Presence of vitamin B$_{12}$ metabolism in the last common ancestor of land plants
Résumé
Vitamin B$_{12}$, or cobalamin, (hereinafter B$_{12}$) is an essential organic cofactor for methionine synthase (METH) that is only synthesised by a subset of bacteria. Plants and fungi have an alternative methionine synthase (METE) that does not need B$_{12}$, and are typically considered not to utilise it. Some algal species (e.g., Chlamydomonas) facultatively utilise B12 because they encode both METE and METH, while others are dependent on B$_{12}$ as they encode METH only. We performed phylogenomic analyses of METE, METH and eleven further B$_{12}$ metabolism proteins across over 1,600 plant and algal genomes and transcriptomes (e.g., from OneKp), demonstrating the presence of B$_{12}$-associated metabolism deep into the streptophytes. METH, and five further accessory proteins (MTRR, CblB, CblC, CblD and CblJ) were detected in the hornworts (Anthocerophyta), and two (CblB and CblJ) were identified in liverworts (Marchantiophyta) in the bryophytes, suggesting a retention of B$_{12}$-metabolism in the last common land plant ancestor. Our data further show more limited distributions for other B$_{12}$-related proteins (MCM, RNR-II), and B$_{12}$-dependency in several algal orders. Finally, considering the collection sites of algae that have lost B$_{12}$ metabolism, we propose freshwater-to-land transitions and symbiotic associations to have been constraining factors for B$_{12}$ availability in early plant evolution.
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