BIOSYNTHESIS AND DISTRIBUTION OF INSECT-MOLTING HORMONES IN PLANTS - A REVIEW

Insect-molting hormones, phytoecdysteroids, have been reported to occu r in over 100 plant families. Plants, unlike insects, are capable of t he biosynthesis of ecdysteroids from mevalonic acid, and in several ca ses the biosynthesis of phytoecdysteroids was also demonstrated to pro ceed via sterols. Spinacia oleracea (spinach) biosynthesizes polypodin e B and 20-hydroxyecdysone, which is the predominant insect-molting ho rmone found in plant species. The onset of ecdysteroid production in s pinach requires the appropriate ontogenetic development within the pla nt, which is related to leaf development. In spinach, lathosterol is t he biosynthetic precursor to ecdysone and 20-hydroxyecdysone. Phosphor ylated ecdysteroid intermediates, particularly ecdysone-3-phosphate, a re required during biosynthesis. Polyphosphorylated forms of ecdystero ids are putative regulatory components of the pathway. During spinach development the 20-hydroxyecdysone is transported from the sites of bi osynthesis to the apical regions. An analysis of the physiological dat a available suggests that different species may synthesize ecdysteroid s in various organs and distribute these ecdysteroids to other sites. Annual plants appear to concentrate ecdysteroids in the apical regions , including flowers and seeds. Perennial plants may recycle their ecdy steroids between their deciduous and their perennial organs over the g rowing season. Further investigations of ecdysteroid biosynthesis and physiology within plants will be required before an acceptable system can be designed to test phytoecdysteroid effectiveness in vivo against insect herbivory.