TaDIR13, a dirigent protein from wheat, promotes lignan biosynthesis and enhances pathogen resistance

Dirigent is proposed to be involved in either lignan or lignin biosynthesis. In the present study, sequences for dirigent and dirigent-like proteins from the wheat genome were analyzed. We obtained wheat dirigent (TaDIR13), and dirigent-like (TaDIR4 and TaJA1) recombinant proteins used for biochemical assays. It was shown that TaDIR13 could effectively direct coniferyl alcohol coupling into (+)-pinoresinol. In contrast, TaDIR4 and TaJA1 did not exhibit this activity. In accordance with this role of TaDIR13, overexpression of TaDIR13 in transgenic tobacco increased total lignan accumulation. Lignan extracts from TaDIR13 transgenic plants had enhanced antibacterial effects to Pseudomonas syringae. Furthermore, these plants did show strong resistance to P. syringae and Phytophthora parasitica. On the contrary, TaDIR4 transgenics did not show noticeable changes in either lignan accumulation or pathogen resistance. Overexpression of TaJA1 did not increase lignan accumulation either. Unlike TaDIR4 plants though, TaJA1 plants exhibited high pathogen resistance; yet, this was achieved through its jacalin-related lectins domain and not its dirigent domain. Lignin content and gene expression were not affected in all transgenic plants. Interestingly, expression of the pinoresinol-lariciresinol reductase gene (NtPLR) was increased in TaDIR13 transgenic plant; this is apparently owing to a favored metabolic flux from coniferyl alcohol into the lignan pathway. These data collectively suggest that TaDIR13 is mainly involved in regulating lignan biosynthesis which, in turn, responsible for the observed roles in pathogen resistance. TaDIR4 and TaJA1, both being dirigent-like proteins, have little functional overlap with dirigent proteins.