A carotenoid-derived hormonal signal that inhibits shoot branching in plants has long escaped identification. Strigolactones are compounds thought to be derived from carotenoids and are known to trigger the germination of parasitic plant seeds and stimulate symbiotic fungi. Here we present evidence that carotenoid cleavage dioxygenase 8 shoot branching mutants of pea are strigolactone deficient and that strigolactone application restores the wild-type branching phenotype to ccd8 mutants. Moreover, we show that other branching mutants previously characterized as lacking a response to the branching inhibition signal also lack strigolactone response, and are not deficient in strigolactones. These responses are conserved in Arabidopsis. In agreement with the expected properties of the hormonal signal, exogenous strigolactone can be transported in shoots and act at low concentrations. We suggest that endogenous strigolactones or related compounds inhibit shoot branching in plants. Furthermore, ccd8 mutants demonstrate the diverse effects of strigolactones in shoot branching, mycorrhizal symbiosis and parasitic weed interaction.

Different transgenic tobacco lines down-regulated for either one or two enzymes of the monolignol pathway were compared for their lignin content and composition, and developmental patterns. The comparison concerned CCR and CAD down-regulated lines (homozygous or heterozygous for the transgene) and the hybrids resulting from the crossing of transgenic lines individually altered for CCR or CAD activities. Surprisingly, the crosses containing only one allele of each antisense transgene, exhibit a dramatic reduction of lignin content similar to the CCR down-regulated parent but, in contrast to this transgenic line, display a normal phenotype and only slight alterations of the shape of the vessels. Qualitatively the lignin of the double transformant displays characteristics more like the wild type control than either of the other transgenics. In the transgenics with a low lignin content, the transformations induced other biochemical changes involving polysaccharides, phenolic components of the cell wall and also soluble phenolics. These results show that the ectopic expression of a specific transgene may have a different impact depending on the genetic background and suggest that the two transgenes present in the crosses may operate synergistically to reduce the lignin content. In addition, these data confirm that plants with a severe reduction in lignin content may undergo normal development at least in controlled conditions.