Agriculture Reference
In-Depth Information
the forage crop stem is thus a key aspect of
its nutritive value, and by consequence also
inl uences the proi tability of the livestock
farm. Forage digestibility is correlated
negatively with concentration in lignin - a
complex polymer that is a constituent of
plant cell walls (Jung and Vogel, 1986).
Lignii cation of plant tissues, which
increases with plant maturation, slows down
the hydrolysis of polysaccharides to simple
sugars and prevents full cell wall digestion in
the rumen of ruminant animals (Fu
et al
.,
2011; Jung
et al
., 2012). Similar barriers due
to lignin exist for the industrial conversion
of sugars contained in cell wall poly-
saccharides to ethanol, which is an important
step for biofuel production.
Dif erent strategies have been explored
to improve the digestibility of forage, such
as chemical pretreatment of fodders or
conventional breeding of forage crops. While
the i rst option is not economically rational,
the latter has resulted in the development of
varieties with improved digestibility. For
maize, the cereal crop that is the most widely
used as forage, conventional breeding has
allowed cell wall digestibility to almost
double (Barrière
et al
., 2009). Spontaneous
mutants in maize and sorghum, known as
brown midrib (
bm
), which feature reduced
lignin concentration and improved digesti-
bility, have also been used to obtain a few
commercial varieties (Oliver
et al
., 2004).
However, this mutation is recessive and thus
the breeding of
bm
hybrids is laborious,
since the mutation should be obtained in all
copies of the gene carried in the plant
genome.
h erefore, transgenic approaches of er
new perspectives for increasing the digesti-
bility of forages such as maize, sorghum,
lucerne, tall fescue or switchgrass (see
Chapter 7). GM techniques allow the
improvement of cell wall digestibility, rather
than total digestibility, by changing the
composition of lignin or, even better, of the
cell wall itself (Jung
et al
., 2012).
events under research (see Table 12.3): nine
are purely forage crops (lucerne, tall fescue
and ryegrass, among others) and six are
cereal crops (maize, sorghum and rice). h e
interest in such plants has grown steadily in
the recent years, as shown for instance by
the high number of i eld trial requests in the
USA. Interestingly, most of the research
ef orts to improve the digestibility of crops
are concentrated in developed countries,
mainly because of the prospects for biofuel
production based on low-lignin GM crops in
these countries. Only one event has reached
the regulatory phase of the pipeline so far
(lucerne event KK 179-5), but at least four
more are in advanced development.
Lucerne is the plant that has received the
most attention and that concentrates the
largest number of events in the advanced
development stage. Indeed, two teams based
in the USA started research to improve its
digestibility more than 10 years ago.
Numerous i eld trials have been conducted
since 2000 to test lucerne lines with a
downregulation of various enzymatic path-
ways that play a key role in lignin
biosynthesis (Reddy
et al
., 2005; Chen
et al
.,
2006). h is has resulted in the production of
transgenic lucerne lines with reduced lignin
content and acceptable agronomic perform-
ance, which display conclusive digestibility
improvement in feeding trials (Reisen
et al
.,
2009). h e i rst GM plant with a reduced
lignin content to reach the commercial
pipeline will likely be a product of these
research ef orts, since one event is currently
in the regulatory pipeline. Stacks with
herbicide-tolerant trait events can also be
expected.
h e downregulation of a lignin
biosynthetic enzyme (CAD) of tall fescue
(
Festuca arundinacea
), an important
perennial forage crop for the cool season, is
also reaching advanced development. While
various i eld trials did not show a signii cant
dif erence regarding the agronomic perform-
ance of this GM tall fescue compared to the
control plants, its lignin content was reduced
signii cantly and this improved its
digestibility by up to 9.5% (Chen
et al
.,
2003). More recently, another team using a
dif erent approach also reported promising
The pipeline for low-lignin GM forage crops
h e pipeline for improved digestibility crops
is a rather active one, with about 15 dif erent
