Biology Reference
In-Depth Information
with high efficiency both the 3- and 5-hydroxylation of the corresponding
aldehydes and alcohols (
Weng et al., 2008b, 2010a
). CYP788A1 was first
shown to encode a F5H with substrate specificities similar to CYP84A1 from
Arabidopsis that was able to complement the fah1/cyp84a1 phenotype (
Weng
et al.,2008b
). Then it became clear that the Selaginella enzyme also has
3-hydroxylase activity, but uses only 4-coumaraldehyde and 4-coumaryl alco-
hol as substrates, while 4-coumaroyl shikimate is not converted. Also free
4-coumarate as well as caffeic acid and cinnamic acid and the respective
aldehydes and alcohols are not substrates for the Selaginella enzyme. The
enzyme was also able to complement the c3
0
h/cyp98a3 phenotype as it allowed
G- and S-lignin production in the cyp84a1/cyp98a3 double mutant when
expressed driven by a C4H promoter (
Weng et al.,2010a
). Taken together,
this shows that the Selaginella C3H/F5H enzyme can bypass a large portion
of the phenylpropanoid pathway as it is currently understood in angiosperms,
namely, HCT, C3
0
H, and CCoAOMT. Instead, Selaginella can generate sina-
poyl alcohol from 4-coumaraldehyde or 4-coumaryl alcohol (
Fig. 1
). This
involves the consecutive action of CYP788A1 and a bifunctional COMT
that first methoxylates the 3-hydroxy product of CYP788 (thereby providing
the second substrate of CYP788) and then the 5-hydroxy product produced by
the F5H activity of CYP788. Indeed, recently, a COMT from Selaginella has
been characterized that is localized in close vicinity to the C3H/F5H gene and
has both activities needed to complete the alternative pathway (
Weng et al.,
2011
). Both the Selaginella C3H/F5H and the COMT genes are only distantly
related to their angiosperm couterpart, and it is more than unlikely that the last
common ancestor of the respective genes had the common enzymatic activities.
Thus, the pathways towards S-lignin evolved independently in the lycopod and
angiosperm lineages, respectively. This is a remarkable example of convergent
evolution deciphered on the molecular genetic level that highlights the adap-
tive advantage of S-lignin in plants. It remains to be noted that not all lycopods
produce S-lignin, while all do produce G-lignin. In this respect, it is notewor-
thy, that the Selaginella genome does contain a CYP98A family member
annotated as CYP98A38. It would be informative to elucidate the biochemical
function of this enzyme to see if the 'canonical' pathway towards G-lignin via
4-coumaroyl-shikimate exists in this plant lineage in parallel.
III. CONCLUDING REMARKS
Interest in understanding lignin biosynthesis on the molecular level has been
rejuvenated in recent years driven by the exciting new possibilities offered by
technological advances of analytical and genomic tools. Also, the prospect of
