OXIDATION/POLYMERIZATION: PEROXIDASES These are large multigene families in plants whose full physiological/biochemical roles and functions still remain poorly understood. Unlike laccases, however, a direct role for per­oxidases in lignification has been demonstrated. That is, downregulation of a peroxidase (TP60) in tobacco (N. tabacum) gave transformants with lignin levels reduced by circa 40­50% (257). Additionally, phloroglucinol-HCl staining suggested that the vasculature had been weakened (Figure 7.12H), although no quantitative structural testing on the plant stems was carried out. Thus, at present, the only oxidative enzyme demonstrated to have a role in monolignol oxidation/lignification is peroxidase. As before, more detailed analyses are required to fully ascertain the effects of peroxidase downregulation on lignification/cell wall structure(s).

7.6.2.1 Summary

Studies of CCR downregulation/mutation gave rise to severely dwarfed phenotypes; in Arabidopsis, typical G/S lignins were biosynthesized albeit at a delayed rate. There was no convincing evidence for replacement of monolignols with other non-monolignol moi­eties — e. g., feruloyl tyramine (60), acetosyringone (61), etc. The studies with CAD, F5H, and COMT were also most informative: while many of the mutants (e. g., CAD, COMT) have been known for almost three-quarters of a century, the biochemical basis of how they (CAD and COMT) disrupt the normal proposed template polymerization (see later) has now apparently come to light, i. e., via limited substrate degeneracy on the proposed lignin-forming template. The effects of this (attempted) degeneracy were not though structurally beneficial, and thus help explain why neither p-hydroxycinnamaldehydes nor 5-hydroxyconiferyl alcohol (4) evolved as substrates proper for lignification. Addition­ally, the F5H mutant (fah1-2) and the C4H-F5H overexpressing lines afforded two lignins with altered G and S levels, as did downregulation/mutation of COMT. Significantly, the patterns of interunit linkage frequency established that (based on monomer/dimer re­lease) lignification was apparently proceeding in similar (controlled) manner in each case. The data are explained through limited (substrate) degeneracy during proposed template polymerization.