Abstract

We use the model liverwort Marchantia polymorpha to understand fundamental questions of evolutionary biology. One of the advantages of this model system is its genome presents low redundancy of transcription factor genes compared to other plant species. During my PhD thesis, I studied the molecular evolution of the class I homeodomain leucine-zipper (C1HDZ) in Marchantia polymorpha. In flowering plants, C1HDZ transcription factor function is primarily associated with abiotic stress responses. However, loss-of-function alleles of MpC1HDZ (Mpc1hdzge) did not exhibit phenotypes associated with abiotic stress. Rather, Mpc1hdzge mutant plants present a reduced number of oil body cells as the most remarkable developmental defect. Oil bodies, a synapomorphy of liverworts, are organelles in specialized cells that accumulate secondary metabolites, but their function and development are poorly understood. Using these plants, we interrogated the biological function of oil bodies. Briefly, we found Mpc1hdzge are more susceptible to herbivory and cannot accumulate terpenoid compounds. Toxic compounds accumulated in oil bodies are an important defence strategy against herbivores, an analogous function to glandular trichomes in flowering plants. In addition, C1HDZ genes were co-opted to regulate separate responses to biotic and abiotic stressors in two distinct land plant lineages. Our findings and other works in the field suggest that the expression pattern of transcription factor expression is key in their functional macroevolution.

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