Cell Death Control: The Interplay of Apoptosis and Autophagy in the Pathogenicity of Sclerotinia sclerotiorum

Abstract

Programmed cell death is characterized by a cascade of tightly controlled events that culminate in the orchestrated death of the cell. In multicellular organisms autophagy and apoptosis are recognized as two principal means by which these genetically determined cell deaths occur. During plant-microbe interactions cell death programs can mediate both resistant and susceptible events. Via oxalic acid (OA), the necrotrophic phytopathogen Sclerotinia sclerotiorum hijacks host pathways and induces cell death in host plant tissue resulting in hallmark apoptotic features in a time and dose dependent manner. OA-deficient mutants are non-pathogenic and trigger a restricted cell death phenotype in the host that unexpectedly exhibits markers associated with the plant hypersensitive response including callose deposition and a pronounced oxidative burst, suggesting the plant can recognize and in this case respond, defensively. The details of this plant directed restrictive cell death associated with OA deficient mutants is the focus of this work. Using a combination of electron and fluorescence microscopy, chemical effectors and reverse genetics, we show that this restricted cell death is autophagic. Inhibition of autophagy rescued the non-pathogenic mutant phenotype. These findings indicate that autophagy is a defense response in this necrotrophic fungus/plant interaction and suggest a novel function associated with OA; namely, the suppression of autophagy. These data suggest that not all cell deaths are equivalent, and though programmed cell death occurs in both situations, the outcome is predicated on who is in control of the cell death machinery. Based on our data, we suggest that it is not cell death per se that dictates the outcome of certain plant-microbe interactions, but the manner by which cell death occurs that is crucial.

Aspartyl Protease-Mediated Cleavage of BAG6 Is Necessary for Autophagy and Fungal Resistance in Plants

“…We observed similar results with S. sclerotiorum where the fungal secreted oxalic acid induces cell death with apoptotic-like features in the host to achieve pathogenic success (Kim et al, 2008). Conversely, mutants that are deficient in oxalic acid production trigger a defense response within the host that includes autophagic PCD (Kabbage et al, 2013). However, we cannot rule out that autophagy as a pro-survival mechanism keeps cells alive in both cases, thus restricting fungal induced cell death and pathogenic development (Munch et al, 2014).…”

“…A key question is how is complex formation linked to protease cleavage and ultimately to resistance? Previous reports have linked autophagy to basal resistance to necrotrophs (Lai et al, 2011;Lenz et al, 2011;Kabbage et al, 2013). Blocking autophagy inhibits such resistance and results in greatly enhanced susceptibility.…”

“…This potential dual function of autophagy (pro-death versus pro-survival) is intriguing but not entirely understood. There is evidence for both outcomes (Hofius et al, 2011;Kabbage et al, 2013), and further work is required to explain the underpinning mechanisms that contrast pro-survival and pro-death regimes. As we discussed previously and substantiated here, we suggest that it is not cell death as such that is key, but rather the mechanism by which cell death occurs that is critical to the outcome of a given plant-pathogen interaction.…”

“…Fluorescence was visualized using an Olympus IX81 inverted fluorescence confocal microscope with excitation and emission wavelengths of 335 and 508 nm, respectively. For TEM, leaf segments were fixed for 5 h at 37°C in 3% glutaraldehyde and 2% formaldehyde in 0.1 M Na cacodylate buffer and then processed according to standard protocols as described previously (Kabbage et al, 2013). For fluorescent protein detection, samples were directly observed by confocal microscopy under the following conditions: RFP, excitation at 561 nm and detection at 580 to 630 nm; YFP, excitation at 495 nm and detection at 510 to 550 nm.…”

“…To cope with both biotic and abiotic environmental pressures, plants have developed a number of defense strategies (Jones and Dangl, 2006). Recent studies have established that control of programmed cell death (PCD) pathways can be an important component that controls the outcome of a given stress response in plants (Dickman et al, 2001;Doukhanina et al, 2006;Lord and Gunawardena, 2012;Kabbage et al, 2013). Apoptosis and autophagy are two principal forms of PCD broadly studied in eukaryotes.…”