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Suspension-cultured
Chenopodium album L. cells are capable of continuous,
long-term growth on a boron-deficient medium. Compared with cultures
grown with boron, these cultures contained more enlarged and detached
cells, had increased turbidity due to the rupture of a small number of
cells, and contained cells with an increased cell wall pore size. These
characteristics were reversed by the addition of boric acid (≥7
μm) to the boron-deficient cells. C. album
cells grown in the presence of 100 μm boric acid entered
the stationary phase when they were not subcultured, and remained
viable for at least 3 weeks. The transition from the growth phase to
the stationary phase was accompanied by a decrease in the wall pore
size. Cells grown without boric acid or with 7 μm boric
acid were not able to reduce their wall pore size at the transition to
the stationary phase. These cells could not be kept viable in the
stationary phase, because they continued to expand and died as a result
of wall rupture. The addition of 100 μm boric acid
prevented wall rupture and the wall pore size was reduced to normal
values. We conclude that boron is required to maintain the normal pore
structure of the wall matrix and to mechanically stabilize the wall at
growth termination.The ultrastructure and physical properties of plant cell walls are
known to be affected by boron deficiency (Kouchi and Kumazawa, 1976;
Hirsch and Torrey, 1980; Fischer and Hecht-Buchholz, 1985; Matoh et
al., 1992; Hu and Brown, 1994; Findeklee and Goldbach, 1996). Moreover,
boron is predominantly localized in the cell wall when plants are grown
with suboptimal boron (Loomis and Durst, 1991; Matoh et al., 1992; Hu
and Brown, 1994; Hu et al., 1996). In radish, >80% of the cell wall
boron is present in the pectic polysaccharide RG-II (Matoh et al.,
1993; Kobayashi et al., 1996), which is now known to exist as a dimer
that is cross-linked by a borate ester between two apiosyl residues
(Kobayashi et al., 1996; O''Neill et al., 1996). Dimeric RG-II is
unusually stable at low pH and is present in a large number of plant
species (Ishii and Matsunaga, 1996; Kobayashi et al., 1996, 1997; Matoh
et al., 1996; O''Neill et al., 1996; Pellerin et al., 1996; Kaneko et
al., 1997). The widespread occurrence and conserved structure of RG-II
(Darvill et al., 1978; O''Neill et al., 1990) have led to the
suggestion that borate ester cross-linked RG-II is required for the
development of a normal cell wall (O''Neill et al., 1996; Matoh, 1997).One approach for determining the function of boron in plant cell walls
is to compare the responses to boron deficiency of growing plant cells
that are dividing and synthesizing primary cell walls with those of
growth-limited plant cells in which the synthesis of primary cell walls
is negligible. Suspension-cultured cells are well suited for this
purpose because they may be reversibly transferred from a growth phase
to a stationary phase. Continuous cell growth phase is maintained by
frequent transfer of the cells into new growth medium (King, 1981;
Kandarakov et al., 1994), whereas a stationary cell population
is obtained by feeding the cells with Suc and by not subculturing them.
Cells in the stationary phase are characterized by mechanically
stabilized primary walls and reduced biosynthetic activity. Here we
describe the responses of suspension-cultured Chenopodium
album L. cells in the growth and stationary phases to boron
deficiency. These cells have a high specific-growth rate, no
significant lag phase, and reproducible changes in their wall pore size
during the transition from the growth phase to the stationary phase
(Titel et al., 1997). 相似文献
7.
8.
