Lignification and Lignin Manipulations in Conifers - Botanical Research - page 58

Fig. 6. Impact of CCR suppression on monolignol biosynthesis in P. abies. CCR
suppression restricted the biosynthesis of both p-coumaryl alcohol and coniferyl
alcohol (dashed arrows).
CAD in angiosperm species seems to be encoded by a small gene family
( Bomati and Noel, 2005; Jourdes et al., 2007; Sibout et al., 2003, 2005 ), and
individual genes differ in their substrate preference, catalytic activity and
expression pattern ( Bomati and Noel, 2005; Kim et al., 2004; Sibout et al.,
2003 ). Suppression of a single lignin-related CAD gene resulted in quite
modest reductions in lignin content in angiosperms, but could still cause
substantial changes in lignin composition, for example, changes in the
S:G-ratio ( Lapierre et al., 2004; Sibout et al., 2003 ). It was speculated that
individual members of the CAD gene family in angiosperms can compensate
for loss in activity caused by suppression of an individual CAD gene, as
suppression of two lignin-related CAD genes in A. thaliana resulted in an
almost 50% loss in lignin content ( Jourdes et al., 2007; Sibout et al., 2005 ).
CAD in conifers, to the contrary, seems to be encoded by single copy gene
( MacKay et al., 1997; Wagner et al., 2005 ) and this explains why the P. taeda
cad-n1 mutant has virtually no measurable CAD activity in developing xylem
( Ralph et al., 1997 ). Surprisingly, the homozygous cad-n1 mutant displayed
only very moderate reductions in lignin content compared to the wild-type
control ( Table I ; Lapierre et al., 2000; MacKay et al., 1997 ). Reduced CAD