Multiple Avirulence Loci and Allele-Specific Effector Recognition Control thePm3Race-Specific Resistance of Wheat to Powdery Mildew

Abstract

In cereals, several mildew resistance genes occur as large allelic series; for example, in wheat (Triticum aestivum and Triticum turgidum), 17 functional Pm3 alleles confer agronomically important race-specific resistance to powdery mildew (Blumeria graminis). The molecular basis of race specificity has been characterized in wheat, but little is known about the corresponding avirulence genes in powdery mildew. Here, we dissected the genetics of avirulence for six Pm3 alleles and found that three major Avr loci affect avirulence, with a common locus_1 involved in all AvrPm3-Pm3 interactions. We cloned the effector gene AvrPm3 a2/f2 from locus_2, which is recognized by the Pm3a and Pm3f alleles. Induction of a Pm3 alleledependent hypersensitive response in transient assays in Nicotiana benthamiana and in wheat demonstrated specificity. Gene expression analysis of Bcg1 (encoded by locus_1) and AvrPm3 a2/f2 revealed significant differences between isolates, indicating that in addition to protein polymorphisms, expression levels play a role in avirulence. We propose a model for race specificity involving three components: an allele-specific avirulence effector, a resistance gene allele, and a pathogenencoded suppressor of avirulence. Thus, whereas a genetically simple allelic series controls specificity in the plant host, recognition on the pathogen side is more complex, allowing flexible evolutionary responses and adaptation to resistance genes.

Hybridization of powdery mildew strains gives rise to pathogens on novel agricultural crop species

“…tritici, which has a length of approximately 1,800 cM (ref. 28). Thus, one expects one recombination event every 10 Mb in a sexual cycle.…”

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

“…tritici . The AvrPm3 a2/f2 gene belongs to a family of 24 sequence‐divergent, but structurally related, secreted effectors (Bourras et al ., ). In transient expression assays in Nicotiana benthamiana , it was demonstrated that AVRPM3 A2/F2 is recognized specifically by the Pm3a and Pm3f alleles.…”

“…spp. ), specifically the Pm3 gene in wheat ( Triticum aestivum ) and the Mla gene in barley ( Hordeum vulgare ), suggest that distinct resistance alleles recognize highly sequence diverse effectors (Bourras et al ., ; Lu et al ., ).…”

LRX Proteins Play a Crucial Role in Pollen Grain and Pollen Tube Cell Wall Development

“…Tobacco infiltration with Agrobacteria (strain GV3103) containing the pBART-560 p35S::LRR11-Citrine construct was performed as described (Bourras et al, 2015).…”