The Role of Thionins in Plant Protection

Thionins are a group of small (5000 Da), sulfur-rich plant proteins found mainly in cereals and mistletoes. Their three-dimensional structures are very compact and amphipathic, stabilized by three or four disulfide bridges. Thionins are usually basic and exert toxicity in various biological systems by destroying membranes. Thionins are synthesized as preproproteins and secreted into vacuoles, protein bodies, and the cell wall. Their antibacterial and antifungal activities point to a role as plant defense proteins. Support for this possible function comes particularly from work on the leaf thionins of barley, showing that these proteins can be induced by several stress factors. Infection of barley with mildew, one of its most devastating pathogens, leads to an incorporation of leaf thionins into papillae in incompatible interactions. The possible role of thionins to enhance the resistance of crop plants by genetic engineering is discussed.