Indian mustard (Brassica
juncea) plants exposed to Pb and EDTA in hydroponic solution
were able to accumulate up to 55 mmol kg−1 Pb in dry shoot
tissue (1.1% [w/w]). This represents a 75-fold concentration of Pb
in shoot tissue over that in solution. A threshold concentration of
EDTA (0.25 mm) was found to be required to stimulate this
dramatic accumulation of both Pb and EDTA in shoots. Below this
threshold concentration, EDTA also accumulated in shoots but at a
reduced rate. Direct measurement of a complex of Pb and EDTA (Pb-EDTA)
in xylem exudate of Indian mustard confirmed that the majority of Pb in
these plants is transported in coordination with EDTA. The accumulation
of EDTA in shoot tissue was also observed to be directly correlated
with the accumulation of Pb. Exposure of Indian mustard to high
concentrations of Pb and EDTA caused reductions in both the
transpiration rate and the shoot water content. The onset of these
symptoms was correlated with the presence of free protonated EDTA
(H-EDTA) in the hydroponic solution, suggesting that free H-EDTA is
more phytotoxic than Pb-EDTA. These studies clearly demonstrate that
coordination of Pb transport by EDTA enhances the mobility within the
plants of this otherwise insoluble metal ion, allowing plants to
accumulate high concentrations of Pb in shoots. The finding that both
H-EDTA and Pb-EDTA are mobile within plants also has important
implications for the use of metal chelates in plant nutritional
research.The synthetic chelate EDTA forms a soluble complex with many
metals, including Pb (Kroschwitz, 1995), and can solubilize Pb from
soil particles (Means and Crerar, 1978). Recently, application of EDTA
to Pb-contaminated soils has been shown to induce the uptake of Pb by
plants (Jøgensen, 1993; Huang and Cunningham, 1996; Blaylock et al.,
1997; Huang et al., 1997), causing Pb to accumulate to more than 1%
(w/w) of shoot dry biomass (Huang and Cunningham, 1996; Blaylock et
al., 1997; Huang et al., 1997). For the in situ remediation of
Pb-contaminated soils it appears that this chelate-assisted
phytoextraction strategy (Salt et al., 1998) may be more effective than
a strategy based on the natural ability of certain wild plant species
for metal hyperaccumulation (Chaney, 1983; Baker et al., 1988).For more than 40 years, synthetic chelates have been used to supply
plants with micronutrients in both soil and hydroponics. Yet the
mechanisms by which chelates enhance metal accumulation are still not
well characterized (Wallace and Wallace, 1992), and what is known
appears contradictory. For example, some evidence suggests that the
Fe-chelate EDTA can be absorbed by plants and translocated to shoots
(Weinstein et al., 1954; Hill-Cottingham and Llyod-Jones, 1961, 1965).
However, Tiffin et al. (1960) concluded that Fe-chelates are excluded
from root tissue, and this was supported by Chaney et al. (1972), who
demonstrated that Fe is taken up by plants only after first being split
from the Fe-chelate complex by the action of a specific plasma
membrane-bound Fe-chelate reductase.To optimize the process of chelate-assisted phytoextraction, it is
important to understand the biological mechanisms responsible for this
process. Because of the stimulatory role of chelate application in the
uptake of Pb and other metals by plants, we have investigated the role
of EDTA in Pb accumulation in plants. In this study we have
demonstrated that the previously described EDTA-enhanced Pb
accumulation in Indian mustard (Brassica juncea) is based on
the ability of EDTA to chelate and transport Pb from soil into shoot
tissue. 相似文献
9.
Sally Wilkinson Janet E. Corlett Ludovic Oger William J. Davies 《Plant physiology》1998,117(2):703-709
The pH of xylem sap from tomato
(Lycopersicon esculentum) plants increased from pH 5.0
to 8.0 as the soil dried. Detached wild-type but not
flacca leaves exhibited reduced transpiration rates when
the artificial xylem sap (AS) pH was increased. When a well-watered
concentration of abscisic acid (0.03 μm) was provided in
the AS, the wild-type transpirational response to pH was restored to
flacca leaves. Transpiration from flacca
but not from wild-type leaves actually increased in some cases when the
pH of the AS was increased from 6.75 to 7.75, demonstrating an absolute
requirement for abscisic acid in preventing stomatal opening and
excessive water loss from plants growing in many different
environments.Jones (1980) and Cowan (1982) were the first to suggest that
plants can “measure” soil water status independently of shoot water
status via the transfer of chemical information from roots to shoots.
Dehydrating roots in drying soil synthesize ABA more rapidly than fully
turgid tissue, and resultant increases in the ABA concentration of
xylem sap flowing toward the still-turgid shoot constitutes a chemical
signal to the leaves (for review, see Davies and Zhang, 1991): the
xylem vessels give up their contents to the leaf apoplast, thereby
increasing the ABA concentration in this compartment. ABA receptors on
the external surface of stomatal guard cells respond to the apoplastic
ABA concentration (Hartung, 1983; Anderson et al., 1994; but see
Schwartz et al., 1994). When bound, the receptors transduce a reduction
in guard cell turgor, which leads to stomatal closure (Assmann, 1993).
This maintains shoot water potential despite the reduction in soil
water availability.Another chemical change related to soil drying in the absence of a
reduction in shoot water status is an increase in the pH of the xylem
sap flowing from the roots (Schurr et al., 1992). The pH of the xylem
and/or apoplastic sap of plants can also change dramatically in
response to soil flooding, diurnal or annual rhythms, and mineral
nutrient supply (Table (TableI)I) in the absence
of concomitant changes in either root or shoot water status. We already
know that, like the increase in xylem ABA concentration described
above, an increase in xylem pH can also act as a signal to leaves to
close their stomata (Wilkinson and Davies, 1997). Since the conditions
that affect xylem/apoplastic pH can also affect transpiration (light
intensity [Cowan et al., 1982]; soil drying [Davies and Zhang,
1991]; nitrate supply [Clarkson and Touraine, 1994]; soil flooding
[Else, 1996]), the possibility exists that the pH change that they
induce could be the means by which they alter stomatal aperture.
