Biology Reference
In-Depth Information
concentrations in the low mM range. Experimental evidence suggests that
coniferin is stored in the vacuoles until the tonoplast disintegrates
during programmed cell death (
Samuels et al., 2002
). The enzyme coniferin
b
-glucosidase has been immunolocalised to lignifying secondary cell walls in
pine suggesting that deglucosylation of coniferin takes place in the cell wall
space during lignification (
Samuels et al. 2002
). However, it is still unclear
what relevance coniferin formation and hydrolysis has for lignification in
conifers. Feeding Pinus contorta with radioactive phenylalanine did not
result in labelling of coniferin or the vacuolar compartment, where coniferin
accumulates, suggesting that glucoside formation plays no significant role in
lignification (
Kaneda et al., 2008
). However, feeding G. biloba plants with
labelled coniferin did result in labelled lignins (
Terashima et al., 2009; Xie
et al., 1994
). The mechanism of transport of coniferin across the tonoplast
has not been identified to date, and the same applies to the transport of
monolignols across the plasma membrane. Recent studies from angiosperm
species make it likely that both of these transport mechanisms involve ABC
transporters (
Liu et al., 2011; Miao and Liu, 2010
), but it still needs to be
determined whether the same applies to conifers.
The cellular biology of lignification in conifers has not been studied in
great detail to date and such studies are complicated by the fact that most
enzymes involved in monolignol biosynthesis in conifers seem to be encoded
by gene families (
Koutaniemi et al., 2007
). Cellular studies of the phenylpro-
panoid pathway in angiosperms indicated that enzymes required for mono-
lignol biosynthesis are likely to be organised in membrane-associated enzyme
complexes. This enables efficient metabolite channelling and pathway regu-
lation (
Achnine et al., 2004; Winkel-Shirley, 1999
) and prevents release
of cinnamic acid that can interfere with auxin function (
Weng and
Chapple, 2010
). Proteomic studies in pine also support the idea that proteins
associated with the phenylpropanoid pathway are organised in the form
of an enzyme complex, as membrane-associated isozymes, for many of the
required enzymatic steps (
Mast et al., 2010
). Such a membrane-associated
protein complex might also incorporate enzymes that have supporting roles
in monolignol biosynthesis. For example, a membrane-associated glutamine
synthetase was identified in the proteomic analysis of pine compression wood
(
Mast et al., 2010
), and could support lignification by recapturing ammonia
released through deamination of
L
-phenylalanine. The precise sub-cellular
localisation of such a lignin-related enzyme complex still seems to be disput-
ed. A recent study in poplar suggested that such enzyme complexes could be
associated with the plasma membrane in angiosperms (
Nilsson et al., 2010
),
but earlier studies favour an association with the ER (
Achnine et al., 2004;
Winkel-Shirley, 1999
).
