Botrytis cinerea is a necrotrophic fungus that causes the gray mold disease in many crops. Host resistance mechanisms to B. cinerea are poorly understood but they appear to be multigenic. In this study, our objective is to understand the molecular mechanisms of tomato defense responses to B. cinerea. It was previously shown that the Arabidopsis BOS1 and BIK1 genes encoding an R2R3MYB transcription factor and a Serine/Threonine kinase proteins, respectively, play key roles in mediating resistance to B. cinerea in Arabidopsis. We extended our study to determine the function of the tomato orthologs, SlBOS1 and SlBIK1, of the Arabidopsis genes. Our overall hypothesis is that SlBOS1 and SlBIK1 are regulatory components of the tomato pathogen response pathways and contribute to B. cinerea resistance. To test this, we generated transgenic Arabidopsis and tomato lines that under- or over-express the SlBIK1 or SlBOS1 genes. In addition, we used a genetic approach to determine the SlBIK1 residues that are required for disease resistance function and cellular localization. Finally, we tested the interaction between SlBOS1- and SlBIK1-mediated responses with other defense and non-defense pathways by utilizing tomato mutant lines impaired in JA/wound response pathways.
In general, SlBIK1 and SlBOS1 are induced by pathogens and oxidative stress, suggesting a role for both genes in biotic and abiotic stress tolerance. Interestingly, suppression of the expression of SlBOS1 gene through RNAi or VIGS resulted in enhanced fungal growth and disease symptoms when assayed for B. cinerea resistance. In addition, SlBOS1 RNAi plants show increased sensitivity to salt stress and reduced sensitivity of root growth inhibition by ABA, suggesting that SlBOS1 is required for ABA response. Tomato 35S:SlBOS1 seedlings were tolerant and survived under increased salt concentrations. However, 35S:SlBOS1 was not sufficient for increased resistance to B. cinerea. Our data implicate that SlBOS1 controls a crosstalk between biotic and abiotic stress responses.
Plants that have reduced levels of SlBIK1 expression show enhanced susceptibility to B. cinerea. SlBIK1 cDNA rescues the B. cinerea and A. brassicicola susceptibility of the Arabidopsis bik1 mutant suggesting that both tomato SlBIK1 and Arabidopsis BIK1 genes are functional homologs. In addition, SlBIK1 RNAi plants show enhanced susceptibility to tobacco hornworm (M. sexta). Our results also indicate that substitution of SlBIK1 in the conserved Gly residue in the N-myristylation and Tyr 246 in the kinase activation domains to Ala, failed to complement the Arabidopsis bik1 phenotype, suggesting that these residues are required for SlBIK1 function in disease resistance in planta. After challenge with B. cinerea, the SlBIK1 RNAi plants show altered expression of both the wound-responsive gene PI II, and the ET-regulated defense gene ACC synthase that suggest SlBIK1 function is required for expression of these genes and defense response to B. cinerea is mediated by JA/wound and ET signaling in tomato. SlBIK1 suppressed plants show defects in fruit structure and produce less seeds. Thus, SlBIK1 is required for both disease resistance and normal fruit and seed development.
The tomato mutants inhibited in wound response, jai1, spr2, acx1 and def1 but not spr1 are susceptible to B. cinerea, supporting the hypothesis that wound- and B. cinerea-response pathways overlap in tomato, but independent from systemin pathway. We also studied how SlBOS1 and SlBIK1 function in the tomato wound/insect response pathway mediated by JA. SlBIK1 but not SlBOS1 expression is induced by wounding and B. cinerea induces the wound response gene, PI II. Taken together, these data suggest that tomato and Arabidopsis share similar mechanisms of resistance to B. cinerea mediated by BOS1 and BIK1 although the molecular details of how they are regulated differ.