Table IpH changes that occur in plant xylem or apoplastic
sap under various conditions It was originally suggested that an increase in xylem sap pH could
putatively enhance stomatal closure by changing the distribution of the
ABA that is present in all nonstressed plants at a low “background”
concentration, without requiring de novo ABA synthesis (Schurr et al.,
1992; Slovik and Hartung, 1992a, 1992b). This hypotheses is built on
the well-known fact that weak acids such as ABA accumulate in more
alkaline compartments (Kaiser and Hartung, 1981). More recently,
Wilkinson and Davies (1997) and Thompson et al. (1997) directly
demonstrated that increases in xylem sap pH reduced rates of water loss
from Commelina communis and tomato (Lycopersicon
esculentum) leaves detached from well-watered plants. This was
found to be mediated by the relatively low endogenous concentration of
ABA (about 0.01 mmol m−3) contained in the xylem vessels
and apoplast of these leaves, a concentration of ABA that did not
itself affect transpiration at a well-watered sap pH of 6.0. The
mechanism by which the combination of high sap pH and such a low
concentration of ABA was able to increase the apoplastic ABA
concentration sufficiently to close stomata was also elucidated: the
mesophyll and epidermis cells of these leaves had a greatly reduced
ability to sequester ABA away from the apoplast when the pH of the
latter was increased by the incoming xylem sap (Wilkinson and Davies,
1997).In contrast to the indirect ABA-mediated effect of pH on stomata, it
was also demonstrated that increasing the pH of the external solution
(from 5.0 to 7.0) bathing isolated abaxial epidermis tissue peeled from
well-watered C. communis leaves actually increased
stomatal aperture (Wilkinson and Davies, 1997). Mechanisms for this
direct effect of pH on guard cells have been speculated on by Thompson
et al. (1997). If this process were to occur in vivo, environments that
increase xylem sap pH could potentially induce excessive water loss
from the plants experiencing them, over and above rates of
transpiration occurring in unstressed plants. The latter may contain
stomata with apertures smaller than the maximum that is possible, even
under favorable local conditions. It was assumed that high-pH-induced
apoplastic ABA accumulation in C. communis in vivo was
sufficient to override the direct stomatal opening effect seen in the
isolated tissue (Wilkinson and Davies, 1997). To test these
possibilities, effects of pH on transpiration rates from leaves of the
flacca mutant of tomato were investigated.
flacca does not synthesize ABA as efficiently as
wild-type tomato (Parry et al., 1988; Taylor et al., 1988). It contains
a very low endogenous ABA concentration (Tal and Nevo, 1973), although
it retains the ability to respond to an application of this hormone
(Imber and Tal, 1970). The results demonstrate not only that ABA
mediates high xylem sap pH-induced stomatal closure but also that it is
necessary to prevent high xylem sap pH-induced stomatal opening and
dangerously excessive water loss. 相似文献
10.
NAD-isocitrate
dehydrogenase (NAD-IDH) from the eukaryotic microalga
Chlamydomonas reinhardtii was purified to
electrophoretic homogeneity by successive chromatography steps on
Phenyl-Sepharose, Blue-Sepharose, diethylaminoethyl-Sephacel, and
Sephacryl S-300 (all Pharmacia Biotech). The 320-kD enzyme was found to
be an octamer composed of 45-kD subunits. The presence of isocitrate
plus Mn2+ protected the enzyme against thermal inactivation
or inhibition by specific reagents for arginine or lysine. NADH was a
competitive inhibitor (Ki, 0.14
mm) and NADPH was a noncompetitive inhibitor
(Ki, 0.42 mm) with respect to
NAD+. Citrate and adenine nucleotides at concentrations
less than 1 mm had no effect on the activity, but 10
mm citrate, ATP, or ADP had an inhibitory effect. In
addition, NAD-IDH was inhibited by inorganic monovalent anions, but
l-amino acids and intermediates of glycolysis and the
tricarboxylic acid cycle had no significant effect. These data support
the idea that NAD-IDH from photosynthetic organisms may be a key
regulatory enzyme within the tricarboxylic acid cycle.IDH catalyzes the oxidative decarboxylation of isocitrate to
produce 2-oxoglutarate. According to the specificity for the electron
acceptor, two enzymes with IDH activity are known, NAD-IDH (EC
1.1.1.41) and NADP-IDH (EC 1.1.1.42) (Chen and Gadal, 1990a).In photosynthetic organisms NADP-IDH has been detected in the cytosol,
chloroplasts, mitochondria, and peroxisomes. Cytosolic NADP-IDH has
been purified from higher plants (Chen et al., 1988) and eukaryotic
algae (Martínez-Rivas et al., 1996), and its cDNA has been
cloned from alfalfa (Shorrosh and Dixon, 1992), soybean (Udvardi et
al., 1993), potato (Fieuw et al., 1995), and tobacco (Gálvez et
al., 1996). This 80-kD isoenzyme is a dimer, and it is likely to be
involved in the synthesis of NADPH for biosynthetic purposes in the
cytosol (Chen et al., 1988), in the synthesis of 2-oxoglutarate for
ammonium assimilation (Chen and Gadal, 1990b), and in the cycling,
redistribution, and export of amino acids (Fieuw et al., 1995).
