How was apical growth regulated in the ancestral land plant? Insights from the development of non seed plants

Land plant life cycles are separated into distinct haploid gametophyte and diploid sporophyte stages. Indeterminate apical
growth evolved independently in bryophyte (moss, liverwort, and hornwort) and fern gametophytes, and tracheophyte
(vascular plant) sporophytes. The extent to which apical growth in tracheophytes co-opted conserved gametophytic
gene networks, or exploited ancestral sporophytic networks, is a long-standing question in plant evolution. The recent
phylogenetic confirmation of bryophytes and tracheophytes as sister groups has led to a reassessment of the nature of
the ancestral land plant. Here, we review developmental genetic studies of apical regulators and speculate on their likely
evolutionary history.