Presence of vitamin B12 metabolism in the last common ancestor of land plants.

Vitamin B₁₂, also known as cobalamin, is an essential organic cofactor for methionine synthase (METH), and is only synthesized by a subset of bacteria. Plants and fungi have an alternative methionine synthase (METE) that does not need B₁₂ and are typically considered not to utilize it. Some algae facultatively utilize B₁₂ because they encode both METE and METH, while other algae are dependent on B₁₂ as they encode METH only. We performed phylogenomic analyses of METE, METH and 11 further proteins involved in B₁₂ metabolism across more than 1600 plant and algal genomes and transcriptomes (e.g. from OneKp), demonstrating the presence of B₁₂-associated metabolism deep into the streptophytes. METH and five further accessory proteins (MTRR, CblB, CblC, CblD and CblJ) were detected in the hornworts (Anthocerotophyta), and two (CblB and CblJ) were identified in liverworts (Marchantiophyta) in the bryophytes, suggesting a retention of B₁₂-metabolism in the last common land plant ancestor. Our data further show more limited distributions for other B₁₂-related proteins (MCM and RNR-II) and B₁₂ dependency in several algal orders. Finally, considering the collection sites of algae that have lost B₁₂ metabolism, we propose freshwater-to-land transitions and symbiotic associations to have been constraining factors for B₁₂ availability in early plant evolution. This article is part of the theme issue 'The evolution of plant metabolism'.