Project Title: Revealing the Plant Immune Response Network
Project Aims: Investigate the role of putative effector proteins from Hyaloperonospora arabidopsidis.
Oomycete pathogens are responsible for many devastating diseases of agricultural crops as well as ornamental and native plants. Within the oomycetes are many important genera of plant pathogens such as Albugo, Bremia, Peronospora and Plasmopora that cause downy mildews and white rusts on several crops. Also existing within the oomycetes are over 60 species in the genus Phytophthora which can cause large scale damage to many important crop species including potatoes, tomatoes, soybeans, peppers and alfalfa. Phytophthora infestans is the causative agent of late blight of potato and tomato, resulting in the deaths of 1.25 million people during the Irish potato famine of 1845.
The oomycetes are a group of eukaryotes which have been described as ‘fungus-like’ although they belong to the kingdom Stramenopiles and are more closely related to brown algae and diatoms than higher fungi. Oomycetes and fungi share several characteristics however there are several important differences; unlike fungi, the cell walls of oomycetes have little or no chitin and are cellulose based which means that chitin synthase inhibitors (such as Nikkomycin and Polyoxin D) have no inhibitory effects and can therefore not be used as a control mechanism against oomycetes. Another major target of fungicides is sterol synthesis; triazole fungicides target the biosynthesis of the fungal-specific sterol ergosterol. Because many oomycetes are sterol prototrophs and do not synthesize ergosterol, these fungicides are cannot be used to control diseases caused by these pathogens.
Alignment of effector proteins which have been identified and cloned from oomycete pathogens has revealed a conserved motif within 32 amino acids of the predicted signal peptides, consisting of the consensus sequence RxLR; arginine, any, leucine, arginine (see figure below). It was noted that this motif was similar with the host-targeting signal(RxLxE/D/Q) which is conserved in numerous, dissimilar effector proteins from malaria parasites (Plasmodium spp.) and required for the export of effector proteins across both a pathogen derived membrane and an invaginated host membrane. It was therefore postulated that this motif may play a similar function in the delivery of oomycete effectors across the haustorial and plant cell membranes. Furthermore is has been shown that RxLR-containing sequences from P. infestans can mediate host translocation of proteins from Plasmodium into an erythrocyte (Bhattacharjee et. al., 2006).
(Adapted from Rehmany et. al., 2005)
Allen, R. L., Bittner-Eddy, P. D., Grenville-Briggs, L. J., Meitz, J. C., Rehmany, A. P., Rose, L. E., & Beynon, J. L. (2004). Host-parasite coevolutionary conflict between Arabidopsis and downy mildew Science (New York, NY), 306(5703), 1957–1960. doi:10.1126/science.1104022
Birch, P. R. J., Boevink, P. C., Gilroy, E. M., Hein, I., Pritchard, L., & Whisson, S. C. (2008). Oomycete RXLR effectors: delivery, functional redundancy and durable disease resistance Current opinion in plant biology, 11(4), 373–379. doi:10.1016/j.pbi.2008.04.005
Bhattacharjee, S., Hiller, N. L., Liolios, K., Win, J., Kanneganti, T.-D., Young, C., Kamoun, S., et al. (2006). The malarial host-targeting signal is conserved in the Irish potato famine pathogen PLoS pathogens, 2(5), e50. doi:10.1371/journal.ppat.0020050
Coates, M. E., & Beynon, J. L. (2010). Hyaloperonospora Arabidopsidis as a pathogen model Annual Review of Phytopathology, 48, 329–345. doi:10.1146/annurev-phyto-080508-094422
Kamoun, S. (2003). Molecular genetics of pathogenic oomycetes Eukaryotic cell, 2(2), 191–199.
Rehmany, A. P., Gordon, A., Rose, L. E., Allen, R. L., Armstrong, M. R., Whisson, S. C., Kamoun, S., et al. (2005). Differential recognition of highly divergent downy mildew avirulence gene alleles by RPP1 resistance genes from two Arabidopsis lines The Plant cell, 17(6), 1839–1850. doi:10.1105/tpc.105.031807