Chloroplast translation in Marchantia and its regulation by light
Barkan lab, University of Oregon.Translation of the chloroplast psbA mRNA in angiosperms is dynamically regulated by light. psbA translation initiation is triggered by the light-induced damage of its gene product, the D1 reaction center protein of photosystem II (PSII). This regulatory circuit tunes the rate of D1 synthesis to D1 photodamage, providing nascent D1 as needed for PSII repair. Several proteins that have been implicated in chlorophyll/D1 transactions during PSII assembly and repair are involved in this regulation. To address whether the light-induced chlorophyll synthesis that is characteristic of angiosperms is relevant to the regulatory mechanism, we examined the effects of light-dark shifts on chloroplast mRNA ribosome occupancies in the liverwort Marchantia polymorpha, which synthesizes chlorophyll in both the dark and the light. As in angiosperms, ribosome occupancy on psbA mRNA decreased rapidly upon shifting plants to the dark, was rapidly restored upon a transfer back to the light, and ribosome occupancy on other chloroplast mRNAs did not change substantively during this light shift regime. This behavior was similar in a Marchantia mutant defective in the chloroplast chlB gene, required for the synthesis of chlorophyll in the dark. These findings show that patterns of light-regulated chloroplast translation are conserved among vascular and non-vascular plants, and suggest that light-induced chlorophyll synthesis is not relevant to light-regulated psbA translation. Additionally, our data support synthetic biology approaches in Marchantia by providing a plastome-wide view of translational efficiency and translational output.