Osmotic Stress in Plants
Transcript of Osmotic Stress in Plants
Székely Gyöngyi
Osmotic Stress in Plants
Gyöngyi SzékelyInstitute of Plant Biology, Biological Research Center,
6726 Szeged, Hungary, Temesvári krt. 62, [email protected]
Abstract
Plants are exposed to diff erent types of environmental stress conditions.
Osmotic stress conditions, such as salinity, drought and low temperature
are important factors limiting plant growth and crop productivity. In re-
sponse to environmental conditions causing osmotic stress, many higher
plants accumulate diff erent protective compounds. One of the most studied
compatible osmolyte is the proline. Th is amino acid has been proposed to
participate in the protection of plasma membrane integrity and to act as a
scavenger of reactive oxygen radicals. In addition, proline can be a source of
reducing power and to serve as a transient storage of carbon and nitrogen
and is required for protein biosynthesis. Th e proline homeostasis is tightly
regulated by its synthesis, degradation and transport. Th is paper summa-
rizes the identifi cation of genes induced by osmotic stress, and emphasizes
the role of proline accumulation during osmotic stress conditions.
Key words: abiotic stress, osmoprotectants, proline.
Introduction
Plants are exposed to diff erent types of environmental stress factors. In
1936 Selye has formulated the concept of stress. Th is concept had a deep
impact on the formation of the modern science. Living organisms, except
plants, possess a nervous system, which determine their normal behaviour.
Adverse environmental conditions, named stress, change this behaviour
at cellular and molecular level. Cellular stress is a term introduced in this
connection, such as osmotic and oxidative stress. To understand the stress
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beginning from molecular level to the level of a whole organism is crucial
to study the mechanisms of signal transduction in the cell.
Th e stress concept in plants has been continuously studied in the past
60 years. Stress is defi ned as inadequate conditions that infl uence plants
metabolism, growth or development, crop yield, or the primary assimila-
tion processes. Th e stress is associated with stress tolerance that is deter-
mined by the capacity of a plant to cope with an extreme condition. Diff er-
ent plants show diff erent sensibility to an unfavorable condition. If tolerance
of the plant increases, the plants are acclimated to stress. Plants can show
adaptation to stress, which is a genetically determined level of resistance
acquired by selection over many generations.
It is important to mention that environmental factors such as tempera-
ture and water availability determine the geographic distribution of plants.
Further connections originated from biotic factors such as predation and
competition; and abiotic factors such as light intensity, nutrients, organic
and inorganic pollutants aff ect the same aspects.
Plants have developed complex survival strategies to survive and adapt.
Th ese strategies have been well studied, but it is also important to under-
stand the molecular and physiological processes underlying plant respons-
es to stress conditions (Ingram and Bartels 1996, Zhu et al. 1997).
Abiotic stress
Abiotic stress factors, such as drought, salinity, extreme temperatures (caus-
ing osmotic stress) and oxidative stress are key determinants limiting plant
growth and crop productivity (Bray et al. 2000). Th ese stress factors are
interconnected, and they induce cellular damages altering the function and
certain structures of the cells (Smirnoff 1998). For instance, under osmotic
stress certain cell signalling pathways are activated, which lead to diff erent
cellular responses, like the production of stress proteins, upregulation of
antioxidants and accumulation of compatible solutes (Wang et al. 2003).
During evolution, plants have evolved specifi c tolerance mechanisms to
adapt to stress conditions.
In the past 50 years genetic engineering techniques have been based
on the transfer of genes involved in signalling and regulatory pathways, or
genes that encode enzymes present in pathways for synthesis of osmopro-
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Székely Gyöngyi
tectants or antioxidants, or genes that encode stress-tolerance-conferring
proteins. Th e importance of these techniques is improving the tolerance of
the plants to maintain the function and structure of cellular components
that is important for plant growth and productivity under continuously
changing environmental conditions. Fig. 1 shows the genes involved in
plant response to drought, salinity or extreme temperatures.
