Evolution of molecular networks underlying plant tissue patterning: insights from conducting tissues

The emergence of land plants involved the progressive elaboration of molecular networks that pattern tissues and define specialized cell types, as illustrated by the evolution of diverse conducting tissues. While tracheophytes developed complex vascular systems with distinct xylem and phloem, bryophytes evolved functionally analogous cells for water and nutrient transport, including hydroids and leptoids in mosses and pegged rhizoids in complex thalloid liverworts. Fossil evidence, such as the early Devonian plant Horneophyton, suggests that multifunctional conducting cells may represent early forms of conducting cells that predate the divergence of modern tracheophyte vascular tissues and bryophyte conducting cells, indicating that key components of their developmental machinery were already present in early land plants. Across plant lineages, conducting tissues have evolved through the redeployment of shared genetic modules, lineage-specific innovations and rewiring of existing networks, shaping diverse patterns of tissue differentiation. A striking example of this divergence is found in certain liverworts, where water-conducting cells have been associated with pegged rhizoids and appear to be controlled by independent developmental mechanisms. A comparative approach is thus essential to understand how these crucial tissues emerged and diversified over millions of years of plant evolution, while also providing a framework for investigating the evolution of other developmental circuits.