3 - Localization and biosynthesis in plants

Summary

Although plant lectins have been known for the last 100 years, we are only beginning to understand their functions in the life-cycle of plants. The fact that lectin genes have been conserved well during evolution, especially in Leguminoseae, and that lectin proteins coded for by the genes are homologous, argues very strongly that lectins may have important, albeit mainly unknown, roles in the life of the plant. In the past, many such roles have been proposed. Thus lectins may act as defensive agents, carbohydrate transporters, recognition agents, storage proteins or growth regulators, etc (for references see Pusztai et al., 1983a; Etzler, 1986). However, experimental evidence in support of such roles is still scanty at best.

Whatever role(s) lectin may play, it is important to find out their localization and distribution in various parts of the plant. Temporal changes in their distribution may also help to formulate ideas about the functions of lectins. In addition, any proposed role will also have to be compatible with their ultrastructural localization and any possible changes in it during the various stages of plant development. Owing to their great importance, studies on the localization of lectins have been reviewed extensively (Pusztai et al., 1983a; Etzler, 1985, 1986).

Localization in the plant and cellular location

Seeds

Lectins are generally most abundant in the seeds of plants, especially in the seeds of Leguminoseae and Graminaceae. However,lectins are by no means confined to seeds. Variable, although usually small amounts of lectins or lectin-like, immunochemically cross-reacting proteins (CRM) are also present in vegetative parts of plants.