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 explants of mangosteen has been studied from a developmental perspective. This analysis indicates that at least three discrete, experimentally distinguishable developmental states, namely, morphogenic competence, caulogenic determination and organ differentiation, were expressed during shoot bud morphogenesis. The state of morphogenic competence in leaf tissues was expressed maximally between days 10 and 12 of leaf development. Competent cells in explants required a minimum of 6 days of BA treatment (20 μM) to become caulogenically determined. Such determined cells would continue shoot organogenesis on medium devoid of growth regulators. Delaying of BA exposure for as short as 2 days caused a dramatic decline in tissue competence. The state of competence and the process of caulogenic determination were adversely affected by IAA, but were insensitive to ethylene or its precursor, ACC. Shoot bud differentiation was greatly enhanced by BA, but selectively delayed by ethylene. IAA also showed an inhibitory effect on shoot bud differentiation, but not mediated through ethylene. The distinct roles of auxin, cytokinin and ethylene on the regulation of shoot bud development in mangosteen leaf explants have been discussed on the basis of the current understanding of the concept of tissue competence, determination and differentiation.