Chloroplastic NADP-IDH has been studied in higher plants (Gálvez
et al., 1994) and eukaryotic algae (Martínez-Rivas and Vega,
1994). It is a 154-kD dimer that has been proposed to be involved in
the supply of NADPH for biosynthetic reactions in the chloroplast when
photosynthetic NADPH production is low (Gálvez et al., 1994). The
mitochondrial NADP-IDH of higher plants may have a physiological role
in the production of NADPH, which can be converted to NADH by a
transhydrogenase or used to reduce glutathione in the mitochondrial
matrix (Rasmusson and Møller, 1990). NADP-IDH activity has also been
detected in peroxisomes from spinach leaves (Yamazaki and Tolbert,
1970).NAD-IDH is localized exclusively in the mitochondria in association
with the TCA cycle. This enzyme has been purified from several
nonphotosynthetic eukaryotes such as fungi (Keys and McAlister-Henn,
1990; Alvarez-Villafañe et al., 1996) and animals (Giorgio et
al., 1970), in which it appears to be a 300-kD octamer. Its key
regulatory role in the TCA cycle is well documented. The NAD-IDH from
yeast is activated by AMP and citrate (Hathaway and Atkinson, 1963),
whereas the animal enzyme is activated by ADP and citrate (Cohen and
Colman, 1972). In addition, the NAD-IDH cDNAs have been cloned from
yeast (Cupp and McAlister-Henn, 1991, 1992) and animals (Nichols et
al., 1995; Zeng et al., 1995). In these organisms, the enzyme is
composed of two (yeast) or more (animals) different subunits encoded by
different genes.To our knowledge, no NAD-IDH from photosynthetic organisms has yet been
purified to homogeneity, mainly because of the low stability of the
enzyme (Oliver and McIntosh, 1995). However, partial purifications have
been reported from pea (Cox and Davies, 1967; Cox, 1969; McIntosh
and Oliver, 1992), potato (Laties, 1983), spruce (Cornu et al., 1996),
and the eukaryotic microalga Chlamydomonas reinhardtii
(Martínez-Rivas and Vega, 1994). Matrix and membrane forms of
the enzyme have been detected in potato (Tezuka and Laties, 1983) and
pea (McIntosh, 1997). Although it is an allosteric enzyme that exhibits
sigmoidal kinetics with respect to isocitrate (Cox and Davies, 1967;
McIntosh and Oliver, 1992) and is activated in vitro by ABA (Tezuka et
al., 1990), the regulatory importance of NAD-IDH in photosynthetic
organisms is still under debate.To elucidate the regulatory significance of NAD-IDH in photosynthetic
organisms and its apparent contribution to the 2-oxoglutarate
supply for ammonium assimilation, we have purified and characterized
the NAD-IDH from C. reinhardtii. 相似文献
11.
12.
13.
Temporal and Spatial Patterns of Accumulation of the Transcript
of Myo-Inositol-1-Phosphate Synthase and Phytin-Containing
Particles during Seed Development in Rice 总被引:2,自引:2,他引:0 下载免费PDF全文
Kaoru T. Yoshida Tomikichi Wada Hiroshi Koyama Ritsuko Mizobuchi-Fukuoka Satoshi Naito 《Plant physiology》1999,119(1):65-72
14.
