Quantitative evaluation of the biological activity of various brassinosteroids using spiral root induction in rice seeds.

Spiral roots are induced in germinated rice seeds through treatment with nanomolar brassinosteroids (BRs) but not with other plant hormones. Here, we determined the minimum effective concentration (MEC) of various BRs to induce spiral roots in germinated rice seeds. The reciprocal logarithm of MEC, pMEC, was used as the BL-like activity index, which was linearly correlated with the reciprocal logarithm of a 50% effective dose (pED50) as evaluated in the rice lamina inclination assay. Furthermore, a ligand-receptor docking simulation was performed against the BL receptor complex, Arabidopsis thaliana BRI1/SERK1, and the binding free energy (ΔGbind) was calculated for the tested BRs. The ΔGbind calculation was performed using the molecular mechanics/generalized Born surface area method on an ensemble of uncorrelated snapshots collected via molecular dynamics to predict biological activity.

Structure modification of nonsteroidal brassinolide-like compound, NSBR1.

Brassinolide (BL) is a possible plant growth regulator in agriculture, but the presence of a steroid skeleton hampers the field application of BL in agriculture because of its high synthetic cost. We discovered NSBR1 as the first nonsteroidal BL-like compound using in silico technology. Searching for more potent BL-like compounds, we modified the structure of NSBR1 with respect to 2 benzene rings and the piperazine ring. The activity of synthesized compounds was measured in Arabidopsis hypocotyl elongation. The propyl group of butyryl moiety of NSBR1 was changed to various alkyl groups, such as straight, branched, and cyclic alkyl chains. Another substituent, F, at the ortho position of the B ring toward the piperazine ring was changed to other substituents. A methyl group was introduced to the piperazine ring. Most of the newly synthesized compounds with the 3,4-(OH)2 group at the A ring significantly elongated the hypocotyl of Arabidopsis.

Discovery of a nonsteroidal brassinolide-like compound, NSBR1.

Fourteen compounds screened from 5 million compounds in silico were submitted to bioassay to find brassinolide (BL) agonists/antagonists against Arabidopsis thaliana. Of these, two N-benzoyl-N'-phenylpiperazine (NBNPP)-type compounds showed antagonistic activity; however, none showed agonistic activity against A. thaliana. The substituents at the benzoyl moiety of NBNPP were changed to OH groups to derive N-(3,4-dihydroxybenzoyl)-N'-(4-butanoyl-2-fluorophenyl)pyrazine, which was named NSBR1. NSBR1 was rationally designed based on docking simulations and molecular dynamics. NSBR1 significantly suppressed the gene expression of CPD and BR6-ox2, which are known as marker genes for the action of BL. This novel NSBR1 was also effective in the rice lamina inclination assay (RLIA), and the activity in terms of the 50% effective dose (ED50) was determined as 0.79 nmol/plant from the dose-response curve for RLIA.

In silico exploration for agonists/antagonists of brassinolide.

Brassinolide (BL) is a plant steroid hormone that is necessary for stem elongation and cell division. To date more than 70 steroidal BL-like compounds, which are collectively named as brassinosteroids, have been identified. However, non-steroidal compounds that mimic BL have not been reported yet, which can be used as plant growth regulators. Twenty-two non-steroidal compounds were screened from the database containing about 5 million compound structures using a pharmacophore-based in silico screening method. The crystal structure (PDB: 4LSX) of the BL receptor was used to generate a pharmacophore model required for in silico screening. Among 22 hit compounds, 15 compounds that are thought to be physicochemically acceptable were submitted to the in vivo rice lamina inclination assay. Although no compound showed BL like activity, three compounds were detected as BL antagonist. The most potent compound was an ester derivative of 1,4-diphenlenedimethanol and isoxazole-4-carboxylic acid, and the other two compounds contain 2-phenylfuran and pyrimidin-2(1H)-one moieties bridged by an ethenyl substructure. The 50% effective doses (ED50) for the antagonistic activity were in a range of 0.6-5nmol per plant. The inhibition of the lamina inclination by the most potent agonist was recovered by the co-application of BL in a dose-dependent manner.