MYB 103 is required for FERULATE ‐5‐ HYDROXYLASE expression and syringyl lignin biosynthesis in A rabidopsis stems

Summary The transcription factor MYB 103 was previously identified as a member of the transcriptional network regulating secondary wall biosynthesis in xylem tissues of A rabidopsis, and was proposed to act on cellulose biosynthesis. It is a direct transcriptional target of the transcription factor SECONDARY WALL ASSOCIATED NAC DOMAIN PROTEIN  1 ( SND 1), and 35 S ‐driven dominant repression or over‐expression of MYB 103 modifies secondary wall thickness. We identified two myb103 T ‐ DNA insertion mutants and chemically characterized their lignocellulose by pyrolysis/ GC / MS , 2 D NMR , FT ‐ IR microspectroscopy and wet chemistry. The mutants developed normally but exhibited a 70–75% decrease in syringyl ( S ) lignin. The level of guaiacyl ( G ) lignin was co‐ordinately increased, so that total K lason lignin was not affected. The transcript abundance of FERULATE ‐5 ‐ HYDROXYLASE ( F 5 H ), the key gene in biosynthesis of S  lignin, was strongly decreased in the myb103 mutants, and the metabolomes of the myb103 mutant and an F 5 H null mutant were very similar. Other than modification of the lignin S to G ratio, there were only very minor changes in the composition of secondary cell‐wall polymers in the inflorescence stem. In conclusion, we demonstrate that F 5 H expression and hence biosynthesis of S  lignin are dependent on MYB 103.