A 135-kD actin-bundling protein was
purified from pollen tubes of lily (Lilium longiflorum)
using its affinity to F-actin. From a crude extract of the pollen
tubes, this protein was coprecipitated with exogenously added F-actin
and then dissociated from F-actin by treating it with
high-ionic-strength solution. The protein was further purified
sequentially by chromatography on a hydroxylapatite column, a
gel-filtration column, and a diethylaminoethyl-cellulose ion-exchange
column. In the present study, this protein is tentatively referred to
as P-135-ABP (Plant 135-kD
Actin-Bundling Protein). By the
elution position from a gel-filtration column, we estimated the native
molecular mass of purified P-135-ABP to be 260 kD, indicating that it
existed in a dimeric form under physiological conditions. This protein
bound to and bundled F-actin prepared from chicken breast muscle in a
Ca2+-independent manner. The binding of 135-P-ABP to actin
was saturated at an approximate stoichiometry of 26 actin monomers to 1
dimer of P-135-ABP. By transmission electron microscopy of thin
sections, we observed cross-bridges between F-actins with a
longitudinal periodicity of 31 nm. Immunofluorescence microscopy using
rhodamine-phalloidin and antibodies against the 135-kD polypeptide
showed that P-135-ABP was colocalized with bundles of actin filaments
in lily pollen tubes, leading us to conclude that it is the factor
responsible for bundling the filaments.Actin filaments, one of the major components of the cytoskeleton,
are organized into a highly ordered architecture and are involved in
various kinds of cell motility. Their architecture is regulated by
several kinds of actin-binding proteins, including cross-linking
proteins, severing proteins, end-capping proteins, and
monomer-sequestering proteins in animal, protozoan, and yeast cells
(Stossel et al., 1985; Pollard and Cooper, 1986; Vandekerckhove
and Vancompernolle, 1992). In plant cells the organization of the actin
cytoskeleton also changes remarkably during the cell cycle or during
developmental processes, and it is suggested that actin-binding
proteins are involved in their dynamic change. However, little is known
about actin-binding proteins in plant cells.Only a low-Mr actin-binding and -depolymerizing
protein, profilin, in white birch (Betula verrucosa;
Valenta et al., 1991), maize (Zea mays; Staiger
et al., 1993; Ruhlandt et al., 1994), bean (Phaseolus
vulgaris; Vidali et al., 1995), tobacco (Nicotiana
tabacum; Mittermann et al., 1995), tomato (Lycopersicon
esculentum; Darnowski et al., 1996), Arabidopsis
(Arabidopsis thaliana; Huang et al., 1996), and lily
(Lilium longiflorum; Vidali and Hepler, 1997), and an ADF in
lily (Kim et al., 1993), rapeseed (Brassica napus; Kim
et al., 1993), and maize (Rozycka et al., 1995; Lopez et al., 1996),
have been identified by biochemical or molecular biological means.The native and recombinant forms of these proteins are capable of
binding to animal or plant actin (Valenta et al., 1993; Giehl et al.,
1994; Ruhlandt et al., 1994; Lopez et al., 1996; Perelroizen et al.,
1996; Carlier et al., 1997). Plant profilin expressed in mammalian
BHK-21 cells (Rothkegel et al., 1996) or profilin-deficient Dictyostelium discoideum cells (Karakesisoglou et al., 1996) was
able to functionally substitute for endogenous profilin in these cells.
The introduction of plant profilin into living stamen hair cells by
microinjection caused the rapid reduction of the number of actin
filaments (Staiger et al., 1994; Karakesisoglou et al., 1996; Ren et
al., 1997). These results indicate that plant profilin and ADF share
many functional similarities with other eukaryote profilins and
ADFs.It is well known that the actin cytoskeleton undergoes dynamic changes
in organization during hydration and activation of the vegetative cells
of pollen grains (Pierson and Cresti, 1992). Before hydration actin
filaments exist as fusiform or spiculate structures (a storage form),
but they are rearranged to form a network upon hydration
(Heslop-Harrison et al., 1986; Tiwari and Polito, 1988). In the growing
pollen tube there are strands or bundles of actin filaments parallel to
the long axis (Perdue et al., 1985; Pierson et al., 1986; Miller et
al., 1996) that are involved in cytoplasmic streaming (Franke et al.,
1972; Mascarenhas and Lafountain, 1972) and transport of vegetative
nuclei and generative cells to the growing tip (Heslop-Harrison et al.,
1988; Heslop-Harrison and Heslop-Harrison, 1989). Characterization of
the function of actin-binding proteins is essential to understanding
the regulation of actin organization during the developmental process
of pollen. Since only a small number of vacuoles containing proteases
develop in pollen grains and pollen tubes at a younger stage, pollen
tubes are suitable materials for isolating and biochemically studying
actin-binding proteins responsible for organizing actin filaments into
various forms.In a previous paper we reported that several components in a crude
extract prepared from lily pollen tubes, including a 170-kD myosin
heavy chain and 175-, 135-, and 110-kD polypeptides, could be
coprecipitated with exogenously added F-actin (Yokota and Shimmen,
1994). We also found that rhodamine-labeled F-actin was tightly bound
to the glass surface treated with the fraction containing the 135- and
110-kD polypeptides (Yokota and Shimmen, 1994). These results suggested
that either one or both of the 135- and 110-kD polypeptides possesses
an F-actin-binding activity. In the present study, we purified the
135-kD polypeptide from lily pollen tubes by biochemical procedures and
then characterized its F-actin-binding properties and distribution in
the pollen tubes. This protein was able to bundle F-actin isolated from
chicken breast muscle and colocalized with actin-filament bundles in
pollen tubes. We refer to this protein as P-135-ABP (Plant
135-kD Actin-Bundling
Protein). 相似文献
15.
