Branching out: new class of plant hormones inhibits branch formation For many years the textbooks recognized five ‘classic’ plant hormones: auxin, gibberellins, ethylene, cytokinin and abscisic acid. To these can be added the brassinosteroids, nitric oxide and jasmonates, among others, as phytohormones or plant growth regulators. Shoot branching is regulated by hormones, with both auxin and cytokinin playing a part. But the existence of mutants with enhanced branching in several species suggested a third factor was involved, a novel plant hormone released from the roots that prevents excessive shoot branching. Two groups now identify a class of chemical compounds called strigolactones — or one of their derivatives — as that missing hormone. Strigolactones are found in root exudates and are reduced in the branching mutants; external application of these chemicals inhibits shoot branching in the mutants. On the cover, a growing bud in the leaf axil of an increased branching mutant of pea (Pisum sativum). [Articles pp. 189, 195; News & Views p. 176] Cover photo: Elizabeth Dun.