Gene co-option across scales shapes plant diversity

Of the different modes of evolution affecting plant diversification - novel genes, expansion and co-option - the latter holds an especially powerful framework for integrative biology. Beyond the intricate detail of any particular plant, a large-scale consideration of the plant tree of life reveals flowing gene histories inside species conduits, morphing and making their way to Darwin's "endless forms most beautiful". A concept that was only tentatively emerging just two decades ago in the animal embryology literature, gene co-option has exploded into a myriad of examples from nature's botanical bounty. Gene co-option - the reuse of existing genes for new developmental functions - is solidifying its role as a central mechanism driving morphological innovation in plant evolution. Moreover, mounting evidence points to similar molecular toolkits being recruited independently across vast evolutionary distances, revealing deep homology of genetic modules underlying analogous structures. In plants, this phenomenon results in phenotypes as diverse as stinky flowers, carnivory, sexual deception, amphibious habit, and prickles. The growing availability of plant genomes and functional model systems across lineages is uncovering the genetic architecture of landmark developmental innovations in plant evolution. This holistic view highlights that homologous genes are being repeatedly tweaked into new networks and repurposed into novel roles, providing renewed support for a tinkering model of evolution at increasingly larger scales. We present a synthesis of diverse examples of developmental genes repurposed during land plant evolution under the unifying theme that gene co-option operates across multiple biological scales, and end with outstanding questions to further probe the genetic mechanisms underlying co-option across these expanding scales.