| Fumonisins and AAL-toxins are classified as sphinganine analogy mycotoxins(SAMTs),which competitively inhibit ceramide synthase,leading to accumulation of free sphingoid bases,thereby causing programmed cell death.Fumonisins are produced by specific Fusarium species,which are common fungal pathogens of maize,and AAL-toxin is the main virulence effector of Alternaria alternata f.sp.lycopersici(AAL),causing stem canker on tomato.In addition,both types of mycotoxins pose a serious threat on animal and human health.Thus,clarification of the mechanism in plant defense against SAMT and SAMT-producing pathogens and identification of key regulatory genes,not only deepens our understanding of plant defense mechanism,but also help us to control the diseases caused by SAMT-producing pathogens and SAMT contamination on crops.On the basis of identification of fumonisin B1 resistant 41(FBR41),we further investigated the role and mechanism of FBR41 in plant defense against SAMT and SAMT-producing pathogens in the present study.The role of brassinosteroid(BR)signaling pathway in tomato defense against AAL-toxin and AAL were also explored.1.fbr41 mutant did not display macroscopic PCD phenotype upon FBI treatment.FBR41 contains a mutation in AtLCB2b,which encodes an essential subunit of SPT.Sequence alignment of amino acid sequence between FBR41 and AtLCB2b showed that lysine residue,which was essential for SPT function,was absent in FBR41;Luciferase complementation assay confirmed the interaction between FBR41 polypeptide and AtLCB1 subunit.Further analysis of the downstream products of SPT showed that free sphingoid bases,effective inducers of PCD,were attenuated upon FB1 treatment in fbr41.Above results suggested that,FBR41 might affect SPT function,attenuating FBI-induced accumulation of free sphingoid bases,thus enhancing the resistance of Arabidopsis to FB1.2.Ectopic overexpression of FBR41 in susceptible tomato cultivars was conducted to investigate the role of FBR41 in tomato resistance to AAL-toxin.FBR41-overexpressing tomato plants displayed less severe symptom upon AAL-toxin,suggesting a positive role of FBR41 in inhibition of SAMT-induced PCD across phylogenetically distant plants.Reduced level of free sphenoid bases was observed in FBR4/-overexpressing tomato plants after AAL-toxin treatment,demonstrating that FBR41 could attenuated AAL-toxin-induced accumulation of free sphingoid bases,thereby improving the resistance of tomato to AAL-toxin.3.Less apoptotic cell death and reduced fungal biomass were observed in FBR41－overexpressing tomato plants after AAL inoculation,suggesting that FBR41 enhanced tomato resistance to AAL.Free sphingoid bases are involved in cell death induction and jasmonate(JA)has a promotional effect on AAL invasion.FBR41-overexpressing tomato plants accumulated less free sphingoid bases and jasmonate(JA)upon AAL treatment,indicating that FBR41 enhanced the resistance of tomato to AAL through inhibition ofjasmonate(JA)biosynthesis to prevent pathogen invasion and attenuation of free sphingoid base accumulation to suppress cell death.4.P234 site in AtBZR1 was conserved in SlLAT61(P239)and SlBZR1L(P232).Overexpressing of SlLAT61P239L(mSlLAT61)and SIBZR1LP232L(mSlBZR1L)in tomato resulted in enhanced BR signaling,including suppressed expression levels of BR biosynthesis genes,reduced plant size and curly leaves.mSlLAT61 and mSIBZR1L overexpressing tomato plants displayed enhanced susceptibility to AAL-toxin and AAL,suggesting that SILAT61/SlBZR1L-mediated BR signaling pathway compromised the resistance of tomato to AAL-toxin and AAL. |
