Diversification of CLE expression patterns and nonmeristematic roles for CLAVATA receptor-like kinases in a moss.

The CLAVATA pathway controls meristematic cell proliferation and multiple nonmeristematic processes in Arabidopsis development. While CLAVATA ancestrally regulates meristematic proliferation in nonseed plant gametophytes, ancestral sporophytic and nonmeristematic functions in land plants are unknown. Here, we analysed the promoter activities of all peptide (PpCLE) and receptor-encoding (PpCLV1a, PpCLV1b and PpRPK2) genes throughout the moss (Physcomitrium patens) life cycle and validated our expression analyses using mutant phenotype data.

CLAVATA modulates auxin homeostasis and transport to regulate stem cell identity and plant shape in a moss.

The CLAVATA pathway is a key regulator of stem cell function in the multicellular shoot tips of Arabidopsis, where it acts via the WUSCHEL transcription factor to modulate hormone homeostasis. Broad-scale evolutionary comparisons have shown that CLAVATA is a conserved regulator of land plant stem cell function, but CLAVATA acts independently of WUSCHEL-like (WOX) proteins in bryophytes. The relationship between CLAVATA, hormone homeostasis and the evolution of land plant stem cell functions is unknown.

What can lycophytes teach us about plant evolution and development? Modern perspectives on an ancient lineage.

All Evo-Devo studies rely on representative sampling across the tree of interest to elucidate evolutionary trajectories through time. In land plants, genetic resources are well established in model species representing lineages including bryophytes (mosses, liverworts, and hornworts), monilophytes (ferns and allies), and seed plants (gymnosperms and flowering plants), but few resources are available for lycophytes (club mosses, spike mosses, and quillworts). Living lycophytes are a sister group to the euphyllophytes (the fern and seed plant clade), and have retained several ancestral morphological traits despite divergence from a common ancestor of vascular plants around 420 million years ago.

An apical hypoxic niche sets the pace of shoot meristem activity.

Complex multicellular organisms evolved on Earth in an oxygen-rich atmosphere; their tissues, including stem-cell niches, require continuous oxygen provision for efficient energy metabolism. Notably, the maintenance of the pluripotent state of animal stem cells requires hypoxic conditions, whereas higher oxygen tension promotes cell differentiation. Here we demonstrate, using a combination of genetic reporters and in vivo oxygen measurements, that plant shoot meristems develop embedded in a low-oxygen niche, and that hypoxic conditions are required to regulate the production of new leaves.