Location of pectin synthesis

All available evidence, including autoradiographic pulse chase studies using wallbiosynthetic precursors (222, 223), immunocytochemical studies using anti-pectin-specific antibodies (224-226), and subcellullar fractionation and topology studies of pectin biosynthetic en­zymes (227-231), indicate that pectin is synthesized in the Golgi and transported to the wall in membrane vesicles. Plant cells, unlike animal cells, have multiple Golgi and thus pectin synthesis occurs simultaneously in numerous Golgi stacks in the cell (225, 232). The synthesized pectin and other macromolecules are targeted to the wall by the movement of Golgi vesicles, presumably along actin filaments that have myosin motors (233).

Immunocytochemical studies also indicate that the synthesis of different regions of the pectic polysaccharides occurs in different Golgi cisternae as pectin moves from the cis, through the medial and to the trans-Golgi. For example, the use of antibodies specific to different regions of HG and RG-I suggests that HG and RG-I synthesis begins in the cis-Golgi (225, 234, 235) and continues with more extensive decoration of the backbones as the polymers move through the medial Golgi (224, 225, 235) and into the trans-Golgi cisternae (225, 235). Additional modifications of the pectic glycan structure also appear to proceed in a more-or-less organized manner with HG (236, 237) and RG-I (106, 234, 238, 239) initially synthesized in less modified forms in the cis — and medial-Golgi and becoming more modified (e. g., methylesterified) (236) in the medial — and trans-Golgi (225, 235,240-242). HG is believed to be transported to the plasma membrane and inserted into the wall as a highly methylesterified polymer (214, 237, 243-245) and once in the wall, HG is deesterified to varying degrees by pectin methylesterases (246) in the wall or at the cell plate (245). The deesterification of HG converts it to a more negatively charged form (240, 247-250) which is then available to bind ions, enzymes, proteins, and other HG molecules through Ca++ salt bridges. It is believed that a spatial partitioning of HG esterification and deesterification occurs in the wall based on localization of esterified HG throughout the cell wall (237, 240-242, 243, 245, 250, 251), while relatively unesterified HG is more restricted to the middle lamella. This conclusion is supported by the frequently observed absence of unesterified HG epitopes in the trans-Golgi vesicles. However, since some cell types, such as melon callus cells (240), contain unesterified HG in the trans-Golgi, it is possible that HG may be inserted into the wall in a relatively unesterified form, at least in some cells. Also, since specific pectic epitopes localize to different Golgi compartments in different cell types (5, 225, 234, 237, 244), it is likely that the specific localization of the diverse pectin biosynthetic enzymes may vary in a cell type, species, and development-specific manner (226,252-255). It must be noted, however, that the interpretation ofimmunocytochemistry results can be difficult since the absence of a signal using an epitope-specific antibody may be due to masking of the epitope by additional glycosylation or some other modification (e. g., methylation, acetylation, feruloylation). Thus, to conclude that a particular pectin biosynthetic event does not occur in a cell, the lack of a particular immunocytochemical signal is not sufficient. Information on the presence of the biosynthetic enzyme activity or the actual wall carbohydrate structure itself is required.