Figure 1. Plant responses to abiotic stress (based on Wang et al. 2003)
1. ábra: A növény válaszai abiotikus stresszre (Wang és mtsai. 2003 alapján)
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Osmotic stress
Salt stress
Th ere are many habitats throughout the world where the level of soil salinity
limits crop yield and restricts the use of arable land. Plants encounter high
concentration of salts close to seashore and in estuaries where seawater and
fresh water mix or replace each other. Sodium and chloride are the major
ions present in the soils and in seawater, but other ions, such as sulphate
and calcium, also contribute to salinity. Salts can be accumulated from irri-
gation water also, which is an extremely important problem in agriculture.
Pure water is removed from the soil by evaporation and transpiration, and
this water loss concentrates solutes in the soil, and salt can reach injurious
level that have an adverse eff ect on plant growth at salt sensitive species.
In fact, there are wild plant species that complete their life cycle in saline
soils, called halophytes; they have evolved anatomical and physiological
adaptations to counter the ion toxicity and water defi cit. Th e glycophytes
are not able to resist salts at the same degree as halophytes, showing signs
of growth inhibition, leaf discoloration and loss of dry weight. Some halo-
phyte species, such as Suaeda maritima (herbaceous seep weed) and Atri-
plex nummularia (giant saltbush) show growth stimulation with Cl- levels
at 300 mM. Spartina sp. (cord grass) and Beta vulgaris (sugar beet) toler-
ate salt (up to 200 mM), but their growth is retarded. Festuca rubra (red
fescue), Puccinellia peisonis (seewinkel), Gossypium sp. (cotton), Hordeum
vulgare (barley) are inhibited by high (100 mM) salt concentrations. Many
fruit trees (citrus, avocado) are very salt sensitive glycophytes, which are
severely inhibited or killed by 80 mM salt concentrations (Greenway and
Munns 1980).
Since salinity aff ects many processes in the plants life cycle, tolerance
has to contain a complex interplay of characters. Salt injury involves os-
motic eff ects and specifi c ion eff ect. Osmotic eff ects occur when dissolved
solutes in the rooting zone generate a low osmotic potential that lowers the
soil water potential. Th is aff ects the water balance of the plants, because
leaves need to develop a lower water potential to maintain a decreasing gra-
dient of water potential between the soil and the leaves. Specifi c ion eff ects
are caused when injurious concentrations of Na+, Cl- or SO4
2- accumulate
in cells. Under normal conditions, the cytosol of plant cells contains 1 to
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Székely Gyöngyi
10 mM Na+ and 100 to 200 mM K+. In this condition the enzymes function
optimally. A high ratio of Na+ / K+ and high concentrations of total salt in-
activate enzymes and inhibit protein synthesis.
Salinity depresses photosynthesis when high concentrations of Na+
and / or Cl- accumulate in chloroplasts. Mainly the carbon metabolism or
photophosphorylation is aff ected, because photosynthetic electron trans-
port is relatively insensitive to salts. Enzymes extracted from halophytes
are as sensitive to the presence of NaCl as enzymes from glycophytes are.
So, the halophytes do not possess salt resistant metabolic machinery, but
other mechanisms are involved in avoiding salt stress, like excluding salt
from shoot meristems and leaves. Halophytes possess a greater capacity of
ion accumulation in shoot cells compared with glycophytes. In some salt
resistant plants (Tamarix sp.) salt is accumulated in salt glands at the sur-
face of the leaves or in salt bush (Atriplex sp.), where becomes crystallized
to avoid damaging eff ects. Many halophytes accumulate ions (Na+, Cl-) in
the vacuole to prevent cytotoxicity and thus contribute to the cell’s osmotic
potential without damaging the salt sensitive enzymes (Taiz and Zeiger
1998).
To improve the salt tolerance of plants, crop improvement strategies
were developed, which are based on the use of molecular engineering tech-
niques and biotechnology, increasing the production of small osmolytes
or stress proteins that protect or reduce the damage caused by salt stress
(Zhu 2001). Several osmolytes, such as ononitol, proline, glycine betaine,
trehalose, ectoine and fructan have been engineered in transgenic plants to
improve salt tolerance or water stress tolerance (Shi et al. 2002).
In 2000 Zhu described some Arabidopsis salt tolerant mutants, for ex-
ample three rs (resistant to salt) mutants, that are capable to germinate un-
der saline conditions; and pst (photoautotrophic salt tolerance) mutant with
increased capacity of plants to detoxify reactive oxygen species, enhancing
plant tolerance to oxidative stress, as well as to salt stress.
