Phloem callose

While all of the natural products just referred to were heteropolysaccharides (although several of the hemicelluloses were largely xylose-based), some attention was paid to compounds composed of single hexoses. A minor amount of research was conducted on cellulose and starch chemistry, and callose, an unusual />-(T 3)-linked glucan, found in small amounts in the sieve plates of the phloem of the grape vine, was characterized. Two papers were published on ivory nut mannan and several on fructose-based polymers. 

The shift from phloem feeding to xylem feeding on resistant rices is not well understood. Auclair et al. (37) speculated that these differences were due to either the presence of a feeding deterrent in tissues adjacent to or within the sieve elements of resistant plants or the formation of callose or slime plugs in phloem in response to GLH probing. 

The 9-d-(1— 3)-glucan of higher plants (which is identical with callose ) is found in pollen tubes and in sieve tubes of phloem during senescence and... 

Deposition of the matrix substances and formation of the microfibrils are accompanied by a sequence of related processes that lead to the development and differentiation of the cell wall this sequence includes expansion of the wall, changes in the composition of the polysaccharides, organization and orientation of the different layers, deposition of callose for formation of pores in phloem, lignification (see p. 299), and other processes. Considerable information has been obtained about the mechanism of some of these processes and the factors that affect them this information has been reviewed by leading molecular biologists, and will very briefly be mentioned here because of its relevance to cell-wall formation and to the constitution of cell-wall polysaccharides of interest to carbohydrate chemists. According to the new concepts, the transformations of the cell wall are effected, or are assisted, by the presence of a variety of enzymes, proteins, and, perhaps, even ribonucleic acid to the extent that primary... 

In the development of phloem, the plasma membranes ends of the cells are joined by elaborate plasmodesmata, which make contact through channels in the cell wall. Callose is deposited around these and the sieve tube is the much specialised chain of cells that results, with residual, functional cytoplasm still present, connected across the pores of the sieve plates. Connections to companion cells also remain though the lateral walls and these must serve to sustain the c5hoplasm of the sieve tubes. 

---