AvrPm2 encodes an RNase‐like avirulence effector which is conserved in the two different specialized forms of wheat and rye powdery mildew fungus

Abstract

Summary There is a large diversity of genetically defined resistance genes in bread wheat against the powdery mildew pathogen Blumeria graminis (B. g.) f. sp. tritici. Many confer race‐specific resistance to this pathogen, but until now only the mildew avirulence gene AvrPm3 a2/f2 that is recognized by Pm3a/f was known molecularly.We performed map‐based cloning and genome‐wide association studies to isolate a candidate for the mildew avirulence gene AvrPm2. We then used transient expression assays in Nicotiana benthamiana to demonstrate specific and strong recognition of AvrPm2 by Pm2.The virulent AvrPm2 allele arose from a conserved 12 kb deletion, while there is no protein sequence diversity in the gene pool of avirulent B. g. tritici isolates. We found one polymorphic AvrPm2 allele in B. g. triticale and one orthologue in B. g. secalis and both are recognized by Pm2. AvrPm2 belongs to a small gene family encoding structurally conserved RNase‐like effectors, including Avr a13 from B. g. hordei, the cognate Avr of the barley resistance gene Mla13.These results demonstrate the conservation of functional avirulence genes in two cereal powdery mildews specialized on different hosts, thus providing a possible explanation for successful introgression of resistance genes from rye or other grass relatives to wheat.

Distinct domains of the AVRPM3A2/F2 avirulence protein from wheat powdery mildew are involved in immune receptor recognition and putative effector function

“…We used the previously validated AVRPM3 A2/F2 ‐A as the positive control for HR induction upon recognition by PM3A and PM3F L456P/Y458H . By visual inspection and fluorescence imaging (Praz et al ., ) of the assayed N. benthamiana leaves at 5 d post‐infiltration, we observed that none of the new variants induced HR in the presence of PM3A or PM3F L456P/Y458H , suggesting that they encode inactive AVR proteins (Fig. S1).…”

“…Another possibility is that AvrPm3 a2/f2 is recognized by another NLR in wheat. Although evidence from our genetic mapping populations indicates that AvrPm3 a2/f2 is not recognized by Pm1a , Pm2 , Pm3b‐e , Pm3g , Pm4a , Pm4b , Pm5a , Pm8 and Pm17, we cannot exclude possible recognition by NLRs other than those we have tested (Parlange et al ., ; Bourras et al ., ; Praz et al ., ; Notes S1; Tables S4–S6). In isolates encoding the recognized AvrPm3 a2/f2 ‐A haplotype, we observed that 71% (54/76) were avirulent on Pm3a wheat, and 17 of these (22%) were also avirulent on Pm3f wheat (Table ).…”

“…All isolates from Europe, Japan and Australia, and a subset of isolates from the USA, China and Israel, were maintained in‐house. Spore samples for DNA extraction and sequencing by PCR were obtained primarily for US and Israeli isolates, whereas complete genomes for a subset of Chinese isolates were used for in silico extraction (Praz et al ., ). European, Japanese and Australian isolates were collected in the field and single spore isolated twice, as described by Brown & Wolfe ().…”

“…At least three replications were performed and the HR was visualized as described by Praz et al . (). HR intensity was calculated using the mean gray value estimated by ImageJ (Schneider et al ., ) and normalizing against the noninfiltrated background.…”

“…Quantitative real‐time PCR experiments were performed as described in Praz et al . () with the modification of a 20‐s extension time. Three independent biological replicates were sampled at 2 d after inoculation.…”