Pectins in plant tissues

The location of various pectin substances in plant tissues is well established. Pectins make up most of the middle lamella in unripe fruit and are to be found in the cell walls and in small proportion in all plant tissues. The genesis and fate of pectins in plant tissues have not been fully determined. 

Van Buren, J.P. 1991. Function of pectin in plant tissue structure and firmness, in The Chemistry and Technology of Pectin, ed., R.H. Walter, New York Academic Press, Inc., pp. 1-22. 

Bartolome, L.G. and Hoff, J.E. 1972. Gas chromatographic methods for the assay of pectin methyesterase, free methanol and methoxy groups in plant tissues, J. Agric. Food Chem., 20(2) 262-266. 

Pectins, or vegetable mucilages, are widely distributed in plant tissues, roots, fruit pulp and rind, including the peel of apples, oranges and lemons. They are not extracted by cold water, but when boiled with weak acids, such as occur in fruits, or when attacked by the enzyme pectase, they are hydroljrsed into substances which form characteristic jellies on cooling. 

Modified aldoses, of wide natural occurrence, are the uronic acids. In these molecules the terminal CH2OH group of a hexose has been oxidised to carboxyl. Polysaccharides consisting almost entirely of uronic acid units are pectic acid (D-galacturonic acid) (LXXVIII), the main component of pectin found in plant tissues, and alginic acid (D-mannuronic (LXXIX) and L-guluronic acid (LXXX)) synthesised by the brown algae. For a full description of these acids see Chapter II, p. 73. 

We also lack any knowledge of the ultimate fate of pectic substances and galacturonic add in plant tissues. Little proof exists whether pectic substances may or may not serve as reserve carbohydrates that can be metabolized by the cell. From the fragmentary information on hand, it is not even certain that these pol mronides, once formed, actually disappear from the tissue. The decreasing proportion of pectin in a tissue can be often explained by dilution with newly formed constituents and higher proportion of waterSuch retention of the pectic polyuronides by the tissue would be in harmony with their known structural function. 

Pectin. Pectin [9000-69-5] is a generic term for a group of polysaccharides, mainly partially methoxylated polygalacturonic acids, which are located in the cell walls of all plant tissues. The main commercial sources of pectin are citms peel and apple pomace, where it represents 20—40% and 10—20% of the dry weight respectively. The pectin is extracted, the extract purified, and the pectin precipitated (50) increased extraction times lead to the production of low methoxyl pectins. 

The differentiation of cells occurs concomitantly to modifications of wall components. The nature of the pectins of the walls changes under the action of enzymes, among which esterases, secreted between the apical meristematic cells and the more basal differentiated cells. The apposition of new layers of pectins with different compositions at the inner surface of the walls is another mechanism by which the cells adapt their immediate environment. Using the 2F4 antibody, we have observed, in plant suspensions as well as in tissues, a third mechanism involved in wall modification. Numerous invaginations of the... 

The plant cell wall is a polymeric mesh consisting of cellulose, hemicellulose, pectin and protein. Cellulose and hemicellulose are integral components of the cell wall, but pectic substances are located mainly in the outer wall regions within the middle lamella (McNeil et ai, 1984). Pectic substances are more susceptible to enzymatic degradation, because they are more exposed than other cell wall components. Therefore, pectin-degrading enzymes may play a central role in the penetration of plant tissue by bacteria. 

The ripening process has been subject of many studies [2,6,8]. Due to the large variety of plant tissues investigated, the results of these studies are quite heterogeneous. In general an increase in water-soluble pectins is observed, which is related to the combined action of... 

Two characteristics of the lyase that we have purified may be significant. First, the small molecular size of the protein may confer it a high mobility that could be helpful to its movement through host cell walls. In second place, it is an endo-type enzyme, fact that has been considered essential for maceration of plant tissues [35]. In this sense it is noteworthy that between the battery of pectic enzymes produced by FORL, this pectin lyase is the only protein that behaves as an endo-type. 

The endo-action of the K. marxianus PG was demonstrated by a extremely rapid attack on plant tissue. This activity appears to be at least equivalent to that of several commercial preparations used for separating plant cells for protoplast preparation (RMC, unpublished data). Most of the endo-PGs produced by plant pathogens and saprophytes have so far been reported to possess macerating activity. PG secreted by K. marxianus CCT 3172 also had a strong activity in reducing the viscosity of cocoa pulp. Cocoa pulp generally contains 1 - 1.5% (w/w) of pectin consisting of 68% esterification and 11.6% methoxyl content [18]. 

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