Annals of Botany 88: 9-18, 2001
© 2001 Annals of Botany Company
Ultrastructural Changes in Coconut Calli Associated with the Acquisition of Embryogenic Competence
GENETROP, IRD, 911 av. Agropolis, BP 5045, 34032, Montpellier cedex, France BIOTROP, CIRAD-AMIS, 2477 avenue du Val de Montferrand, BP 5035, 34032, Montpellier cedex 1, France
Received: 6 November 2000 ; Returned for revision: 9 January 2001 . Accepted: 23 February 2001
Ultrastructural studies of 2,4-D (2,4 dichlorophenoxyacetic acid) induced coconut calli and of untreated controls enabled us to characterize early events in cellular reorganization leading to embryogenic cell individualization and subsequent development into proembryos. Embryogenic cells were characterized by special features that chiefly affected the nucleus, cytoplasm and cell wall: deep invaginations of the nuclear envelope, proliferation of dictyosomes, with emission of Golgi vesicles, directly related to an increase in cell wall thickness. Modification of the cell wall structure was studied and particular attention was paid to the cytolocalization of ß-1,4-glucans, and of callose and pectin epitopes, using gold-conjugated probes. The first changes (detected 714 d after 2,4-D increase) involved the closure of plasmodesmata, breaking of symplastic continuity, and callose deposition. The acquisition of embryogenic competence was linked to the appearance of an outer layer of fibrillar material containing pectin epitope (mainly un-methyl-esterified), fully coating the embryogenic cells (21 d after the induction treatment). Some of the ultrastructural changes observed during the reprogramming of somatic cells towards embryogenesis can be likened to those accompanying the maturation of female gamete cells in many plant species. The possible significance of these observations is discussed. Copyright 2001 Annals of Botany Company
Callose, cell wall structure, Cocos nucifera L., cytological events, embryogenic cells, pectin, somatic embryogenesis
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