AUTHOR=Bourras Salim , McNally Kaitlin E. , Müller Marion C. , Wicker Thomas , Keller Beat 

TITLE=Avirulence Genes in Cereal Powdery Mildews: The Gene-for-Gene Hypothesis 2.0

JOURNAL=Frontiers in Plant Science

VOLUME=7

YEAR=2016

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2016.00241

DOI=10.3389/fpls.2016.00241

ISSN=1664-462X

ABSTRACT=<p>The gene-for-gene hypothesis states that for each gene controlling resistance in the host, there is a corresponding, specific gene controlling avirulence in the pathogen. Allelic series of the cereal mildew resistance genes <italic>Pm3</italic> and <italic>Mla</italic> provide an excellent system for genetic and molecular analysis of resistance specificity. Despite this opportunity for molecular research, avirulence genes in mildews remain underexplored. Earlier work in barley powdery mildew (<italic>B.g. hordei</italic>) has shown that the reaction to some <italic>Mla</italic> resistance alleles is controlled by multiple genes. Similarly, several genes are involved in the specific interaction of wheat mildew (<italic>B.g. tritici</italic>) with the <italic>Pm3</italic> allelic series. We found that two mildew genes control avirulence on <italic>Pm3f</italic>: one gene is involved in recognition by the resistance protein as demonstrated by functional studies in wheat and the heterologous host <italic>Nicotiana benthamiana</italic>. A second gene is a suppressor, and resistance is only observed in mildew genotypes combining the inactive suppressor and the recognized <italic>Avr</italic>. We propose that such suppressor/avirulence gene combinations provide the basis of specificity in mildews. Depending on the particular gene combinations in a mildew race, different genes will be genetically identified as the “avirulence” gene. Additionally, the observation of two LINE retrotransposon-encoded avirulence genes in <italic>B.g. hordei</italic> further suggests that the control of avirulence in mildew is more complex than a canonical gene-for-gene interaction. To fully understand the mildew–cereal interactions, more knowledge on avirulence determinants is needed and we propose ways how this can be achieved based on recent advances in the field.</p>