Lectins as Plant Defense Proteins

Many plant species contain carbohydrate-binding pro- teins, which are commonly referred to as either lectins or agglutinins. Generally speaking, lectins are proteins that bind reversibly to specific mono- or oligosaccharidcs. Since the initial discovery of a hemagglutinating factor in castor bean extracts by Stillmark in 1888, several hundred of these proteins have been isolated and characterized in some detail with respect to their carbohydrate-binding specific- ity, molecular structure, and biochemical properties. Lec- tins from different plant species often differ with respect to their molecular structure and specificity. It is important, therefore, to realize that a11 plant lectins are artificially classified together solely on the basis of their ability to recognize and bind carbohydrates. Moreover, the question arises whether proteins with a completely different struc- ture and sugar-binding specificity fulfill the same physio- logical role. No conclusive answer can be given to this question as yet, for the simple reason that the role of most plant lectins is not known with certainty. There is, how- ever, growing evidence that most lectins play a role in the plant's defense against different kinds of plant-eating or- ganisms. The idea that lectins may be involved in plant defense is not new. In an earlier review, Chrispeels and Raikhel (1991) critically assessed the defensive role of the phytohemagglutinin family and a number of chitin-bind- ing proteins. During the last few years important progress has been made in the study of plant lectins in general and in the understanding of their effects on other organisms in particular. In this Update we summarize the recent devel- opments that support the defensive role of plant lectins and, in addition, discuss earlier work in this field against the background of our present knowledge of this group of plant proteins.