16.
17.
Kent D. Chapman Swati Tripathy Barney Venables Arland D. Desouza 《Plant physiology》1998,116(3):1163-1168
Recently, the biosynthesis of an
unusual membrane phospholipid,
N-acylphosphatidylethanolamine (NAPE), was found to
increase in elicitor-treated tobacco (Nicotiana tabacum
L.) cells (K.D. Chapman, A. Conyers-Hackson, R.A. Moreau, S. Tripathy
[1995] Physiol Plant 95: 120–126). Here we report
that before induction of NAPE biosynthesis,
N-acylethanolamine (NAE) is released from NAPE in
cultured tobacco cells 10 min after treatment with the fungal elicitor
xylanase. In radiolabeling experiments [14C]NAE (labeled
on the ethanolamine carbons) increased approximately 6-fold in the
culture medium, whereas [14C]NAPE associated with cells
decreased approximately 5-fold. Two predominant NAE molecular species,
N-lauroylethanolamine and
N-myristoylethanolamine, were specifically identified by
gas chromatography-mass spectrometry in lipids extracted from culture
medium, and both increased in concentration after elicitor treatment.
NAEs were found to accumulate extracellularly only. A microsomal
phospholipase D activity was discovered that formed NAE from NAPE; its
activity in vitro was stimulated about 20-fold by mastoparan,
suggesting that NAPE hydrolysis is highly regulated, perhaps by
G-proteins. Furthermore, an NAE amidohydrolase activity that catalyzed
the hydrolysis of NAE in vitro was detected in homogenates of tobacco
cells. Collectively, these results characterize structurally a new
class of plant lipids and identify the enzymatic machinery involved in
its formation and inactivation in elicitor-treated tobacco cells.
Recent evidence indicating a signaling role for NAPE metabolism in
mammalian cells (H.H.O. Schmid, P.C. Schmid, V. Natarajan [1996] Chem
Phys Lipids 80: 133–142) raises the
possibility that a similar mechanism may operate in plant cells.NAPE is a widespread, albeit minor, membrane phospholipid in
animal and plant tissues (Schmid et al., 1990; Chapman and Moore,
1993). Its unusual structural features (a third fatty acid moiety
linked to the amino head group of PE) impart stabilizing properties to
membrane bilayers (Domingo et al., 1994; LaFrance et al., 1997). NAPE
and its hydrolysis products, NAEs, are known to accumulate in
vertebrate tissues under pathological conditions (for review, see
Schmid et al., 1990). Recently, there has been renewed interest in NAEs
because of the contention that anandamide
(N-arachidonylethanolamine) is an endogenous ligand for the
cannabinoid receptor in mammalian brain (Devane et al., 1992; Fontana
et al., 1995; Schmid et al., 1996). The likely route for NAE formation
in neural and nonneural tissues, although the matter of some debate, is
via the signal-mediated hydrolysis of NAPE (DiMarzo et al., 1994;
Schmid et al., 1996; Sugiura, et al., 1996).In plants little is known regarding the catabolism of NAPE. In
cottonseed microsomes NAPE was metabolized to NAE or NAlysoPE by PLD-
or PLA-type activities, respectively (Chapman et al., 1995b). However,
the metabolic fate of NAPE in vivo and the factors that regulate NAPE
hydrolysis remain largely unknown. We previously noted that the
biosynthesis of NAPE was increased in elicitor-treated cell suspensions
of tobacco (Nicotiana tabacum L.). Here we extend our
investigations with this model system to examine NAPE catabolism by
plant cells in vivo. NAE was released from NAPE, and it accumulated
extracellularly. We identified by GC-MS these tobacco NAEs as
N-lauroylethanolamine and
N-myristoylethanolamine. These NAEs were increased in
elicitor-treated cell suspensions. Furthermore, we detected the
enzymatic machinery capable of the release and the degradation of NAEs
in tobacco cells. To our knowledge this represents the first
identification of the NAE molecular species in plant cells. It is
tempting to speculate that NAPE hydrolysis in elicitor-treated plant
cells may be involved in a signaling pathway analogous to that found in
mammalian cells. 相似文献
18.
