Abstract
During plant infection, fungi secrete effector proteins in coordination with distinct infection stages. Thus, the success of plant infection is determined by precise control of effector gene expression. We analysed the PWL2 effector gene of the rice blast fungus Magnaporthe oryzae to understand how effector genes are activated specifically during the early biotrophic stages of rice infection. Here, we used confocal live-cell imaging of M. oryzae transformants with various PWL2 promoter fragments fused to sensitive green fluorescent protein reporter genes to determine the expression patterns of PWL2 at the cellular level, together with quantitative reverse transcription PCR analyses at the tissue level. We found PWL2 expression was coupled with sequential biotrophic invasion of rice cells. PWL2 expression was induced in the appressorium upon penetration into a living rice cell but greatly declined in the highly branched hyphae when the first-invaded rice cell was dead. PWL2 expression then increased again as the hyphae penetrate into living adjacent cells. The expression of PWL2 required fungal penetration into living plant cells of either host rice or nonhost onion. Deletion and mutagenesis experiments further revealed that the tandem repeats in the PWL2 promoter contain 12-base pair sequences required for expression. We conclude that PWL2 expression is (a) activated by an unknown signal commonly present in living plant cells, (b) specific to biotrophic stages of fungal infection, and (c) requires 12-base pair cis-regulatory sequences in the promoter.