A chromosome-scale genome assembly of the flax rust fungus reveals the two unusually large effector proteins, AvrM3 and AvrN

Rust fungi comprise thousands of species many of which cause disease on important crop plants. The flax rust fungus Melampsora lini has been a model species for the genetic dissection of plant immunity since the 1940s, however the highly fragmented and incomplete reference genome has so far hindered progress in effector gene discovery. Here, we generate a fully-phased, chromosome-scale assembly of the two nuclear genomes of M. lini strain CH5, resolving an additional 320 Mbp of sequence compared to the previous short-read assembly. The 482 Mbp dikaryotic genome is at least 79% repetitive with a large proportion ([~]40%) of the genome comprised of young, highly similar transposable elements. The assembly resolves the known effector gene loci some of which carry complex duplications that were collapsed in the previous assembly. Using a genetic map followed by manual correction of gene models, we identify the AvrM3 and AvrN genes which encode unusually large fungal effector proteins and trigger defense responses when co-expressed with the corresponding resistance genes in Nicotiana tabacum. We locate the genes linked to the tetrapolar mating system on chromosomes 4 and 9, but in contrast to the cereal rusts which have one pheromone receptor gene per haplotype, in flax rust three pheromone receptor genes are found with two of them closely linked on one haplotype. Taken together, we show that a high-quality assembly is crucial for resolving complex gene loci and given the increasing number of fungal effectors of large size, the commonly applied criterion for effector candidates as being small proteins as needs to be reconsidered.