Lidia Osuna Jean-N?el Pierre María-Cruz González Rosario Alvarez Francisco J. Cejudo Cristina Echevarría Jean Vidal 《Plant physiology》1999,119(2):511-520
Phosphoenolpyruvate
carboxylase (PEPC) activity was detected in aleurone-endosperm extracts
of barley (Hordeum vulgare) seeds during germination,
and specific anti-sorghum (Sorghum bicolor)
C4 PEPC polyclonal antibodies immunodecorated constitutive
103-kD and inducible 108-kD PEPC polypeptides in western analysis. The
103- and 108-kD polypeptides were radiolabeled in situ after imbibition
for up to 1.5 d in 32P-labeled inorganic phosphate. In
vitro phosphorylation by a Ca2+-independent PEPC protein
kinase (PK) in crude extracts enhanced the enzyme''s velocity and
decreased its sensitivity to l-malate at suboptimal pH and
[PEP]. Isolated aleurone cell protoplasts contained both
phosphorylated PEPC and a Ca2+-independent PEPC-PK that was
partially purified by affinity chromatography on blue dextran-agarose.
This PK activity was present in dry seeds, and PEPC phosphorylation in
situ during imbibition was not affected by the cytosolic
protein-synthesis inhibitor cycloheximide, by weak acids, or by various
pharmacological reagents that had proven to be effective blockers of
the light signal transduction chain and PEPC phosphorylation in
C4 mesophyll protoplasts. These collective data support the
hypothesis that this Ca2+-independent PEPC-PK was formed
during maturation of barley seeds and that its presumed underlying
signaling elements were no longer operative during germination.Higher-plant PEPC (EC 4.1.1.31) is subject to in vivo
phosphorylation of a regulatory Ser located in the N-terminal domain of
the protein. In vitro phosphorylation by a
Ca2+-independent, low-molecular-mass (30–39 kD)
PEPC-PK modulates PEPC regulation interactively by opposing metabolite
effectors (e.g. allosteric activation by Glc-6-P and feedback
inhibition by l-malate; Andreo et al., 1987), decreasing
significantly the extent of malate inhibition of the leaf enzyme
(Carter et al., 1991; Chollet et al., 1996; Vidal et al., 1996; Vidal
and Chollet, 1997). These metabolites control the rate of
phosphorylation of PEPC via an indirect target-protein effect (Wang and
Chollet, 1993; Echevarría et al., 1994; Vidal and Chollet,
1997).Several lines of evidence support the view that this protein-Ser/Thr
kinase is the physiologically relevant PEPC-PK (Li and Chollet, 1993;
Chollet et al., 1996; Vidal et al., 1996; Vidal and Chollet, 1997). The
presence and inducible nature of leaf PEPC-PK have been established
further in various C3, C4,
and CAM plant species (Chollet et al., 1996). In all cases, CHX proved
to be a potent inhibitor of this up-regulation process so that apparent
changes in the turnover rate of PEPC-PK itself or another, as yet
unknown, protein factor were invoked to account for this observation
(Carter et al., 1991; Jiao et al., 1991; Chollet et al., 1996).
Consistent with this proposal are recent findings about PEPC-PK from
leaves of C3, C4, and CAM
plants that determined activity levels of the enzyme to depend on
changes in the level of the corresponding translatable mRNA (Hartwell
et al., 1996).Using a cellular approach we previously showed in
sorghum (Sorghum bicolor) and hairy crabgrass
(Digitaria sanguinalis) that PEPC-PK is
up-regulated in C4 mesophyll cell protoplasts
following illumination in the presence of a weak base
(NH4Cl or methylamine; Pierre et al., 1992;
Giglioli-Guivarc''h et al., 1996), with a time course (1–2 h) similar
to that of the intact, illuminated sorghum (Bakrim et al., 1992) or
maize leaf (Echevarría et al., 1990). This light- and
weak-base-dependent process via a complex transduction chain is likely
to involve sequentially an increase in pHc, inositol
trisphosphate-gated Ca2+ channels of the
tonoplast, an increase in cytosolic Ca2+, a
Ca2+-dependent PK, and PEPC-PK.Considerably less is known about the up-regulation of PEPC-PK and
PEPC phosphorylation in nongreen tissues. A sorghum root PEPC-PK
purified on BDA was shown to phosphorylate in vitro both recombinant
C4 PEPC and the root
C3-like isoform, thereby decreasing the enzyme''s
malate sensitivity (Pacquit et al., 1993). PEPC from soybean root
nodules was phosphorylated in vitro and in vivo by an endogenous PK
(Schuller and Werner, 1993; Zhang et al., 1995; Zhang and Chollet,
1997). A Ca2+-independent nodule PEPC-PK
containing two active polypeptides (32–37 kD) catalyzed the
incorporation of phosphate on a Ser residue of the target enzyme and
was modulated by photosynthate transported from the shoots (Zhang and
Chollet, 1997). Regulatory seryl phosphorylation of a heterotetrameric
(α2β2) banana fruit
PEPC by a copurifying, Ca2+-independent PEPC-PK
was shown to occur in vitro (Law and Plaxton, 1997). Although
phosphorylation was also detected in vivo and found to concern
primarily the α-subunit, PEPC exists mainly in the dephosphorylated
form in preclimacteric, climacteric, and postclimacteric fruit.In a previous study we showed that PEPC undergoes regulatory
phosphorylation in aleurone-endosperm tissue during germination of
wheat seeds (Osuna et al., 1996). Here we report on PEPC and the
requisite PEPC-PK in germinating barley (Hordeum vulgare)
seeds. PEPC was highly phosphorylated by a
Ca2+-independent Ser/Thr PEPC-PK similar to that
found in other plant systems studied previously (Chollet et al., 1996);
however, the PK was already present in the dry seed and its activity
did not require protein synthesis during imbibition. 相似文献
19.
