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Penetration and Establishment of Phakopsora pachyrhizi in Soybean Leaves as Observed by Transmission Electron Microscopy

July 2011 , Volume 101 , Number  7
Pages  894 - 900

H. H. Edwards and M. R. Bonde

First author: Western Illinois University, Department of Biological Sciences, 1 University Circle, Macomb 61455; and second author: United States Department of Agriculture–Agricultural Research Service, 1301 Ditto Ave., Fort Detrick, MD 21702.

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Accepted for publication 23 February 2011.

For over 30 years, it has been known that Phakopsora pachyrhizi is unusual in that it penetrates from urediniospores directly through the leaf cuticle without entering stomates. This unusual mode of penetration suggests that disease resistance mechanisms might exist for soybean rust that do not exist for most rust diseases. As a result, we decided to conduct a histological study using transmission electron microscopy to further elucidate the mechanisms of penetration and early establishment of P. pachyrhizi in soybean leaves. Based on our study, it was concluded that P. pachyrhizi utilizes primarily mechanical force, perhaps with the aid of digestive enzymes, to penetrate the cuticle on the leaf surface. However, the lack of deformation lines in micrographs indicated that digestive enzymes, without mechanical force, are used by the penetration hypha to penetrate the outer and inner epidermal cell walls. Digestive enzymes, again indicated by the lack of deformation lines, are used by haustorial mother cells to breach the walls of mesophyll cells to form haustoria. The possibility exists for eventual determination of the precise roles of pressure and digestive enzymes in the development of soybean rust and elucidation of some of the determinants of resistance and susceptibility to this important plant disease.

This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 2011.