MAX3/CCD7 is a carotenoid cleavage dioxygenase required for the synthesis of a novel plant signaling molecule.

Plant development is exquisitely environmentally sensitive, with plant hormones acting as long-range signals that integrate developmental, genetic, and environmental inputs to regulate development. A good example of this is in the control of shoot branching, where wide variation in plant form can be generated in a single genotype in response to environmental and developmental cues. Here we present evidence for a novel plant signaling molecule involved in the regulation of shoot branching. We show that the MAX3 gene of Arabidopsis is required for the production of a graft-transmissible, highly active branch inhibitor that is distinct from any of the previously characterized branch-inhibiting hormones. Consistent with its proposed function in the synthesis of a novel signaling molecule, we show that MAX3 encodes a plastidic dioxygenase that can cleave multiple carotenoids. We conclude that MAX3 is required for the synthesis of a novel carotenoid-derived long-range signal that regulates shoot branching.