Th e SOS (salt overly sensitive) salt stress signalling pathway was de-
scribed to have regulatory function in salt tolerance, fundamental of which
is the control of ion homeostasis. Zhu et al. (2000) identifi ed three groups
of ion hypersensitive mutants (salt overly sensitive: sos1, sos2, sos3). Genetic
and physiological analyses indicated, that the mentioned genes are compo-
nents of a Ca-dependent signal pathway regulating ion homeostasis and
salt tolerance. SOS1 encodes a plasma membrane Na+/H+ antiporter, SOS2
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encodes a sucrose non-fermenting-like (SNF) kinase, while SOS3 encodes
a Ca+-binding protein.
Shen et al. (2001) reported the characterization of ESI47 (early salt
stress induced) gene, which encodes a protein kinase that is induced by salt
stress and ABA (abscisic acid) in the salt tolerant Lophopyrum elongatum
(wheat grass). Th ey also identifi ed the Arabidopsis homologs of ESI47 and
demonstrated that salt stress and ABA diff erentially regulate them.
Drought stress
Drought acts as an important selective factor on the growth of plants and
crop productivity. Th ese stress factors cause plant responses at physiologi-
cal level as well as molecular and cellular levels.
As a physiological response, drought causes specifi c limitation on leaf
expansion (Matthews et al. 1984). Th e earliest responses to stress appear to
be mediated by biophysical events rather than by changes in chemical reac-
tions due to dehydration. Limitation of the size and number of individual
leaves appears as an early response to water defi cit. Water defi cit stimu-
lates cell shrinking, leaf abscission, root extension into deeper, moist soil,
stomatal closure (Radin and Hendrix 1988), and also limits the photosyn-
thetic activity of the leaf (Boyer 1970). Since the accumulation of ions dur-
ing osmotic adjustment appears to occur mainly within the vacuoles, other
compatible solutes must accumulate in the cytoplasm to maintain water
potential equilibrium in the cell. Osmotic adjustment promotes dehydra-
tion tolerance but does not have a major eff ect on productivity (McCree
and Richardson 1987).
Th e response to drought stress at molecular and cellular level includes
perception of the signal, signal transduction to cytoplasm and/or nucleus,
changes in gene expression, and fi nally the response resulting in tolerance
to drought stress (Shinwari et al. 2002).
Genes used to improve drought stress tolerance were those that en-
code enzymes required for the biosynthesis of diff erent osmoprotectants,
or those that encode enzymes for modifying membrane lipids. Many genes
are induced by both dehydration and low temperature (causing physiologi-
cal and developmental changes), and their mRNA levels are reduced if stress
ceases. Th ese suggest the existence of similar biochemical processes func-
tioning in dehydration- and cold-stress responses (Shinozaki and Yamagu-
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Székely Gyöngyi
chi-Shinozaki 2000). Th e plant stress-inducible genes function in protecting
cells by producing important metabolic proteins and osmoprotectants, and
regulate genes that are involved in transducing the stress (drought, cold)
response signal. In Arabidopsis, these genes are the CAS (cold acclimation-
specifi c), KIN (cold-inducible), LTI (low temperature- induced) described
by Yamaguchi-Shinozaki et al. (1992), DHN (dehydrin), LEA (late embryo
abundant), RAB (responsive to ABA) described by Welin et al. (1994), COR
(cold regulated) described by Th omashow et al. (1999), ERD (early respon-
sive to dehydration), and RD (responsive to desiccation) genes, described
by Shinozaki and Yamaguchi-Shinozaki (2000). Overexpression of some of
these genes leads to enhanced stress tolerance in transgenic plants, sug-
gesting that their gene products function in stress tolerance.
Analyses of the expression patterns of dehydration and cold inducible
genes revealed diff erences in the timing of their induction and in their re-
sponsiveness to ABA. A great number of the genes that are induced by
exogenous ABA treatment are also induced by cold or dehydration in ABA-
defi cient (aba) or ABA-insensitive (abi) Arabidopsis mutants. At least four
independent signal pathways function during dehydration: two are ABA-
dependent and two are ABA-independent. In addition, two ABA-indepen-
dent pathways are also involved in cold responsive gene expression. Th ere
is a common signal transduction pathway between dehydration and low
temperature induced stress involving the DRE/CRT (dehydration-respon-
sive element/C-repeat) cis-acting element, and two additional signal trans-
duction pathways may function only in dehydration or in cold response
(Shinozaki and Yamaguchi-Shinozaki 2000). Molecular analysis of these
members of the signal-transduction pathways should provide better un-
derstanding of the drought stress.
