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Cytochrome b5 is an obligate electron shuttle protein for syringyl lignin biosynthesis in Arabidopsis

Mingyue Gou, Xiaoman Yang, Yunjun Zhao, Xiuzhi Ran, Yanzhai Song, Chang-Jun Liu
Plant Cell 2019 April 8
In monolignol biosynthetic pathway, three cytochrome P450-catalzyed reactions essentially determine the structural characteristics of lignin precursors. Among them, ferulate 5-hydroxylase (F5H) is specifically responsible for syringyl (S) lignin subunit formation. NADPH Cytochrome P450 oxidoreductase (CPR) is commonly regarded as the electron carrier for the P450-catalyzed reactions in monolignol biosynthesis. Here we show that cytochrome b5 isoform D (CB5D) is an indispensable electron shuttle intermediate specific for S-lignin biosynthesis. Arabidopsis CB5D localizes on endoplasmic reticulum membrane and physically associates with monolignol P450 enzymes. Disrupting CB5D in Arabidopsis results in >60% reduction of S-lignin subunits but no impairment on G-lignin formation compared to the wild type, which sharply contrasts to the impairment of both G- and S-lignin synthesis after disruption of ATR2, the Arabidopsis CPR. The defect of S-lignin synthesis in cb5d mutants is rescued with expression of the gene encoding CB5D but not with the CB5D mutant variants devoid of its electron shuttle property. While disruption of ATR2 suppresses both C4H and F5H catalytic activities, elimination of CB5D specifically depletes the latter's activity. Our study demonstrates that CB5D functions as an obligate electron shuttle intermediate specifically augmenting F5H-catalyzed reaction thus controlling S-lignin biosynthesis.


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