An ultrastructural study of Pittosporum tobira Ait. callus: the status of intercellular connections

Callus was obtained from stem expiants of Pittosporum tobira Ait. plants in Murashige and Skoog's medium enriched with NAA and BAP. This callus, which showed no external morphogenetic activity, was investigated in an ultrastructural study with the main purpose to study possible relationships between this characteristic and the status of the intercellular connections. The callus was comprised of small meristem-like cells, with cytoplasm rich in organdíes, and large parenchymatous cells, in some of which starch (storage products) accumulated; young and mature phloem and xylem cells were also frequently found. In all the various cell types an abundance of junctions (plasmodesmata) was observed. The plasmodesmata were well differentiated, showed characteristics correlated to the extent of cell specialization and function, and were distributed in a manner which secured a good flux of solutes in the symplast. The results are discussed in relation to callus cell organization and to possible cell morphogenetic competence in this in vitro system.