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.