ABSTRACT
Brassinosteroids (BRs) are plant steroidal hormones that play important roles in many stages of plant growth. Several plant species produce ecdysteroids, which are known as insect molting steroid hormones. In this study, we evaluated the biological activities of three hydroxysteroidal compounds, 20-hydroxyecdysone (ECD), 7,8-dihydro-8α-20-hydroxyecdysone (DHECD), and 7,8-dihydro-5α,8α-20-hydroxyecdysone (α-DHECD), and compared their activities with that of brassinolide (BL), the most potent BR. In rice, DHECD and α-DHECD enhanced the degree of lamina inclination, as do BRs. In Arabidopsis thaliana, DHECD and α-DHECD increased hypocotyl length in the wild-type, and also partially overcame the hypocotyl shortening in the wild-type caused by 0.3µM brassinazole, a specific BR biosynthesis inhibitor. DHECD and α-DHECD partially reduced dwarfism in the BR-biosynthesis-deficient mutant det2. Treatment with DHECD or α-DHECD downregulated the expression of the BR biosynthesis genes DWF4 and CPD, which are generally, suppressed by BR, and upregulated the expression of TCH4 and SAUR-AC1, which are generally promoted by BR. However, their regulated activities were less effective than BL. Moreover, the 10-4M DHECD and α-DHECD induced the accumulation of dephosphorylated BIL1/BZR1 that enhanced BR signaling as a master transcription factor. In contrast, ECD did not affect rice lamina bending, Arabidopsis hypocotyl elongation, the expression levels of BR-related genes and BIL1/BZR1 phosphorylation status. Based on these results, we hypothesize that both DHECD and α-DHECD have functional activities similar to those of BR.
Subject(s)
Arabidopsis/drug effects , Biomimetic Materials/pharmacology , Brassinosteroids/pharmacology , Ecdysterone/pharmacology , Oryza/drug effects , Plant Growth Regulators/pharmacology , Steroids, Heterocyclic/pharmacology , Arabidopsis/genetics , Arabidopsis/growth & development , Arabidopsis/metabolism , Arabidopsis Proteins/genetics , Arabidopsis Proteins/metabolism , Biomimetic Materials/chemical synthesis , Cytochrome P-450 Enzyme System/genetics , Cytochrome P-450 Enzyme System/metabolism , DNA-Binding Proteins , Ecdysterone/analogs & derivatives , Ecdysterone/metabolism , Gene Expression Regulation, Developmental , Gene Expression Regulation, Plant , Glycosyltransferases/genetics , Glycosyltransferases/metabolism , Hypocotyl/drug effects , Hypocotyl/genetics , Hypocotyl/growth & development , Hypocotyl/metabolism , Nuclear Proteins/genetics , Nuclear Proteins/metabolism , Oryza/genetics , Oryza/growth & development , Oryza/metabolism , Phosphorylation/drug effects , Real-Time Polymerase Chain Reaction , Seeds/drug effects , Seeds/genetics , Seeds/growth & development , Seeds/metabolism , Signal Transduction , Steroid Hydroxylases/genetics , Steroid Hydroxylases/metabolism , Triazoles/antagonists & inhibitors , Triazoles/pharmacologyABSTRACT
Strigolactones (SLs) and karrikins (KARs) regulate photomorphogenesis. GR24, a synthetic SL and KAR1, a KAR, inhibit the hypocotyl elongation of Arabidopsis thaliana in a weak light. GR24 and KAR1 up-regulate the expression of STH7, encoding a transcription factor belonging to the double B-box zinc finger subfamily. In this study, we used STH7-overexpressing (STH7ox) lines and functionally defective STH7 (STH7-SRDX) mutants to investigate roles of SLs and KARs in photomorphogenesis of Arabidopsis. Hypocotyl elongation of STH7-SRDX mutants was less sensitive to both GR24 and KAR1 treatment than that of wild-type Arabidopsis under weak light conditions. Furthermore, the chlorophyll and anthocyanin content was increased in STH7ox lines when de-etiolated with light and GR24-treated plants had enhanced anthocyanin production. GR24 and KAR1 treatment significantly increased the expression level of photosynthesis-related genes LHCB1 and rbcS. The results strongly suggest that SL and KAR induce photomorphogenesis of Arabidopsis in an STH7-dependent manner.