Strigolactones, a Novel Carotenoid-Derived Plant Hormone

Strigolactones (SLs) are carotenoid-derived plant hormones and signaling molecules. When released into the soil, SLs indicate the presence of a host to symbiotic fungi and root parasitic plants. In planta, they regulate several developmental processes that adapt plant architecture to nutrient availability. Highly branched/tillered mutants in Arabidopsis, pea, and rice have enabled the identification of four SL biosynthetic enzymes: a cis/trans-carotene isomerase, two carotenoid cleavage dioxygenases, and a cytochrome P450 (MAX1). In vitro and in vivo enzyme assays and analysis of mutants have shown that the pathway involves a combination of new reactions leading to carlactone, which is converted by a rice MAX1 homolog into an SL parent molecule with a tricyclic lactone moiety. In this review, we focus on SL biosynthesis, describe the hormonal and environmental factors that determine this process, and discuss SL transport and downstream signaling as well as the role of SLs in regulating plant development.