Biology Reference
In-Depth Information
Unique to grasses is the intercalary meristem or node, which functions in the
development of the plant, particularly in the regulation of water and nutrient
transport and the preservation of the structural integrity of the stem (
Niklas,
1997, 1998
).
Seca et al.(2000)
show that lignified cells in the node of Arundo
donax are comprised of more condensed lignins and are richer in p-coumaric
esters and ferulic esters than internodes (8% vs. 1.2%;
Seca et al., 2000
). On the
molecular level, little is known about the genetic regulators of node lignifica-
tion. Rice (Oryza sativa) studies of the brittle culm 5 (bc5) show that this gene is
required for the proper formation and lignification of the rice node specifically
in fibre cells (
Aohara et al., 2009
). Other studies have supported the findings by
demonstrating that genes in the brittle culm family, when mutated, lead to the
downregulation of several cell wall related components including a decrease in
lignin deposition in the wall (reviewed in
Anterola and Lewis, 2002; Kotake
et al., 2011; Li et al., 2003; Wang et al., 2010
).
III. LIGNIN BIOSYNTHESIS AND GENETICALLY
ENGINEERED PLANTS
Based on recent phylogenetic studies, expansion of the lignin biosynthesis
gene families happened primarily after the speciation of mono- and dicoty-
ledons (
Xu et al., 2009
). Genes encoding enzymes for specific events of
hydroxylation or O-methylation of the phenolic core of monolignols are
found for both lineages however, several differences have been suggested.
For example, the presence of a tyrosine ammonia lyase (TAL) activity that
could partially substitute or complement phenylalanine ammonia lyase
(PAL) activity in grasses (
Higuchi et al., 1967; Neish, 1961
). Unfortunately,
this assumption was not confirmed with any recent data or with transgenic
plants and mutants. Similarly, the abundance of p-hydroxycinnamic acids in
grass cell walls as compared to dicotyledons suggests that different pathways
or/and gene regulators are active in these two kingdoms. Until now, mechan-
isms responsible for these differences in cell wall composition were poorly
understood. Several genes encoding feruloyl transferases have been identified
in rice (
Piston et al., 2010a,b
). Nevertheless, due to the increased interest in
alternative energies generated from lignocellulosic plants, several studies
using transgenic grasses dedicated to biofuels (or digestibility by animals)
have helped to decipher some key steps of the lignin pathway. The following
paragraphs describe phenotypes observed in plants with genes altered
in specific steps of the lignin biosynthesis pathway (summarized in
Table I
).
Noteworthy, gene families in Brachypodium and maize are cited as model
references for C3 and C4 plants, respectively.
