Figure 1.

Electron Micrograph of a Stem Rust Haustorium in a Wheat Mesophyll Cell.

The rust haustorium develops after penetration of the host cell wall by a fungal hypha but remains external to the host cell. Host-derived nutrients cross the host and then fungal plasma membranes to enter the haustorium. There is evidence that proteins, including Avr proteins, are secreted by the fungus across the haustorial membrane by type I secretion and are then taken up by the plant cell using as yet undefined transport mechanisms. Host resistance results from recognition of Avr proteins by host cytoplasmic R proteins. In the absence of R protein recognition, it is believed that Avr proteins are effectors of pathogen virulence. (Figure courtesy of Peter Dodds, Jeff Ellis, and Tony Pryor.)