Ave1-like orthologs in Venturia: another expanded effector family emerges

Effectors are secreted by pathogens to modify plant physiology and establish disease. Plant immune receptors have evolved to recognise effectors and counter attack with defence responses. Most fungal effectors are lineage-specific, i.e. they are unique to a species, or to physiological races within a species. The availability of many whole genome sequences has revealed that some effectors are found in a discontinuous distribution within the fungal kingdom; a few phytopathogenic fungi (Colletotrichum higginsianum, Cercospora beticola, Fusarium oxysporum) possess an ortholog of Ave1 from Verticillium dahliae, an effector that activates Ve1-mediated resistance in tomato. A subset of these orthologs were shown to activate Ve1-mediated resistance in tomato. Unusually, Ave1 also shares similarity to an ortholog in the phytopathogenic bacterium Xanthomonas axonopodis, as well as to a widespread family of plant natriuretic peptides and expansins, involved in plant homeostasis and plant cell wall modification (de Jonge & van Esse et al. 2012). We have identified an expanded Ave1-like gene family in apple and pear scab fungi, Venturia inaequalis and V. pirina. These species also have expanded gene families with similarity to the Leptosphaeria maculans effector AvrLm6. V. pirina has 14 unique hits (best,1.43e-18) to VdAve1. V. inaequalis has 17 unique hits (best,1.07e-22) to VdAve1. The distribution of Ave1 orthologs is suggestive of one or more cross-kingdom gene transfer events. We are characterising Venturia Ave1-like genes to investigate the mode of gene multiplication; seek evidence of horizontal gene transfer; and determine the role of Ave1-like genes in pathogenicity. Ave1-like genes from non-Venturia fungi (and the bacterial gene) do not contain predicted introns, however, several (not all) V. inaequalis Ave1-like genes are predicted to contain introns. Codon usage bias among fungal, plant, and bacterial Ave1 orthologs, are being compared with the aim of inferring the kingdom of origin of the Venturia Ave1 orthologs. At least two ViAve1 orthologs are up-regulated during infection of apple. To determine whether the Venturia Ave1 proteins also activate a Ve1-mediated hypersensitive response, each has been co-expressed with tomato Ve1 in Nicotiana tabacum, using an Agrobacterium tumefaciens-mediated transient transformation assay.