(A) The shikimate-initiated synthesis pathway of serotonin and lignin. The key enzyme genes with increased expression in the lrd6-6 mutant detected by RNA sequencing are shown in red. Data in the brackets are the gene expression fold-changed determined by qRT-PCR with gene specific primers (Fold-changed ± SD), asterisks denote statistical gene expression increases in lrd6-6 mutant compared to Kitaake by Student’s t-test (*, P < = 0.05; **, P < = 0.01). (B) Comparison of the expression of OsWRKY14, which regulates synthesis of serotonin directly, between Kitaake and lrd6-6. (C) Content determination of lignin in Kitaake and lrd6-6. The bars represent the means of data from a representative experiment, and the error bars indicate the SEM (n = 4). Asterisks denote a significant difference between the lrd6-6 mutant and the wild type Kitaake (Student’s t-test; **, P < 0.01).