Shoot branching is a major determinant of plant architecture and is highly regulated by endogenous and environmental cues. Two classes of hormones, auxin and cytokinin, have long been known to have an important involvement in controlling shoot branching. Previous studies using a series of mutants with enhanced shoot branching suggested the existence of a third class of hormone(s) that is derived from carotenoids, but its chemical identity has been unknown. Here we show that levels of strigolactones, a group of terpenoid lactones, are significantly reduced in some of the branching mutants. Furthermore, application of strigolactones inhibits shoot branching in these mutants. Strigolactones were previously found in root exudates acting as communication chemicals with parasitic weeds and symbiotic arbuscular mycorrhizal fungi. Thus, we propose that strigolactones act as a new hormone class—or their biosynthetic precursors—in regulating above-ground plant architecture, and also have a function in underground communication with other neighbouring organisms.
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We are grateful to S. Ishikawa for sequencing the d17-1 allele; K. Fujiwara for assistance in preparing plant materials; N. Makita and H. Sakakibara for their advice on rice hydroponic culture; and Y. Tsuchiya for advice on germination assays. We thank the Salk Institute and the Arabidopsis Biological Resource Center for providing Arabidopsis T-DNA insertion lines; T. Yokota, K. Yoneyama and X. Xie for sharing information on strigolactone analysis; M. Maekawa for propagating rice seeds; and K. Mori, P. McCourt and A. Gabar Babiker for providing (+)-strigol and 2'-epi-orobanchol, (+)-GR24, and S. hermonthica seeds, respectively. This work was supported in part by grants from the MEXT of Japan (1820810 to K.Y., 19678001 to K.S. and 19780040 to Sa.Y.) and the MAFF of Japan (Genomics for Agricultural Innovation, IPG0001 to J.K.). M.U. is supported by the RIKEN Special Postdoctoral Researchers Program.
Author Contributions M.U. and T.A. developed and performed rice branching assays. T.A. and J.K. prepared rice genetic materials. M.U. performed Arabidopsis branching assays. A.H. carried out LC/MS–MS analysis. Sa.Y. and K.S. designed and performed S. hermonthica infection assays. K.A. synthesized labelled epi-5DS. N.T.-K. and H.M. carried out qRT–PCR analysis. K.Y. and K.A. provided strigolactones and assisted strigolactone analysis by A.H. Y.K., K.S., K.Y. and J.K. contributed to the experimental design. Sh.Y. directed the project and designed the experiments. Sh.Y. and M.U. wrote the manuscript.
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Umehara, M., Hanada, A., Yoshida, S. et al. Inhibition of shoot branching by new terpenoid plant hormones. Nature 455, 195–200 (2008). https://doi.org/10.1038/nature07272
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