Steroid signaling in plants and insects—common themes, different pathways

Outside of mammals, two model systems have been the focus of intensive genetic studies aimed at defining the molecular mechanisms of steroid hormone action—the flowering plant, Arabidopsis thaliana, and the fruit fly, Drosophila melanogaster. Studies in Arabidopsis have benefited from a detailed description of the brassinosteroid (BR) biosynthetic pathway, allowing the effects of mutations to be linked to specific enzymatic steps. More recently, the signaling cascade that functions downstream from BR production has been defined, revealing for the first time how the hormone can exert its effects on gene expression through a cell surface receptor and phosphorylation cascade. In contrast, studies of steroid hormone action in Drosophila began in the nucleus, with a detailed description of the transcription puffs activated by the steroid hormone 20-hydroxyecdysone (20E) in the giant polytene chromosomes. Subsequent genetic studies have revealed that these effects are exerted through nuclear receptors, much like mammalian hormone signaling. Most recently, genetic studies have begun to elucidate the ecdysteroid biosynthetic pathway which, until recently, remained largely undefined. Our current understanding of steroid hormone signaling in Arabidopsis and Drosophila provides a number of intriguing parallels as well as distinct differences. At least some of these differences, however, appear to be due to deficiencies in our understanding of these pathways. Below we discuss recent breakthroughs in defining the molecular mechanisms of BR biosynthesis and signaling in plants, and we compare and contrast this pathway with what is known about the mechanisms of ecdysteroid action in Drosophila. We raise some current questions in these fields, the answers to which may reveal other similarities in steroid signaling in plants and animals.