co-injected with mevastatin i n either phase II or III of fruit growth. I n vivo application of ABA reduced fruit growth and mesocarp HMCR activity and accelerated fruit abscission, effects that were reversed by co-treatment with isopen-tenyladenine. Together, these observations indicate that ABA accumulation down-regulates mesocarp HMCR activity and fruit growth, and that i n situ cytokinin biosynthesis modulates these effects during phase I of fruit ontogeny, whereas both cytokinins and sterols seem to perform this function during the later phases. HMGR catalyzes the irreversible conversion of HMG-COA to MVA, the committed step in isoprenoid biosynthe-sis in all eukaryotic organisms (Goldstein and Brown, 1990). For plant growth and development, synthesis of isoprenoids is fundamental because the pathway supplies compounds that are essential for full morphogenic expression. This class of compounds is of structural significance, e.g. carotenoids and the side chain of chlorophylls and plastoquinone for photosynthesis, the side chain of ubiqui-none for respiration, sterols for membrane structure, and phytoalexins for defense. The pathway also supplies sev

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