Submitted to: Molecular Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/3/2002
Publication Date: N/A
Citation: Interpretive Summary: Mutant and transgenic plants provide useful insights into the biochemical processes occurring in normal plants. It has recently been found that plants will change the makeup and structure of their lignin (the polymer that holds fibers together in woody and forage plants) if they are not able to produce lignins normally. When one of the lignin genes, COMT, is deficient, the resulting plants are more digestible by ruminant animals. The COMT gene has been altered in transgenic plants (such as aspen or poplar) but is also found to be depleted in natural mutants of corn such as one of the brown midrib mutants. Here we have identified and isolated a mutant of the popular model plant arabidopsis that is deficient in this gene. We have characterized the gene activity and preliminarily characterized the lignin to show that the now typical signatures of COMT-deficiency are observed in this mutant; COMT-deficient plants cannot make sinapyl alcohol, one of the normal components of lignin, but are instead able to supplement their lignin production by using the immediate precursor to the normal component, so-called 5-hydroxyconiferyl alcohol. This flexibility of the plant not only allows it to remain viable but provides researchers in the forage industry with new opportunities to change lignin to produce more utilizable plants (e.g. as forages for ruminant animals). Such studies are at the heart of efforts to improve the utilization of valuable plant resources.
Technical Abstract: A promoter-trap screen allowed us to identify an Arabidopsis line expressing GUS in the root vascular tissues. T-DNA border sequencing showed that the line was mutated in the COMT 1 gene (AtOMT1) and therefore deficient in OMT1 activity. In this knockout mutant and relative to the wild type, lignins lack syringyl (S) units and contain more 5-hydroxyguaiacyl units, the precursors of S-units. This altered lignin structure is associated with an improved cell wall digestibility of the dry stems. The sinapoyl ester pool is modified with a two-fold reduction of sinapoyl-malate in the leaves and stems of mature plants as well as in seedlings. In addition, the soluble phenolics extracted from the seedlings contain unusual derivatives assigned to 5-OH-feruloyl malate and to 5-OH-feruloyl glucose. Therefore, AtOMT1 enzymatic activity appears to be involved not only in lignin formation but also in the biosynthesis of sinapate esters. In addition, a deregulation of other monolignol biosynthetic gene expression can be observed in the Atomt1 mutant. A poplar cDNA encoding a caffeic acid OMT was successfully used to complement the Atomt1 mutant and restored both the level of S units and of sinapate esters to the control level. However, the over-expression of the OMT in wild-type Arabidopsis did not increase the S-lignin content, suggesting that OMT is not a limiting enzyme for S-unit biosynthesis.