Orobanchaceae root parasitic weeds cause serious damage in agriculture worldwide. These parasitic species disperse large numbers of small seeds per individual to ensure that the offspring encounter the host plants. On the other hand, the seeds retain a small amount of storage materials because of their limited size. Accordingly, germinated root parasitic weeds are not able to survive more than several days after germination without the hosts.
We identified the trisaccharide planteose as a storage carbohydrate in Orobanchaceae root parasitic plants through metabolomics. Imaging mass spectrometry revealed that planteose is stored mainly in endosperm in Orobanche minor seeds. Sugar profiling during germination indicated that the first step of planteose metabolism is the hydrolysis of α-galactosidic bond releasing sucrose. Three genes encoding α-galactosidase found in the transcriptome of germinating O. minor seeds were further characterized. The time course of the expression of the gene OmAGAL2 among the three candidates fitted well with the time course of planteose hydrolysis. OmGAL2 expressed heterologously in Escherichia coli was confirmed to hydrolyze planteose under acidic conditions.
Chemical library screening was conducted to find OmAGAL2 inhibitors which might be a novel technology to control root parasitic weeds. As a result, 28 compounds with various structures were selected. These OmAGAL2 inhibitors were shown to exhibit inhibitory activity against radicle elongation of O. minor. An aromatic thiourea, PI-28, was selected for further structural optimization and a series of its derivatives were synthesized. Structure–activity relationship study revealed the important structural features for control of O. minor seed germination.