Biology Reference
In-Depth Information
mutant: although shoot lignin is almost completely devoid of S- and
G-lignins (confirming the ref8 results described above), these plants still
produce detectable levels of soluble sinapoyl conjugates and display an
aberrant lignification phenotype in roots with substantial amounts of
G- and S-units (
Abdulrazzak et al., 2006
). Thus, a CYP98A3-independent
pathway exists in Arabidopsis, but activation of this alternative pathway
cannot complement a defect in the 'normal' developmental lignin pathway.
One possibility is that meta-hydroxylated phenylpropanoids are released
from the spermidine conjugates produced via CYP98A8/A9 in Arabidopsis
(
Matsuno et al., 2009
). These genes are expressed predominantly in anthers
during normal development but are slightly induced by environmental stres-
ses. It thus seems possible that hydroxycinnamoyl conjugates not normally
produced as lignin intermediates can be recruited 'at times of despair'.
C. COMMITTED TO S-LIGNIN: THE 5-HYDROXYLASE OF THE AROMATIC RING
Hydroxylation in the 5-position of the aromatic ring, after methoxylation of
the 3-hydroxy group added by C3
0
H, is required for S-lignin biosynthesis
and also for the biosynthesis of a diverse range of sinapate conjugates. The
accumulation of such a conjugate, namely, sinapoyl malate, in Arabidopsis
leaves was the basis of a simple, yet very effective screen for phenylpropanoid
mutants. Sinapoyl malate causes a blue-green fluorescence under UV light in
intact leaves, and numerous mutants have been identified with a reduced
epidermal fluorescence phenotype caused by an absence of sinapoyl malate
(
Ruegger and Chapple, 2001
). One of these mutants is almost completely
devoid of S-lignin and accumulates almost exclusively G-units instead. The
mutant also accumulates free ferulic acid and is unable to incorporate
ferulate into sinapate in feeding assays, while 5-hydroxyferulate can be
converted (
Chapple et al., 1992
). Therefore, a lack of F5H activity in the
mutant was suggested and the mutant was called fah1 for ferulic acid hydrox-
ylase after it was shown to be caused by a mutation in a novel family of
cytochrome P450 monooxygenase, namely, CYP84A (
Meyer et al., 1996
).
1. Another twist in the pathway: F5H actually is a coniferaldehyde/ coniferyl
alcohol 5-hydroxylase
When the Arabidopsis CYP84A1 (FAH1/F5H) was expressed in yeast, it
came as a surprise that the enzyme only poorly catalyses the 5-hydroxylation
of ferulate with kinetic properties that make a physiological function as
F5H unlikely. Instead, coniferaldehyde and also coniferyl alcohol were
5-hydroxylated with much higher efficiency (
Humphreys et al., 1999
). At
the same time, CYP84A4 from sweetgum (Liquidambar styraciflua) was also
