Page 177 - Advances in Textile Biotechnology

158 Advances in textile biotechnology

upon the plant line analyzed, and that soluble phenolic content increased.
In addition, GC–MS analysis of cell wall fractions indicated reductions in
the amount of pectin and hemicellulose cell wall sugars. In contrast, no
changes were observed in the overall cellulose content of stem cell walls.
Interestingly, the reductions in lignin content were associated with modifi -
cations in whole stem mechanical properties (increased Young’s modulus
and tensile stiffness) possibly as a result of the changed ratio between cel-
lulose and lignin/pectins/hemicelluloses. Initial tests also showed that CAD
down-regulated lines were approximately twice as sensitive as control
plants to infection with the fungal pathogen F. oxysporum sp. lini. Although
such an observation would tend to suggest that the CAD engineering
leading to improvement in stem mechanical properties could be offset by
increased sensitivity to plant pathogens, further investigations are neces-

sary. In particular, it would be interesting to confirm that the improved

mechanical properties of in vitro whole flax stems are equally associated

with modifications in the mechanical properties of individual fi bres.

In another study (Day et al., 2009), flax plants were engineered to down-
regulate another lignin biosynthetic gene – caffeoyl coenzyme A O-methyl
transferase (CCoAOMT). The CCoAOMT gene codes for a lignifi cation
enzyme involved in the control of both overall lignin monomer content
(quantity), and lignin structure. Previous work has shown that fl ax lignin is
poor in lignin syringyl units (S-units) and is therefore chemically closer to
the lignin type found in conifers (Day et al., 2005a). Analyses of greenhouse-
grown plants showed that CCoAOMT down-regulation was associated with
reductions (8–18%) in stem lignin content, as well as with slight modifi ca-
tions in the structure of the lignin polymer. Although the mechanical prop-
erties of the flax stems were not evaluated, microscopic examination of stem

cross-sections revealed that CCoAOMT down-regulation was associated
with decreased cell wall thickness and reduced mechanical strength as
indicated by the irregular outline of certain xylem cells. In addition,
CCoAOMT down-regulation also appeared to be associated with increased
xylem cell size. However, despite careful examination of engineered plants,

no evident modifications in the structure of bast fibres were observed.

Other cell-wall associated genes such as those associated with pectin remod-
elling could also represent potential targets for engineering in fl ax
(Al-Qsous et al., 2004).

Another engineering strategy used in flax aimed to improve retting effi -

ciency (Musialak et al., 2008). As indicated above, fl ax fibres occur as bundles
in the outer stem tissue, and have to be isolated during industrial processing.
The first step in this process is called retting in which plants are left in the

field after harvesting (dew-retting) and are colonized by a variety of soil

micro-organisms. During this process, bacteria and fungi produce hydrolytic
enzymes that start to degrade the intercellular layers between the fi bre

© Woodhead Publishing Limited, 2010