Ultrastructural changes in coconut calli associated with the acquisition of embryogenic competence

Ultrastructural studies of 2,4-D (2,4 dichlorophenoxyacetic acid) induced c oconut calli and of untreated controls enabled us to characterize early eve nts in cellular reorganization leading to embryogenic cell individualizatio n and subsequent development into proembryos. Embryogenic cells were charac terized by special features that chiefly affected the nucleus, cytoplasm an d cell wall: deep invaginations of the nuclear envelope. proliferation of d ictyosomes, with emission of Golgi vesicles, directly related to an increas e in cell wall thickness. Modification of the cell wall structure was studi ed and particular attention was paid to the cytolocalization of beta -1,4-g lucans. and of callose and pectin epitopes, using gold-conjugated probes. T he first changes (detected 7-14 d after 2,4-D increase) involved the closur e of plasmodesmata, breaking of symplastic continuity and callose depositio n. The acquisition of embryogenic competence was linked to the appearance o f an outer layer of fibrillar material containing pectin epitope (mainly un -methyl-esterified), fully coating the embryogenic cells (21 d after the in duction treatment). Some of the ultrastructural changes observed during the reprogramming of somatic cells towards embryogenesis can be likened to tho se accompanying the maturation or female gamete cells in many plant species . The possible significance of these observations is discussed. (C) 2001 An nals of Botany Company.