Final Activity Report Summary - MILDEWAVRGENES (Evolution of elicitor (avirulence) genes in powdery mildew fungi. (Diversity and evolution of avirulence in powdery mildews)) The fungus 'blumeria graminis f.sp. hordei' (Bgh) is the causal agent of powdery mildew of barley, a highly damaging disease. Plant resistance genes (R) are widely used to control powdery mildews, but these are rapidly overcome by parasite mutants that evade recognition. It is not known how this rapid evolution occurs without apparent loss of parasite fitness (Bronson & Ellingboe, 1986, Brown, 2003). Interactions between Bgh and barley are governed in a gene-for-gene manner (Flor, 1971), such that a barley plant is only resistant to Bgh genotypes with the corresponding avirulence (Avr) gene. Over 25 independent Avr genes have been described in Bgh isolates (Brown; Jessop, 1995, Jensen et al., 1995) and we recently isolated AVRa10 and AVRk1 (Ridout et al., 2006). We showed that they induced resistance responses when expressed in host cells of Mla10 and Mlk1 barley varieties, and that they also enhanced infection success on susceptible host plant cells. AVRa10 and AVRk1 were paralogues and belonged to a large gene family. The findings implied that the mildew fungus had a repertoire of avirulence genes, which could function as effectors and contribute to parasite virulence. Multiple copies of AVR-effector genes might enable populations of Bgh to rapidly overcome host R genes whilst maintaining virulence. The objective of this work was to obtain and analyse the sequences of other members of the AVR family in different isolates and formae speciales (ff. spp), providing information about host adaptation and parasite evolution. The results of my work revealed that: 1. the mildew AVR family was very large and highly conserved. There was a high conservation of the family between the members of the same genome, from different isolates and from different ff. spp, which possibly reflected their important function as effectors. The regions flanking the core of the protein sequence were less conserved, which might indicate adaptation to evade recognition by R proteins in the host. There was a close association between AVRk1-like sequences and the retrotransposon CgT1 sequence in the genomes of both Bgh and Blumeria graminis f.sp. tritici (Bgt). Retrotransposons in both bacterial and fungal genomes were believed to contribute to effector and AVR gene expansion and diversification (Gout et al., 2006, Rohmer et al., 2004, Skamnioti; Ridout, 2005). 2. AVR genes could be related to the adaptation of B.g to different crops. The AVR family was present in other ff. spp of Bg, but not in other powdery mildew species. AVRa10 -like genes existed before the specialisation of b. graminis onto different cereal hosts. The absence of AVRk1-like genes in the f. sp avenae suggested that the proliferation of this subfamily occurred after the specialisation of B.g. on oat. Also, the topology of the phylogenetic tree indicated that the isolation of the ff. spp secali, tritici and agropyri from the common ancestor with hordei was synchronised with the proliferation of the AVRk1 subfamily. This work illustrated the evolution of crop-pathogen interactions in the domestication of cereal crops. The results helped us to understand how the parasite could adapt to new crop varieties, thus they could potentially help to develop new strategies for breeding.
