Abstract
Plant caffeic acid 3-O-methyltransferase (COMT) has been implicated in the lignin biosynthetic pathway through catalyzing the multi-step methylation reactions of hydroxylated monomeric lignin precursors. However, genetic evidence for its function in plant disease resistance is poor. Sharp eyespot, caused primarily by the necrotrophic fungus Rhizoctonia cerealis, is a destructive disease in hexaploid wheat (Triticum aestivum L.). In this study, a wheat COMT gene TaCOMT-3D, is identified to be in response to R. cerealis infection through microarray-based comparative transcriptomics. The TaCOMT-3D gene is localized in the long arm of the chromosome 3D. The transcriptional level of TaCOMT-3D is higher in sharp eyespot-resistant wheat lines than in susceptible wheat lines, and is significantly elevated after R. cerealis inoculation. After R. cerealis inoculation and disease scoring, TaCOMT-3D-silenced wheat plants exhibit greater susceptibility to sharp eyespot compared to unsilenced wheat plants, whereas overexpression of TaCOMT-3D enhances resistance of the transgenic wheat lines to sharp eyespot. Moreover, overexpression of TaCOMT-3D enhances the stem mechanical strength, and lignin (particular syringyl monolignol) accumulation in the transgenic wheat lines. These results suggest that TaCOMT-3D positively contributes to both wheat resistance against sharp eyespot and stem mechanical strength possibly through promoting lignin (especially syringyl monolignol) accumulation.
Highlights
Lignin is a phenolic cell wall polymer, which is composed of guaiacyl, syringyl and p-hydroxyphenyl units derived from the monolignol precursors (p-coumaryl, coniferyl, and sinapyl alcohols), respectively (Supplemental Fig. S1)[10,11,12]
Microarray-based comparative transcriptomic assay was used to identify differentially expressed probes between sharp eyespot-resistant wheat line CI12633/Shanhongmai and the susceptible wheat line Wenmai 6 inoculated with R. cerealis R0301
The probe A_99_P198406, 100% matching to 3′-terminal sequence of a wheat cDNA sequence with accession number AK332908, displayed significantly transcriptional increase in sharp eyespot-resistant wheat lines (CI12633 and Shanhongmai) than in the susceptible wheat line Wenmai 6 at 4, 7, and 21 dpi with R. cerealis R0301 (Fig. 1A), respectively
Summary
Lignin is a phenolic cell wall polymer, which is composed of guaiacyl, syringyl and p-hydroxyphenyl units derived from the monolignol precursors (p-coumaryl, coniferyl, and sinapyl alcohols), respectively (Supplemental Fig. S1)[10,11,12]. In diploid wheat (Triticum monococcum), transient knock-down of monolignol pathway enzymes [phenylalanine ammonia-lyase (PAL), caffeoyl-CoA O-methyltransferase (CCoAOMT), caffeic acid 3-O-methyltransferase (COMT) and cinnamyl alcohol dehydrogenase (CAD)] individually and pair-wise led to decreased basal immunity or penetration resistance to the fungal pathogens Blumeria graminis f. The probe with Agilent GeneChip number A_99_P198406, being homologous to certain plant COMTs, was identified to be in response to R. cerealis R0301 through comparative transcriptomic analysis on the sharp eyespot-resistant wheat line CI12633 and the susceptible wheat line Wenmai 6. This COMT gene located on wheat chromosome 3DL, designated as TaCOMT-3D, was cloned from the resistant wheat line CI12633. The functional characterization proved that TaCOMT-3D positively contributed to syringl monolignol biosynthesis, wheat resistance response to R. cerealis, and the stem mechanical strength
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