Differential regeneration response in two cotyledon types of Vigna radiata: Histomorphological analysis and effect of b-arabinogalactan

A unique differential regeneration response of the two cotyledon types, 'Cot F (attached to the embryonal axis) and 'Cot' was reported earlier in Vigna radiata. The histological study revealed that there is a temporal difference in meristemoid development between 'Cot E' and 'Cot'. The meristematic tissue differentiate directly from the epidermal/sub-epidermal layers of 'Cot E', whereas, callus mediated differentiation occurs in the 'Cot'. It was observed that the frequency of shoot differentiation in 'Cot' explants increased distinctively when 20 mg l(-1) of beta-Arabinogalactan (beta-ABG) was supplemented in the medium and an altered mode of regeneration was noted like that of 'Cot E'. While, under the same condition, the regeneration frequency decreased substantially in 'Cot E' and the explants became necrotic. The results indicate that the de-embryonated 'Cot F grown in vitro contain more endogenous arabinogalactan proteins (AGPs) than 'Cot', presumably due to the wound formation during excision, as AGP is wound inducible. And perhaps due to the feed back inhibition, addition of beta-ABG to the differentiation-inducing medium either resulted in necrosis of 'Cot F or sharp decrease in regeneration efficiency. It was postulated that glycosylation of cytosolic peptides occurs when beta-ABG was supplemented externally in the culture medium and resulted in AGP synthesis. Presence of AGP in the cotyledonary tissues was demonstrated through gel electrophoretic study and also verified by histochemical localization in these explant types. This is the first report showing presence of AGP in V. radiata. The dose dependent phytohormone like effect of beta-ABG suggests its role as precursor for a novel 'growth regulator', AGP.