Melanin-independent accumulation of turgor pressure in appressoria of Phakopsora pachyrhizi

Authors: CHANG, HAO - University Of Illinois; MILLER, LOU ANN - University Of Illinois; Hartman, Glen

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/16/2014
Publication Date: 9/1/2014
Publication URL: http://handle.nal.usda.gov/10113/60202
Citation: Chang, H., Miller, L., Hartman, G.L. 2014. Melanin-independent accumulation of turgor pressure in appressoria of Phakopsora pachyrhizi. Phytopathology. 104:977-984.

Interpretive Summary: In many plant pathogenic fungi, penetration of plants occurs by mechanical force formed when a melanized appressorium accumulates high turgor pressure. Accumulation of turgor pressure in appressoria produces a mechanical force allowing for the direct penetration of the fungus through the plant cell epidermis. Melanin synthesis has been reported to function as an impermeable barrier to leakage, which allow appressoria to accumulate high turgor pressure. Deficiency of melanized appressoria has been shown to reduce turgor pressure and compromise the infection process. The transparent appressoria of soybean rust pathogen, Phakopsora pachyrhizi, raises the questions of whether melanin occurs in the appressorial cell wall and if the appressoria accumulate turgor pressure via an impermeable barrier. We found that two melanin biosynthesis inhibitors did not reduce turgor pressure or compromise the infection process. Transmission electron microscope images also showed the absence of melanin layer between the appressorial cell wall and plasma membrane. This is the first report showing that turgor pressure accumulation of P. pachyrhizi appressoria was independent of melanin. This research will be useful to soybean pathologists, mycologists, and other scientists interested in knowing how fungal pathogens penetrate plants and cause disease.

Technical Abstract: In some plant pathogenic fungi, turgor pressure accumulation in appressoria produces a mechanical force enabling the direct penetration of hyphae through the plant cell epidermis. Melanin has been reported to function as an impermeable barrier to osmolytes, which allow appressoria to accumulate high turgor pressure. Deficiency of melanin in appressoria has been shown to reduce turgor pressure and compromise the infection process. The transparent appressoria of the soybean rust pathogen, Phakopsora pachyrhizi, raises the questions of whether melanin occurs in the appressorial cell wall and if the appressoria accumulate turgor pressure via an impermeable barrier. We used two melanin biosynthesis inhibitors and found that these melanin inhibitors did not reduce turgor pressure or compromise the infection process. In addition, the turgor pressure of P. pachyrhizi appressoria was 5 to 6 MPa based on extracellular osmolytes used to simulate different osmotic pressures independent of melanin synthesis. Transmission electron microscopy also showed the absence of a melanin layer between the appressorial cell wall and plasma membrane. This is the first report showing that turgor pressure accumulation of P. pachyrhizi appressoria was independent of melanin.