A role for Ca in the cellular differentiation of root cap cells: A re-examination of root growth control mechanisms

Ultrastructural investigations of root caps (Zea mays) have shown a correlation between the depletion of extracellular Ca²⁺ resources by treatment of intact roots with 50 mM EGTA and changes in the activity of peripheral cap cells, involving reductions in the quantity and changes in the appearance of Golgi apparatus-derived material. In EGTA-treated roots the development of the Golgi apparatus of peripheral cap cells was severely altered and there was no evidence of a granular secretory product, which was encountered in the vesicles of dictyosomes of control roots. Vesicles of dictyosomes of EGTA-treated roots were small and the development of dictyosomes was reminiscent of that encountered in central cap cells of control roots. A decrease in amyloplast numbers brought about by EGTA was concomitant with reduced secretory activity. EGTA treatment was also associated with a redistribution of amyloplasts located in central cap cells. Decreased cap volume arising from EGTA treatment was considered indicative of a decline in overall cap activity. Proposals are made with regard to the integration of intercellular activities in the response of plant roots to stimulus-modulated signals. It is suggested that alterations in peripheral cap cell secretory activity arising from the redistribution of Ca²⁺ may account for the anisotropic growth response of gravireactive roots.