Small RNA-Regulated Networks and the Evolution of Novel Structures in Plants

The evolution of plants on land has produced a great diversity of organs, tissues, and cell types.Many of the genes identified as having a role in the development of such structures in flowering plants are conserved across all land plants, including in clades that diverged before the evolution of the structure in question. This suggests that novel organs commonly evolve via the cooption of existing developmental gene regulatory networks (GRNs). Although numerous examples of such cooptions have been identified, little is known about why those specific GRNs have been coopted. In this review, we discuss the properties of GRNs that may favor their cooption, as well as the mechanisms by which this can occur, in the context of plant developmental evolution.We especially focus on small RNA (sRNA)-regulated and auxin-signaling GRNs as intriguing models of regulatory network recruitment. The plethora of multicellular species that inhabit the earth today—with of all their tissue complexity—evolved from a handful of unicellular ancestors. As evolution pro-ceeded, the appearance of novel organs and tissue types allowed organisms to occupy new niches. For example, the evolution of wings allowed insects to expand into the air column, and flowers allowed angiosperms to attrac