49. Brown, Jr. R. M. 1974. The Golgi apparatus and endomembrane system; its role in the biosynthesis, transport, and secretion of cell wall constituents in Pleurochrysis. International Symposium on Plant Cell Differentiation. Lisbon, Portugal, August 1974 (M.S. Pais, ed.). Portugal Acta Biologica 14:369-384.

49. Introduction

The biological importance of the plant cell wall has been long recognized, especially in view of its fundamental importance in the morphology, growth, and development of the plant cell (LOEWUS, 1973). As in any specialized domain of scientific research, the new advances are made as a result of the judicious choice and <<exploitation>>, of suitable experimental systems. Rigorous testing of many essential hypothetical questions relating to cell wall biogenesis have not met success using higher plant systems, yet some of the unicellular plants, i.e., the algae offer unparalleled test systems of analyzing cell wall development. Thus, a basic prerequisite for understanding and eventually elucidating mechanisms of cell wall biogenesis is coupled with the necessity of being able to clearly recognize the morphology of the cell wall constituents, not only within the intact deposited cell wall but also within the cell during the biogenic and transport phases. Most higher plant cell walls do not meet this test inasmuch as they consist of a randomly distributed cellulosic microfibrillar phase embedded within an amorphous network, usually composed of pectic substances. Such characteristic wall morphology makes it very difficult if not often impossible to complete a morphometric analysis of cell wall development. This writer certainly would not deny the impressive advances made in cell wall structure and composition (KEEGSTRA, et al., 1973), and the role of the GOLGI apparatus and other cytoplasmic organelles in cell wall deposition (NORTH-COTE and PICKETT-HEAPS, 1966; NORTHCOTE, 1972; WHALEY et al., 1972). These and other studies have contributed greatly to our understanding of cell growth and differentiation. However, many critical tests still remain before we are to understand the intracellular wall assembly sites, the dynamics of assembly and mechanisms of transport and exocytosis of cell wall constituents. The groundwork of MANTON (1966) and her colleagues (PARKE, MANTON, and CLARK, 1958) established the basis for our investigations with Pleurochrysis, a marine haptophycean alga. MANTON'S studies demonstrated the role of the GOLGI apparatus in the biosynthesis of elaborate scales. Never before had such a complex role of the GOLGI apparatus been so clearly implicated, and the challenging reports of MANTON and her colleagues stimulated this investigator to analyse and document a similar process in a heretofore previously uninvestigated alga Pleurochrysis. Professor HAROLD C. BOLD of the University of Texas suggested and encouraged a detailed ultrastructural investigation of this alga which led to the initial published report on this subject (BROWN, 1969). With the parade of technical advances in cytology, cytochemistry, and biochemistry, we have continued to make steady progress in the utilization of Pleurochrysis and related haptophycean algae as model systems for the study of cell wall biogenesis (BROWN et al., 1969; BROWN et al., 1970; BROWN and FRANKE, 1971; FRANKE and BROWN, 1971; HERTH et al., 1972; BROWN et al, 1973; BROWN, 1974.) In this communication, the basic life history of Pleurochrysis will be outlined and the biogenic pathway of scale formation will be described. Evidence supporting the role of the GOLGI apparatus and other membranes of the endomembrane system in scale biosynthesis, assembly, and exocytosis will be documented. Finally, the writer will attempt to relate what has been learned from Pleurochrysis to the more generalized process of cell wall biogenesis, particularly as it may relate to the regulation and fate of the differentiating plant cell.

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