• Journal of Plant Biotechnology
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• v.34 no.3
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• pp.243-251
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• 2007

It has been recently demonstrated the presence of not only $C_{28}-BRs$ biosysnthesis, but also $C_{27}-$ and $C_{29}-BRs$ biosynthesis in plants, suggesting that BRs biosynthesis are complicatedly connected to produce biologically active BR (s). This prompted us to investigation of metabolism of a $C_{29}-BR$, 28-homoCS in seedlings of rice from which $C_{29}-BRs$ such as 28-homoTE and 28-homoTY have been identified. In vitro enzyme conversion study using a crude enzyme solution prepared from rice seedlings revealed that 28-homoCS is converted into both CS and 26-nor-28-homoCS, but their reversed reaction did not occur. This indicated that 28-homoCS is biosynthetically converted into more biologically active $C_{28}-BR$, CS by C-28 demethylation and biodegraded into 26-nor-28-homoCS by C-26 demethylation. Next, bio-conversion of 28-homoCS to 28-homoBL was examined by the same enzyme solution. No 28-homoBL as a metabolite of 28-homoCS was detected, meaning that biosynthetic reaction for 28-homoCS to 28-homoBL is not contained, and main connection of $C_{28}-BRs$ and $C_{29}-BRs$ biosynthesis is between CS and 28-homoCS in the rice seedling. This study is the first demonstrated that $C_{29}-BRs$ and $C_{28}-BRs$ bionsynthetic pathways are connected, and that $C_{29}-BRs$ biosynthetic pathway is an alternative biosynthetic pathway to produce more biologically active $C_{28}-BR$, CS in plant.