PLANT PHYSIOLOGY , Vol 114, Issue 2 511-518, Copyright © 1997 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Metabolic Control of Avocado Fruit Growth (Isoprenoid Growth Regulators and the Reaction Catalyzed by 3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase)
A. K. Cowan, C. S. Moore-Gordon, I. Bertling and B. N. Wolstenholme
Department of Horticultural Science, University of Natal, Pietermaritzburg, Private Bag X01, Scottsville 3209, South Africa
The effect of isoprenoid growth regulators on avocado (Persea americana
Mill. cv Hass) fruit growth and mesocarp 3-hydroxy-3-methylglutaryl
coenzyme A reductase (HMGR) activity was investigated during the course of
fruit ontogeny. Both normal and small-fruit phenotypes were used to probe
the interaction between the end products of isoprenoid biosynthesis and the
activity of HMGR 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. In small fruit a
70% reduction in microsomal HMGR activity was associated with an increased
mesocarp abscisic acid (ABA) concentration. Application of mevastatin, a
competitive inhibitor of HMGR, 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-pyridyl)-N-phenylurea, but were not significantly affected by
either gibberellic acid or stigmasterol. However, stigmasterol appeared to
partially restore fruit growth when co-injected with mevastatin in either
phase II or III of fruit growth. In vivo application of ABA reduced fruit
growth and mesocarp HMGR activity and accelerated fruit abscission, effects
that were reversed by co-treatment with isopentenyladenine. Together, these
observations indicate that ABA accumulation down-regulates mesocarp HMGR
activity and fruit growth, and that in 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.