20.
Arabidopsis has
inducible responses for tolerance of O2 deficiency. Plants
previously exposed to 5% O2 were more tolerant than the
controls to hypoxic stress (0.1% O2 for 48 h) in both
roots and shoots, but hypoxic acclimation did not improve tolerance to
anoxia (0% O2). The acclimation of shoots was not
dependent on the roots: increased shoot tolerance was observed when the
roots of the plants were removed. An adh
(alcohol dehydrogenase) null mutant
did not show acclimation of the roots but retained the shoot survival
response. Abscisic acid treatment also differentiated the root and
shoot responses; pretreatment induced root survival in hypoxic stress
conditions (0.1% O2) but did not induce any increase in
the survival of shoots. Cycloheximide blocked both root and shoot
acclimation, indicating that both acclimation mechanisms are dependent
on protein synthesis.The supply of O2 to plant tissues may be
restricted under certain environmental conditions (Hook and Crawford,
1978). When air spaces normally present in the soil become saturated
with water, the root environment becomes hypoxic or anoxic as a result
of O2 consumption by respiring roots and
microorganisms and the insufficient diffusion of
O2 through water (Armstrong, 1979).
O2 deficiency is thought to be a major
determinant in the adverse effects of waterlogging on crops and other
plant species (Jackson et al., 1991). Plants have evolved inducible
metabolic mechanisms to cope with these ephemeral,
low-O2-stress conditions. When exposed to
low-O2 conditions, plants switch to the
expression of “anaerobic” polypeptides (Sachs et al., 1980, 1996).
The induction of these proteins may be responsible for the tolerance to
O2 deficiency that would otherwise be lethal. A
number of anaerobic polypeptides have been identified as enzymes
involved in glycolysis and ethanol fermentation (for a recent review,
see Vartapetian and Jackson, 1997), and this supports the view that
when O2 is limiting, oxidative catabolism of
sugars is hindered and ethanolic fermentation acts as an alternative
energy-producing pathway.Ethanol is the main end product of anaerobic metabolism in plants
(Smith and ap Rees, 1979; Good and Muench, 1993). Unlike lactate, which
is also generated under O2 deficiency, ethanol is
a relatively nontoxic end product (Jackson et al., 1982) and does not
lead to the acidification of the cytoplasm, a major determinant in
intolerance to O2 deficiency (Roberts et al.,
1984, 1985). The induction of glycolytic enzymes probably reflects the
need for increased glycolysis to compensate for the lower ATP yield of
ethanol fermentation.The importance of ethanol fermentation is supported by studies of
adh (alcohol
dehydrogenase) null mutants in a number of
species (Schwartz, 1966; Harberd and Edwards, 1982; Jacobs et al.,
1988; Matsumura et al., 1995), which report reduced tolerance to
O2 deficiency in these plants.Some plant tissues exposed to a period of mild hypoxia show more
tolerance to subsequent hypoxic or anoxic stress than plants kept in
fully aerated conditions before the stress (for review, see Drew, 1997;
see also more recent work on tomato [Germain et al., 1997] and rice
[Ellis and Setter, 1999]).In this study we examined the survival of Arabidopsis plants after
exposure to anoxic or hypoxic stress. Our results demonstrate that
hypoxic pretreatment protects against hypoxic stress and that different
mechanisms of acclimation to hypoxic stress are operative in root and
shoot tissues. 相似文献