| Summary: | A plant's response to pathogen infection is tailored dependent on infection strategy. Successful plant pathogens employ various infection strategies to avoid or reduce plant defense responses for the establishment of host compatibility. Autophagy is a non-selective degradation pathway conserved in eukaryotic organisms, which has been implicated in the regulation of cell survival or cell death, depending on cell type and stimulus. In <i>Arabidopsis thaliana</i>, an autophagic response has been reported to be activated during nutrient deprivation. Cellular contents, such as cytoplasm and organelles, are sequestered into double-membraned autophagosomes and delivered to the vacuole for degradation; degradative products, such as amino acids, are released back into the cell and reutilized to maintain cellular function. In this study, the response of the autophagy pathway was investigated in <i>A. thaliana</i> leaf tissues upon biotrophic <i>Erysiphe cichoracearum</i> and hemibiotrophic <i>Colletotrichum higginsianum</i> infections. Expression of some autophagy genes was induced in <i>A. thaliana</i> at 9 days post infection with <i>E. cichoracearum</i> and, 3 and 5 days post infection with <i>C. higginsianum</i>. Using a transgenic <i>A. thaliana</i> plant line over expressing autophagosome associated protein autophagy-8e (<i>ATG8e</i>) conjugated to green fluorescent protein (GFP) (<i>ATG8e-GFP</i>), confocal analysis revealed that autophagosomes specifically accumulated at the infection sites during <i>E. cichoracearum</i> and <i>C. higginsianum</i> invasions. These results indicate that the plant autophagic pathway responds to an interaction between <i>A. thaliana</i> and fungal pathogens. None of the defense signaling molecules including salicylic acid, jasmonic acid, ethylene, hydrogen peroxide and nitric oxide consistently triggered expression of autophagy genes. The insensitivity to defense signaling molecules and the delayed induction of autophagy genes compared to expression of pathogenesis-related genes suggest that the activation of this pathway does not contribute to host resistance responses during the infection process. In <i>A. thaliana</i> mutants, <i>atg4a/b, atg5-1, atg9-1</i> and <i>atg9-6</i> deficient for the autophagic response, virulence of <i>E. cichoracearum</i> was retarded whereas pathogenesis of <i>C. higginsianum</i> was accelerated. Taken together, these data suggest that the autophagy pathway is a potential host susceptibility factor for pathogen infection, possibly involved in establishing/facilitating biotrophy in <i>A. thaliana</i>.
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