Sci. Signal., 16 September 2008
Plant Hormones New Ramifications of Strigolactone Signaling?
Elizabeth M. Adler
Science Signaling, AAAS, Washington, DC 20005, USA
Plants not only grow upward, through the apical shoot, but also branch laterally through subapical shoots. Lateral branching is regulated by both auxins and cytokinins; mutational studies have suggested that a third class of hormone—derived from carotenoids—also regulates shoot branching (see Klee). Now two groups propose that strigolactones (carotenoid-derived compounds known to signal to symbiotic fungi that colonize plant roots and to plants that parasitize other plants roots) or their derivatives represent these unidentified branch-inhibiting hormones. Umehara et al. used mass spectrometric analysis to analyze strigolactones and found that the strigolactone content of roots and root exudates of the highly branched rice seedlings with mutations in the genes encoding the enzymes CAROTENOID CLEAVAGE DIOXYGENASE 7 (CCD7) and CCD8 was lower than that of wild-type seedlings. Culture media from these mutants (as well as from bushy Arabidopsis plants with mutations affecting CCD7 or CCD8) were less effective at eliciting germination of the seeds of a root parasite (an assay of strigolactone concentration) than media from wild-type plants, and strigolactone treatment rescued the branching phenotype in plants with mutations affecting CCD7 or CCD8 but not in branching mutants that produced strigolactones. In the second study, Gomez-Roldan et al. found that root exudates from peas with mutations in the genes encoding CCD7 and CCD8 were less effective than those from wild-type peas at stimulating mycorrhizal fungal hyphae branching and root parasite seed germination, and confirmed their loss of strigolactones with mass spectrometric analysis. Exogenous strigolactone inhibited shoot branching in pea and Arabidopsis ccd mutants but not in strigolactone-producing mutants. Thus, both groups conclude that strigolactones or their derivatives not only signal to organisms that interact with plant roots but also act to regulate lateral branching.
V. Gomez-Roldan, S. Fermas, P. B. Brewer, V. Puech-Pagès, E. A. Dun, J.-P. Pillot, F. Letisse, R. Matusova, S. Danoun, J.-C. Portais, H. Bouwmeester, G. Bécard, C. A. Beveridge, C. Rameau, S. F. Rochange, Strigolactone inhibition of shoot branching. Nature 455, 189-194 (2008). [PubMed]
M. Umehara, A. Hanada, S. Yoshida, K. Akiyama, T. Arite, N. Takeda-Kamiya, H. Magome, Y. Kamiya, K. Shirasu, K. Yoneyama, J. Kyozuka, S.Yamaguchi, Inhibition of shoot branching by new terpenoid plant hormones. Nature 455, 195-200 (2008). [PubMed]
H. Klee, Hormones branch out. Nature 455, 176-177 (2008). [PubMed]
Citation: E. M. Adler, New Ramifications of Strigolactone Signaling? Sci. Signal. 1, ec322 (2008).
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