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Strigolactones (SLs) are a class of plant hormones that regulate diverse physiological processes, including shoot branching and root development. They also act as rhizosphere signaling molecules to stimulate the germination of root parasitic weeds and the branching of arbuscular mycorrhizal fungi. Although various types of cross talk between SLs and other hormones have been reported in physiological analyses, the cross talk between gibberellin (GA) and SLs is poorly understood. We screened for chemicals that regulate the level of SLs in rice (Oryza sativa) and identified GA as, to our knowledge, a novel SL-regulating molecule. The regulation of SL biosynthesis by GA is dependent on the GA receptor GID1 and F-box protein GID2. GA treatment also reduced the infection of rice plants by the parasitic plant witchers weed (Striga hermonthica). These data not only demonstrate, to our knowledge, the novel plant hormone cross talk between SL and GA, but also suggest that GA can be used to control parasitic weed infections.

Strigolactones are a class of plant hormones that inhibit axillary bud outgrowth and are released from plant roots to act as a rhizosphere communication signal. The Orobanchaceae parasitic plant perceives strigolactone as its germination signal, indicating host presence. After germination, the plant parasitises the host plant and suppresses host growth by draining photosynthetic products, water and other essential nutrients. Because of this way of life, this parasite threatens crop production in sub-Saharan Africa with infestation in crop fields and crop devastation. Crop protection in such areas is among the most concerning problems to be dealt with as immediately as possible. With respect to crop protection from , many strigolactone agonists have been developed and used in research to reveal biology, and have contributed to development of crop protection methods. However, an effective method has yet to be found. In a previous study, we reported debranones as a group of strigolactone mimics that inhibit axillary buds outgrowth with moderate stimulation activity for germination. Debranones would be accessible because they are simply synthesized from commercially available phenols and bromo butenolide. Taking this advantage of debranones for research, we tried to find the debranones stimulating seed germination. To modulate functional selectivity and to enhance germination inducing activity of debranones, we studied structure-activity relationships. We investigated effects of substituent position and functional group on debranone activity and selectivity as a strigolactone mimic. As a result, we improved stimulation activity of debranones for seed germination by chemical modification, and demonstrated the pharmacophore of debranones for selective modulation of distinct strigolactone responses.