The Regulatory Mechanism Of Putrescine Induced DON Biosynthesis In Fusarium Graminearum During Its Infection On Wheat

Fusarium head blight(FHB)caused by the plant pathogen Fusarium graminearum is a devastating disease and results in substantial yield losses as well as serious mycotoxin contaminations.The Type B trichothecene deoxynivalenol(DON)is the most frequently detected mycotoxin in cereal grains worldwide.Besides its direct biological toxicity to mammalians,DON has also been reported to be an important virulence factor for F.graminearum,playing a critical role in the spread of the pathogen within host tissues.Importantly,F.graminearum can produce enormous amounts of DON during infection on wheat heads,whereas little is produced during vegetative growth.However,the corresponding mechanisms by which trigger DON accumulation during the F.graminearum infection are mostly lacking.In fungi,the clustered arrangement of secondary metabolite genes allows epigenetic modifications to efficiently modulate larger genomic regions in response to environment stresses.This regulatory model is also applicable to DON biosynthesis in F.graminearum.Recent studies revealed that both histone acetylation states and methylation states are important for promoting FgTRIs transcription and DON production.These promoted us to investigate the role of these and other epigenetic regulations in regulating host induced DON production in F.graminearum.In this study,we showed the regulation mechanism of DON biosynthesis during the F.graminearum infection on wheat using the phenotypic analysis,genetic transformation,molecular biological assayas.We found that:1)Wheat produced massive defence compound putrescine,which induced the hyper-transcription of DON biosynthesis genes(FgTRIs)and thus leading to DON accumulation in F.graminearum during infection.2)The fungal secondary nitrogen source utilization regulator FgAreA is necessary for putrescine-induced DON up-regulation.FgAreA enters into the nucleus in responding to putrescine,then facilitates the FgTRIs transcription and DON production.3)FgAreA regulated DON production upon putrescine via facilatating histone H2 B mono-ubiquitination(H2B ub1)enrichment on FgTRIs.Importantly,we identified a specific DNA-binding domain basic region leucine zipper(bZIP)domain within H2 B ub1 E3 ligase FgBre1 that was required for the recognition of FgTRIs genes.Moreover,the binding of FgBre1 to FgTRIs relied on FgAreA-mediated chromatin rearrangement on FgTRIs.Thus,FgAreA enters into the nucleus in responding to putrescine,and then facilitates H2 B ub1 enrichment on FgTRIs,consequently resulting in up-regulation of FgTRIs transcription and DON production.4)Under putrescine treatment,H2 B ub1 enhanced histone 3 lysine 4 di-,tri-methylation(H3K4 me2/me3)through strengthening the interaction of FgBre2 and FgSet1 in the H3K4 methylation complex COMPASS,thus co-operated with H3K4 me to modulate FgTRIs transcription and DON biosynthesis.H3K4 me is regulated by the methyltransferase complex COMPASS that consists of seven subunits FgSet1,FgBre2,FgSwd1,FgSpp1,FgSwd2,FgSwd3,FgSdc1.Western blotting assays showed that FgSet1,FgSwd1,FgSwd3,FgSdc1 and FgBre2 were COMPASS key components involved in H3K4 me generation,DON production and FgTRIs transcription.Besides,H2 B ub1 occurance is the prerequisite of H3K4 me2/me3.H2 B ub1 conjugating enzyme Fg Rad6 and ligase FgBre1 are able to recruite FgBre2 to target chromosome region to interact with FgSet1,subsequently induces H3K4 me2/me3.5)We analysed the biological functions of H2 B ub1 in F.graminearum.H2 B ub1 is important for hyphae growth,DON biosynthesis,FgTRIs transcription,cell wall damage responses via its role in H3K4 me regulation.Additionally,it is also able to regulate oxidative damage responses via its control in histone 3 lysine 79 methylation(H3K79 me).Taken together,our findings uncover that two histone modifications together regulate mycotoxin production upon putrescine during infection,which broaden our understanding on transcriptional regulation of fungal secondary metabolites as well as the roles of defensive compounds produced by host during phytopathogen-host interaction.