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

Appressoria of some plant-pathogenic fungi accumulate turgor pressure that produces a mechanical force enabling the direct penetration of hyphae through the epidermis. Melanin functions as an impermeable barrier to osmolytes, which allows appressoria to accumulate high turgor pressure. Deficiency of melanin in appressoria reduces turgor pressure and compromises the infection process. In Phakopsora pachyrhizi, the soybean rust pathogen, the appressoria are hyaline. Our objective was to ensure the absence of a melanin layer specifically between the appressorial cell wall and plasma membrane, as well as to determine the turgor pressure of P. pachyrhizi appressoria. We demonstrated that two melanin biosynthesis inhibitors neither reduced turgor pressure nor compromised the infection process. Transmission electron microscopy also showed the absence of a melanin layer between the appressorial cell wall and plasma membrane. In addition, the turgor pressure of P. pachyrhizi appressoria was 5 to 6 MPa, based on extracellular osmolytes used to simulate different osmotic pressures. This is the first report showing that turgor pressure accumulation of P. pachyrhizi appressoria was independent of melanin.