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Wheat stem rust effectors: Genomics and functional assay

Puccinia graminis f. sp. tritici (Pgt) is one of the most destructive pathogens of wheat. Fungal secreted proteins termed effectors play an important role in modulating the host cellular environment and suppressing the plant defense response to enable fungal growth. They also become targets of plant resistance (R) proteins. We have taken a genomics approach to initially identify candidate effectors. We have built a draft genome for a founder Australian Pgt isolate of pathotype (pt.) 21-0 (collected in 1954) by next generation DNA sequencing. A combination of reference-based assembly using the genome of the previously sequenced North American Pgt isolate CDL 75-36-700-3 (p7a) and de novo assembly resulted in a 92 Mbp reference genome for Pgt isolate 21-0. This draft genome was subsequently used to build a pan-genome based on five Australian Pgt isolates. Transcriptomes from germinated urediniospores and haustoria were separately assembled for pt. 21-0 and comparison of gene expression profiles showed differential expression in ~10% of the genes in germinated urediniospores as well as haustoria. A total of 1,924 secreted proteins were predicted from the 21-0 transcriptome, of which 586 were classified as haustorial secreted proteins (HSPs). We are currently exploring effector gene expression during infection of wheat to reduce this candidate list based on a common expression profile identified for Avr genes in the flax rust fungus. Comparison of 21-0 with two presumed clonal field derivatives (collected in 1982 and 1984) that had evolved virulence on four additional resistance genes (Sr5, Sr11, Sr27, SrSatu) identified mutations in 13 HSP effector candidates. These candidate effectors are being assessed for recognition in wheat accessions with the corresponding R genes using a bacterial type three secretion delivery system based on an engineered Pseudomonas fluorescence strain (Upadhyaya NM et al. Mol Plant Microbe Interact 27:255-264).

CSIRO Agriculture, Canberra, Australia
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The effector repertoire of Puccinia striiformis subdues host-plant immunity

Rust outbreaks cause severe yield lossespose a serious threat to global food security. As biotrophic pathogens, rust fungi produce effectors to suppress host immunity. In this study, we used a systemic approach to identify and characterize effectors in Puccinia striiformis f. sp. tritici. Among secreted proteins encoded by the Pst genome, we identified 150 putative effectors that are Cys-rich or up-regulated during hostinfection. A systematic screen showed that 14 of them suppressed programmed cell death (PCD) triggered by BAX and INF1 in Nicotiana benthamiana, and all have high intra- and inter-species polymorphisms at the protein level. Although these 14 effectors individually made only minor contributions to Pst virulence, delivery of them into wheat plants via a bacterial type-III secretion system efficiently suppressed PTI and ETI. Interestingly, three of them, Pst_4941, Pst_4884, and Pst_5578, triggered HR-like PCD in the AvSYr1NIL, but only Pst_4941 also caused PCD in the AvSYr7 NIL, suggesting that they may function as avirulence factors. This study suggests that Pst effectors function to suppress PTI and ETI.

State Key Laboratory of Crop Stress Biology for Arid Areas and College of Agronomy, Northwest A&F University, P. R. China
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