Penetration and establishment of Phakopsora pachyrhizi in soybean leaves as observed by transmission electron microscopy

Authors: EDWARDS, H - Western Illinois University; Bonde, Morris

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/23/2011
Publication Date: 7/1/2011
Citation: Edwards, H.H., Bonde, M.R. 2011. Penetration and establishment of Phakopsora pachyrhizi in soybean leaves as observed by transmission electron microscopy. Phytopathology. 101:894-900.

Interpretive Summary: Phakopsora pachyrhizi is a fungus that causes a disease known as soybean rust. The pathogen is unusual for a rust disease because it enters the leaf directly through the surface of the leaf without entering pores in the leaf, the more common means of entry. In order to better understand the initiation of soybean rust, a study was conducted to observe the process under high magnification using an instrument called an electron microscope. Our experiments showed that the fungus breached the leaf surface over cross walls separating adjacent soybean leaf cells. Penetration through the waxy layer covering the surface was by means of digestion, pressure, or combination of the two. Then the fungus digested its way through the outer leaf cell wall using enzymes. Once in the leaf, the pathogen continued to penetrate internal leaf cells by digesting holes in the cell walls. An understanding of the mechanism by which the fungus enters the leaf and continues to spread is an important step in developing a better means to control the disease.

Technical Abstract: Transmission electron microscopy revealed that the usual location of appressorial formation by P. pachyrhizi on the leaf surface of soybean was over the anticlinal wall depression between adjacent epidermal cells. A fibril-like matrix appeared to act as an anchor for the appressorium to attach to the cuticle. From the underside of the appressorium, a penetration hypha formed that subsequently passed through a penetration pore, created by the pathogen, extending through the cuticle and outer epidermal cell wall. Penetration of the cuticle appeared to be by digestion, pressure, or a combination of the two. Penetration of the epidermal cell wall was by digestion, shown by the lack of deformation lines. The penetration hypha then passed through papilla material, expanded, and transversed the host epidermal cell lumen. Penetration through the lower epidermal cell wall was by enzymatic digestion. As the penetration hypha entered the intercellular space between mesophyll cells, a cross wall formed in the penetration hypha giving rise to a developing primary hypha. The primary hypha proceeded to grow between mesophyll cells, and at some point walled off a bleb-like haustorial mother cell. The mother cell became tightly appressed to a host mesophyll cell, and gave rise to a small penetration peg, which digested a hole through the mesophyll cell wall. Upon penetration of the mesophyll cell wall, the penetration peg invaginated into the mesophyll protoplast pushing the mesophyll plasma membrane ahead of the peg. The penetration peg swelled to form a haustorial lobe, which developed into a haustorium.