Cell Elongation in the Red Alga Griffithsia: Control by Light, Ions, and an Endogenous Glycoprotein Hormone

Abstract

In cells which elongate by tip growth, wall loosening and wall synthesis are localized at the apex of the growing cell. This localization of biochemical and developmental activities is reflected in an accumulation of specific organelles and of sites of exocytosis at the growing point (Sievers and Schnepf, 1981; Gooday, 1983). We are studying the control of cytoplasmic localization and localized growth in the red alga Griffithsia pacifica Kylin (Rhodophyta, Ceramiales). In this alga, there are two types of cells that elongate by tip growth: ordinary rhizoid apical cells and specialized repair shoot cells. The growing apex of both types of cells has a characteristic organization. The apical dome contains a highly refractile zone; behind the refractile zone is a deeply pigmented chloroplast band; basal to this band, the cytoplasm is nearly colorless. U1trastructural examination of growing tips of rhizoids and repair shoot cells reveals that the refractile zone contains nuclei, endoplasmic reticulum, ribosomes and numerous active dictyosomes. Each dictyosome has a mitochondrion on its forming face. The cisternae of the dictyosomes are inflated. Large, dictyosome-derived vesicles can be seen to fuse with the plasmalemma in the apical elongation zone. This type of organization is similar to that found in tip-growing cells in other organisms (Sievers and Schnepf, 1981).