Biology Reference
In-Depth Information
and providing stability to the root during growth and water/nutrient transport
(
Schreiber et al., 2000
). In the maize root epidermis, several genes of the lignin
biosynthesis pathway are expressed early in development, i.e., OMT
(O-methyltransferase) and CCoAOMT (caffeoyl-CoA O-methyltransferase;
Civardi et al., 1999; Collazo et al., 1992
). Also, the Z. mays proline-rich protein
(ZmPRP) and peroxidase2 (ZmPox2) were shown to have an expression pat-
tern similar to genes of the lignin biosynthesis pathway in both the epidermis
and xylem (
de Obeso et al., 2003; Vignols et al., 1999
); however, the exact
functions of these genes remain unknown. On a chemical level,
Zeier et al.
(1999)
demonstrated that within the root, the total lignin content increases
throughout the development and elongation of this organ and that the increases
are associated with lignin compositional changes in cell walls of primary roots.
Additionally, some studies show that lignin is highly deposited in root cells such
as the epidermis and xylem and that roots may contain high levels of pCA
(
Dokken and Davis, 2007; Hatfield and Chaptman, 2009
). However, grasses
have a unique root system (embryonic roots, lateral roots, coleoptiles roots,
crown roots or adventitious roots;
Osmont et al., 2007
), and more studies are
needed to fully characterize lignification of this tissue.
In stems, chemical analysis of lignin deposition indicates that most cells
undergo lignification at a precise stage of development that could be different
from each type of cell (
Buxton and Redfearn, 1997
); the first lignified cells
being the xylem and fibre cells (
Terashima et al., 1993
). Interestingly, lignin
deposition in stems is comparable to roots. Like roots, high amounts of pCA
is found in stems as compared to dicotyledons and likely associated with the
production of S unit lignins (
Seca et al., 2000
). One main role of lignification
in grass stems is lodging resistance and flexibility (
Ma, 2009
).
Sindhu et al.
(2007)
showed a trade-off between lignin deposition and stem flexibility in
the brittle stalk 2 (bk2) mutation (in maize), a mutation resulting in a higher
number of S unit lignin causing an increase of lignin production ultimately
decreasing stem flexibility (
Sindhu et al., 2007
). These results complemented
previous studies which also attributed the bk2 phenotype to a decrease in
cellulose content in fibre cells (
Ching et al., 2006
). Noteworthy, bk2 was also
shown to alter leaf flexibility.
Lignins accumulate in leaves primarily in the vasculature zone mostly with
G and S units (
Vincent et al., 2005
). Thus in maize leaves,
Vincent et al.
(2005)
demonstrated a role for caffeic acid OMT (COMT) in regulating
the zone of lignification and that under drought stress induction, the zone
of lignification shifts to more basal regions of the leaf. In addition, COMT
was shown to modulate the amount of lignin deposition into the cell wall,
suggesting that lignification may be a dynamic and adaptive process (
Vincent
et al., 2005
).
