Reactive oxygen species and autophagy play a role in survival and differentiation of the phytopathogen Moniliophthora perniciosa

Abstract

The hemibiotrophic basidiomycete Moniliophthora perniciosa causes “witches’ broom disease” in cacao ( Theobroma cacao). During plant infection, M. perniciosa changes from mono to dikaryotic life form, an event which could be triggered by changes in plant nutritional offer and plant defense molecules, i.e., from high to low content of glycerol and hydrogen peroxide. We have recently shown that in vitro glycerol induces oxidative stress resistance in dikaryotic M. perniciosa. In order to understand under which conditions in parasite-plant interaction M. perniciosa changes from intercellular monokaryotic to intracellular dikaryotic growth phase we studied the role of glycerol on mutagen-induced oxidative stress resistance of basidiospores and monokaryotic hyphae; we also studied the role of H 2O 2 as a signaling molecule for in vitro dikaryotization and whether changes in nutritional offer by the plant could be compensated by inducible fungal autophagy. Mono-/dikaryotic glycerol or glucose-grown cells and basidiospores were exposed to the oxidative stress-inducing mutagens H 2O 2 and Paraquat as well as to pre-dominantly DNA damaging 4-nitroquinoline-1-oxide and UVC irradiation. Basidiospores showed highest resistance to all treatments and glycerol-grown monokaryotic hyphae were more resistant than dikaryotic hyphae. Monokaryotic cells exposed to 1 μM of H 2O 2 in glycerol-media induced formation of clamp connections within 2 days while 1 mM H 2O 2 did not within a week in the same medium; no clamp connections were formed in H 2O 2-containing glucose media within a week. Lower concentrations of H 2O 2 and glycerol, when occurring in parallel, are shown to be two signals for dikaryotization in vitro and may be also during the course of infection. Q-PCR studies of glycerol-grown dikaryotic cells exposed to oxidative stress (10 mM H 2O 2) showed high expression of Mp SOD2 and transient induction of ABC cytoplasmic membrane transporter gene Mp YOR1 and autophagy-related gene Mp ATG8. Expression of a second ABC transporter gene Mp SNQ2 was 14-fold induced after H 2O 2 exposure in glucose as compared to glycerol-grown hyphae while Mp YOR1 did not show strong variation of expression under similar conditions. Glucose-grown dikaryotic cells showed elevated expression of Mp ATG8, especially after exposure to H 2O 2 and 4-nitroquinoline-1-oxide. During different stages preceding basidiocarp formation Mp ATG8 and the two catalase-encoding genes Mp CTA1 and Mp CTT1 were expressed continuously. We have compiled our results and literature data in a model graph, which compares the in vitro and in planta development and differentiation of M. perniciosa with the help of physiological and morphological landmarks.