Auxin, Cytokinin and Ethylene Differentially Regulate Specific Developmental States Associated with Shoot Bud Morphogenesis in Leaf Tissues of Mangosteen (Garcinia mangostana L.) Cultured in Vitro

Hormonal regulation of de novo shoot bud formation in leaf explantsof mangosteen has been studied from a developmental perspective.This analysis indicates that at least three discrete, experimentallydistinguishable developmental states, namely, morphogenic competence,caulogenic determination and organ differentiation, were expressedduring shoot bud morphogenesis. The state of morphogenic competencein leaf tissues was expressed maximally between days 10 and12 of leaf development. Competent cells in explants requireda minimum of 6 days of BA treatment (20 µM) to becomecaulogenically determined. Such determined cells would continueshoot organogenesis on medium devoid of growth regulators. Delayingof BA exposure for as short as 2 days caused a dramatic declinein tissue competence. The state of competence and the processof caulogenic determination were adversely affected by IAA,but were insensitive to ethylene or its precursor, ACC. Shootbud differentiation was greatly enhanced by BA, but selectivelydelayed by ethylene. IAA also showed an inhibitory effect onshoot bud differentiation, but not mediated through ethylene.The distinct roles of auxin, cytokinin and ethylene on the regulationof shoot bud development in mangosteen leaf explants have beendiscussed on the basis of the current understanding of the conceptof tissue competence, determination and differentiation. (Received August 12, 1996; Accepted October 31, 1996)