Identification of a Protein from Rust Fungi Transferred from Haustoria into Infected Plant Cells

Abstract

The formation of haustoria is one of the hallmarks of the interaction of obligate biotrophic fungi with their host plants. In addition to their role in nutrient uptake, it is hypothesized that haustoria are actively involved in establishing and maintaining the biotrophic relationship. We have identified a 24.3-kDa protein that exhibited a very unusual allocation. Rust transferred protein 1 from Uromyces fabae (Uf-RTP1p) was not only detected in the host parasite interface, the extrahaustorial matrix, but also inside infected plant cells by immunofluorescence and electron microscopy. Uf-RTP1p does not exhibit any similarity to sequences currently listed in the public databases. However, we identified a homolog of Uf-RTP1p in the related rust fungus Uromyces striatus (Us-RTP1p). The localization of Uf-RTP1p and Us-RTP1p inside infected plant cells was confirmed, using four independently raised polyclonal antibodies. Depending on the developmental stage of haustoria, Uf-RTP1p was found in increasing amounts in host cells, including the host nucleus. Putative nuclear localization signals (NLS) were found in the predicted RTP1p sequences. However, functional efficiency could only be verified for the Uf-RTP1p NLS by means of green fluorescent protein fusions in transformed tobacco protoplasts. Western blot analysis indicated that Uf-RTP1p and Us-RTP1p most likely enter the host cell as N-glycosylated proteins. However, the mechanism by which they cross the extrahaustorial membrane and accumulate in the host cytoplasm is unknown. The localization of RTP1p suggests that it might play an important role in the maintenance of the biotrophic interaction.

Translocation ofMagnaporthe oryzaeEffectors into Rice Cells and Their Subsequent Cell-to-Cell Movement

“…Mutation of the oomycete translocation motif RXLR-dEER eliminated this translocation. Kemen et al (2005) detected a haustoriumsecreted protein, Uf-RTP1p, in nuclei of invaded host cells by immunolocalization, but mutational analysis is difficult for the obligate pathogen Uromyces fabae. Our fluorescent translocation signal in the rice cytoplasm is faint, especially compared with bright BIC fluorescence (Figures 5C and 5D; see Supplemental Figure 6 online).…”

Insights into Nonhost Disease Resistance: Can They Assist Disease Control in Agriculture?

“…Virulent Bgh strains induce susceptibility to host cell penetration and haustorium formation by the normally avirulent strain (Lyngkjaer et al, 2001), consistent with the idea that the first strain delivers effector molecules that induce a susceptible state. Studies of genes expressed in rust haustoria are beginning to provide insights into proteins delivered from these pathogens to infected host cells (Kemen et al, 2005;Catanzariti et al, 2006), and intensive genomics studies of Bgh now in progress could identify mildew effectors (Both et al, 2005;Zhang et al, 2005). Characterization of fungal pathogen effectors and their host targets is now a research priority.…”

Two modes of pathogen recognition by plants

“…These Avr proteins are small (150 aa) and contain a signal sequence that is removed (giving rise to a 127-aa mature peptide). They are presumably delivered across the haustorial membrane, through the extrahaustorial matrix, and into the plant cell (15,16). Importantly, AvrL567 proteins contain particular amino acid residues that are under selection for sequence divergence.…”