Growth in liverworts of the Marchantiales is promoted by epiphytic methylobacteria

Liverworts, the most basal lineage of extant land plants, have been used as model systems in the reconstruction of adaptations to life on land. In this study, we used gemmae (specialized propagules) that were isolated from mature gemma cups of two distantly related species of liverworts, Marchantia polymorpha L. and Lunularia cruciata L. (order Marchantiales). We show that methylobacteria (genus Methylobacterium), microbes that inhabit the surfaces of land plants where they secrete phytohormones (cytokinines), promote the growth of isolated gemmae cultivated on agar plates. As a control, two species of higher plants, maize (Zea mays L.) and sunflower (Helianthus annuus L.) were raised aseptically from sterile seeds (i.e., caryopses, achenes). Inoculation of these propagules with methylobacteria was without effect on growth in the above-ground phytosphere (expansion of stems and leaves). We conclude that normal development in Marchantia and Lunularia is dependent on (and possibly regulated by) epiphytic methylobacteria, whereas representative higher plants grow at optimal rates in the absence of these prokaryotic epiphytes.