Plant a-amylase inhibitors and their interaction with insect a-amylases structure, function and potential for crop protection

Abstract

Insect pests and pathogens (fungi, bacteria and viruses) are responsible for severe crop losses. Insects feed directly on the plant tissues, while the pathogens lead to damage or death of the plant. Plants have evolved a certain degree of resistance through the production of defence compounds, which may be aproteic, e.g. antibiotics, alkaloids, terpenes, cyanogenic glucosides or proteic, e.g. chitinases, b-1,3-glu- canases, lectins, arcelins, vicilins, systemins and enzyme inhibitors. The enzyme inhibitors impede digestion through their action on insect gut digestive a-amylases and pro- teinases, which play a key role in the digestion of plant starch and proteins. The natural defences of crop plants may be improved through the use of transgenic technology. Current research in the area focuses particularly on weevils as these are highly dependent on starch for their energy supply. Six di€erent a-amylase inhibitor classes, lectin-like, knottin-like, cereal-type, Kunitz-like, c-purothionin-like and thaumatin-like could be used in pest control. These classes of inhibitors show remarkable structural variety leading to di€erent modes of inhibition and di€erent speci®city pro®les against diverse a-amylases. Speci®city of inhibition is an important issue as the introduced inhibitor must not adversely a€ect the plant's own a-amylases, nor the nutritional value of the crop. Of particular interest are some bifunctional inhibitors with additional favourable properties, such as proteinase inhibitory activity or chitin- ase activity. The area has bene®ted from the recent deter- mination of many structures of a-amylases, inhibitors and complexes. These structures highlight the remarkable variety in structural modes of a-amylase inhibition. The continuing discovery of new classes of a-amylase inhibitor ensures that exciting discoveries remain to be made. In this review, we summarize existing knowledge of insect a-am- ylases, plant a-amylase inhibitors and their interaction. Positive results recently obtained for transgenic plants and future prospects in the area are reviewed.