Characterization, expression, and functional analysis of the pathogenesis-related gene PtDIR11 in transgenic poplar

Lignins and lignans are important for plant resistance to pathogens. Dirigent (DIR) proteins control the regio- and stereo-selectivity of coniferyl alcohol in lignan and lignin biosynthesis. DIR genes have been implicated in defense-related responses in several plant species, but their role in poplar immunity is unclear. We cloned PtDIR11 from Populus trichocarpa; we found that overexpression of PtDIR11 in poplar improved the lignan biosynthesis and enhanced the resistance of poplar to Septotis populiperda. PtDIR11 has a typical DIR domain; it belongs to the DIR-b/d family and is expressed in the cell membrane. Reverse transcription quantitative polymerase chain reaction (RT-qPCR) analysis showed that PtDIR11 expression was highest in stems, followed by leaves and roots. Furthermore, PtDIR11 expression was induced by S. populiperda, salicylic acid (SA), jasmonate (JA), and ethylene (ET) stresses. The recombinant PtDIR11 protein inhibited the growth of S. populiperda in vitro. Overexpressing (OE) PtDIR11 in "Nanlin 895" poplar enhanced growth. The OE lines exhibited minimal changes in lignin content, but their total lignan and flavonoid contents were significantly greater than in the wild-type (WT) lines. Overexpression of PtDIR11 affected multiple biological pathways of poplar, such as phenylpropanoid biosynthesis. The methanol extracts of OE-PtDIR11 lines showed greater anti-S. populiperda activity than did lignin extracts from the WT lines. Furthermore, OE-PtDIR11 lines upregulated genes that were related to phenylpropanoid biosynthesis and genes associated with the JA and ET signal transduction pathways; it downregulated genes that were related to SA signal transduction compared with the WT line under S. populiperda stress. Therefore, the OE transgenic plants analysis revealed that PtDIR11 is a good candidate gene for breeding of disease resistant poplar.