Alternation of generations - unravelling the underlying molecular mechanism of a 165-year-old botanical observation.

Characteristically, land plants exhibit a life cycle with an 'alternation of generations' and thus alternate between a haploid gametophyte and a diploid sporophyte. At meiosis and fertilisation the transitions between these two ontogenies take place in distinct single stem cells. The evolutionary invention of an embryo, and thus an upright multicellular sporophyte, in the ancestor of land plants formed the basis for the evolution of increasingly complex plant morphologies shaping Earth's ecosystems. Recent research employing the moss Physcomitrella patens revealed the homeotic gene BELL1 as a master regulator of the gametophyte-to-sporophyte transition. Here, we discuss these findings in the context of classical botanical observations.