CELLULAR ASPECTS OF RUST INFECTION STRUCTURE DIFFERENTIATION. Spore Adhesion and Fungal Morphogenesis

, Rust fungi are important plant pathogens, infecting taxonomically diverged plant taxa such as angiospermes, gymnospermes and also some pteridophytes. Though members of all families of spermatophytes are infected by rusts, it is interesting that members of the Poaceae, Leguminosae and Compositae are the dominating hosts. Many of the plants are economically important crops such as maize, wheat, barley, oat, peas and beans. The success of rust fungi is based on their producdtion of spores which spread over great distances. More than 50,000 uredospores can be prodced from a single pustule, and anemochoric transportation over more than 1000 km has been reported [71]. The coffee rust fungus Hemileia vastatrix was fIrst discovered in plantations in Ceylon in 1868. Ten years later most coffee plantations had been destroyed, and tea was produced instead of coffee. Likewise, due to destruction of coffee plantations in Java by the rust fungus, caoutchouc rubber production replaced that of coffee. So far, H. vastatrix has destroyed plantations in 25 countries, and since 1970 it is also present in Brazil and Colombia, both of which are among the most important coffee-producing states of the world. In North America wheat stem rust epidemics have been a severe problem from the 1-8th century on, and regional epidemics occurded in 1904, 1916, 1923, 1925, 1935, 1937, 1953 and 1954 [82]. These epidemics, caused by Puccinia graminis f. sp. tritici, were severe and resulted in signifIcant crop failures in the U.S.A. These two examples clearly indicate the economic importance of rust fungi and encourage the study of mechanisms that provide the basis for establishment of a successful pathosystem. In order to produce large spore numbers it is necessary to withdraw nutrients from the host over an extended time, and this requires maintainance of integrity of the inefcted host cells and tissues. This is possible as the fungus causes only very limited destruction of the host cell walls, so that· even heavily infected plants stay alive for weeks after the onset of sporulation, and new inoculum is efficiently generated. All fungi that use this strategy are obligate biotrophs. One of the most striking difficulties of working with obligate biotrophs is that they have an absolute requirement for living host cells, and only few exceptions of axenically cultured rusts exist [28,67].