Biology Reference
In-Depth Information
the primary substrates for 3-hydroxylation of the phenolic moiety. In contrast,
4-coumarate, 4-coumaroyl-CoA, or the corresponding aldehyde and alcohol
were poorly or not metabolized ( Franke et al.,2002a;Nairet al.,2002;Schoch
et al.,2001 ). The Arabidopsis CYP98A3 converts the shikimate ester most
efficiently, but the quinate ester of 4-coumarate is also converted with appre-
ciable activity. This defined CYP98A3 as 4-coumaroyl-shikimate/quinate-
3 0 -hydroxylase (C3 0 H, because quinate and shikimate are cyclic as well, the
phenolic moiety becomes annotated as the prime [ 0 ]ring).Thus,C3 0 Halso
catalyses the final step of the biosynthesis of chlorogenic acid (caffeoyl-
quinate), an abundant antioxidant that accumulates in many plants
( Petersen et al.,2009 ). However, functional proof that C3 0 H is also the central
3-hydroxylase of the phenylpropanoid pathway came from a phenotypic
analyses of the ref8 mutant: Soluble sinapoyl malate and sinapoyl choline
levels are drastically reduced in the mutant leaves and seeds, respectively. In
the ref8 mutant, total lignin content was reduced to 20-40% of wild-type levels
and both G- and S-units were found only in trace amounts ( Franke et al.,
2002b ). Instead, the mutant accumulates almost exclusively 4-coumarate-
derived H-units, which are found only in minute amounts in wild-type lignin.
Instead of incorporating H-lignin into cell walls that normally produce G/S
lignin, it now appears that regular H-lignin biosynthesis is taking place early in
inflorescence stem development of the ref8 mutant, while only small amounts
of H monolignols are incorporated into walls that normally would produce S-
or G-lignins ( Patten et al.,2010 ). The inability of the ref8 mutant to produce
G- and S-lignins clearly established that the 3-hydroxylation of the monolignol
pathway occurs at the level of the shikimate ester of 4-coumarate in Arabi-
dopsis rather than on the level of the free acid or CoA-ester. In parallel, a
transferase belonging to the BAHD superfamily had been identified in tobacco
that was shown to catalyse the synthesis of 4-coumaroyl-shikimate (and
-quinate) from 4-coumaroyl-CoA ( Hoffmann et al.,2003 ). The same enzyme
also efficiently catalyses the interconversion between caffeoyl-shikimate and
caffeoyl-CoA and was thus named hydroxycinnamoyl-CoA: shikimate/qui-
nate hydroxycinnamoyltransferase (HCT). HCT downregulation also causes
reduction of G- and S-lignins in several plants and leads to a lignin mainly
composed of H-units ( Besseau et al.,2007;Puet al.,2009;Shadleet al.,2007 ).
Together, the discovery of HCT and C3 0 H immediately suggested that
caffeoyl-CoA is synthesized from 4-coumaroyl-CoA via coumaroyl-shikimate
and caffeoyl-shikimate instead of from free caffeic acid ( Fig. 1 ). An involve-
ment of C3 0 H in both G- and S-lignin units has been confirmed in other species
both on the enzyme activity level and by reverse genetic approaches. Down-
regulation of C3 0 H in alfalfa (M. sativa) and hybrid poplar (Populus grand-
identata
alba) resulted in strong reduction in total
lignin and a drastic
Search WWH ::




Custom Search