Biology Reference
In-Depth Information
p-hydroxycinnamoyl-CoA:quinate/shikimate p-hydroxycinnamoyltransferase
(HCT) and 4-coumarate 3-hydroxylase (C3H) and involves products from
the shikimate pathway. The synthesis of coniferyl alcohol (and G lignin)
subsequently requires caffeoyl-CoA O-methyltransferase (CCoAOMT),
CCR and CAD and the synthesis of sinapyl alcohol (and S lignin) requires
in addition twomore enzymes, ferulate (coniferaldehyde)-5-hydroxylase (F5H)
and caffeic acid/5-hydroxyconiferaldehyde O-methyltransferase (COMT).
It should be underlined that the general phenylpropanoid pathway also
generates a huge range of various phenolic compounds including flavonoids,
stilbenes, etc. (
Vogt, 2010
). Monolignols, in addition to forming lignins, may
also produce (neo)lignans and oligolignols by dimerization or oligomerization
(
Davin et al., 2008
). Consequently, stress-induced modifications of the phenyl-
propanoid metabolism do not necessary imply changes in lignin synthesis.
The phenylpropanoid pathway itself is supplied with phenylalanine
through the shikimate pathway. This pathway drives approximately 20% of
carbon fixed by photosynthesis and feeds various pathways (
Bentley, 1990;
Herrmann and Weaver, 1999
). Lignin synthesis therefore requires a good
coordination between both shikimate and phenylpropanoid metabolism. In
fact, stimulation of the phenylpropanoid pathway may only be effective if
substrate availability from the shikimate pathway is sufficient. Moreover,
shikimate pathway intermediates are involved in HCT- and C3H-catalysed
reactions and can therefore exert metabolic control on monolignol biosyn-
thesis by controlling the channelling of shikimate pathway intermediates.
In lignified cells, lignin constitutes a major component of cell walls togeth-
er with cellulose and hemicellulose with a proportion of approximately 2:1:1
(cellulose, lignin, hemicellulose). The lignin content of cell walls is usually
expressed with regards to the relative abundance of other major components.
Consequently, a variation in cellulose synthesis will affect not only the
cellulose content but also the measured lignin content, even though lignin
biosynthesis per se may not have been affected. Therefore, changes in
lignin content must be interpreted carefully and analysed together with
other parameters such as histological analyses and/or enzymatic activities
involved in the lignin biosynthesis pathway and/or cellulose and hemicellu-
lose biosynthesis pathways. Lignins are deposited within the carbohydrate
matrix of the cell wall and can form chemical bonds with other cell wall
components. The quantification of lignin is difficult because lignins are
covalently linked with cell wall carbohydrates, proteins, phenolics, or other
compounds. These compounds may interfere with determination of lignins
leading to over- or underestimation (
Brinkmann et al., 2002; Dence, 1992
).
Lignins are essential for plant development and the production and depo-
sition of these polymers are highly regulated (
Rogers and Campbell, 2004
).
