Abstract
Switchgrass (Panicum virgatum) has been developed into a model lignocellulosic bioenergy crop. Downregulation of caffeic acid O-methyltransferase (COMT), a key enzyme in lignin biosynthesis, has been shown to alter lignification and increase biofuel yield in switchgrass. Methylenetetrahydrofolate reductase (MTHFR) mediates C1 metabolism and provides methyl units consumed by COMT. It was predicted that co-silencing of MTHFR and COMT would impact lignification even more than either of the single genes. However, our results showed that strong downregulation of MTHFR in a COMT-deficient background led to altered plant growth and development, but no significant change in lignin content or composition was found when compared with COMT plants. Another unexpected finding was that the double MTHFR/COMT downregulated plants showed a novel lesion-mimic leaf phenotype. Molecular analyses revealed that the lesion-mimic phenotype was caused by the synergistic effect of MTHFR and COMT genes, with MTHFR playing a predominant role. Microarray analysis showed significant induction of genes related to oxidative and defense responses. The results demonstrated the lack of additive effects of MTHFR and COMT on lignification. Furthermore, this research revealed an unexpected role of the two genes in the modulation of lesion-mimic cell death as well as their synergistic effects on agronomic performance.
Highlights
As an essential cofactor, tetrahydrofolate (THF) mediates the transfer of one-carbon (C1) units in various methylated anabolic pathways, mainly those for lignin, alkaloids and betaines (Hanson et al, 2000)
Downregulation of caffeic acid O-methyltransferase (COMT) in switchgrass resulted in significant reduction of lignin content, which led to large increase in biofuel production (Fu et al, 2011; Baxter et al, 2014)
Because Methylenetetrahydrofolate reductase (MTHFR) is directly involved in the production of the methyl donor, SAM, and lignin content is reduced in the maize bm2 mutant, it was expected that simultaneous downregulation of COMT and MTHFR would lead to further alteration of lignin biosynthesis
Summary
Tetrahydrofolate (THF) mediates the transfer of one-carbon (C1) units in various methylated anabolic pathways, mainly those for lignin, alkaloids and betaines (Hanson et al, 2000). Methylenetetrahydrofolate reductase (MTHFR) catalyzes the reduction of 5, 10-methylene-THF to 5-methyl-THF, which is the most reduced C1 derivative (Roje et al, 1999; Hanson et al, 2000). The latter provides a methyl group to generate methionine (Met). MTHFRs have been cloned and identified in Arabidopsis (Arabidopsis thaliana), maize (Zea mays) and tobacco (Nicotiana tabacum) (Roje et al, 1999; Hung et al, 2013). The bm mutant with reduced transcriptional level of MTHFR showed decreased lignin content and altered lignin composition (Tang et al, 2014)
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