Cold stress
Besides salt and drought stress, low temperature is another important en-
vironmental factor limiting plant distribution and crop yield. Th ere are two
types of cold stress: chilling and freezing. Chilling is the temperature where
sensitive species (wheat, maize, soybean, tobacco, cucumber, sweet potato
and cotton) are injured. Th is temperature is too low for normal growth,
but not low enough for ice crystals to form. Plants gain resistance if they
are acclimated to cold. Biochemical studies revealed a positive correlation
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between cold tolerance and a high proportion of cis-unsaturated fatty acids
in the phosphatidylglycerol molecules of chloroplast membranes (Murata
1983, Sakamoto et al. 2004). Chilling damage decreases if exposure to low
temperature is slow and gradual. Chilling injury causes change in mem-
brane properties and membrane lipids. Resistant plants contain more un-
saturated fatty acids compared to chill sensitive species. Freezing occurs at
lower temperatures than chilling and causes plant death by forming intra-
cellular ice crystals (Taiz and Zeiger 1998).
Low temperature induces the accumulation of so-called antifreeze
proteins, which are bind to the surface of ice crystals and limit the crystal
growth. Acclimation to freezing involves the installation of dormant state
at woody plants. In some woody species (oak, elm, rhododendron, apple,
pear, peach), acclimation to freezing involves suppression of ice crystal for-
mation at temperatures far below the freezing point (Burke and Stushnoff
1979).
In the future it is important to develop new methods to analyse the
complex networks that control the stress responses of higher plants. Ge-
netic approaches are important for understanding not only the functions
of stress-inducible genes, but also the complex signalling processes of the
dehydration- and cold-stress responses.
Figrue 2. Structures of some
osmoprotectants in plants
2. ábra: Ozmoprotektánsok
szer kezete növényekben.
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Székely Gyöngyi
Osmoprotectants
Many plants, marine algae, bacteria and other organisms accumulate or-
ganic solutes in response to stress (Yancey et al. 1982). Osmotic adjustment
is maintained between cytoplasm and the vacuole by the synthesis of com-
patible osmoprotectants (or osmolytes), such as sugar alcohols (glycerol,
methylated inositols), sugars (raffi nose, trehalose, fructans), quaternary
amino acid derivatives (proline, glycine betaine, proline betaine, alanine
betaine), tertiary amines (1,4,5,6-terahydro-2-methyl-carboxyl pyrimi-
dine), and sulphonium compounds (choline osulfate) (Yokoi et al. 2002).
Th e accumulated solutes vary with the organism and among plant species.
Compatible osmoprotectants are accumulated to high levels without
interfering with normal biochemical reactions of the cell (Bohnert et al.
1995), and have the capacity to maintain the activity of enzymes present
Figure 3. Proline biosynthesis and
degradation in higher plants
Abbreviations: GSA: glutamyl-semial-
dehyde, P5C: pyrroline-5-carboxylate,
P5CS: delta-1-pyrroline-5-carboxyl-
ate synthetase, P5CR: 1-pyrroline-5-
carboxylate reductase, PDH: proline
dehydrogenase, P5CDH: P5C-dehy-
drogenase, OAT: ornithine amino-
transferase.
3. ábra: Prolin bioszintézise és deg radá-
lódása magasabbrendű növényekben.
Rövidítések: GSA: glutamil-szemialde-
hid, P5C: pirolin-5-karboxilát, P5CS:
delta-1-pirolin-5-karboxilát szin te-
táz, P5CR: 1-pirolin-5-karboxi lát re-
duktáz, PDH: prolin dehidrogenáz,
P5CDH: P5C-dehidrogenáz, OAT:
ornitin aminotranszferáz.
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in saline conditions. Osmolytes can act as osmoprotectants, e.g. glycine
betaine retain thylakoid and plasma membrane integrity under extreme
temperature and saline conditions (Rhodes and Hanson 1993).
Osmoprotectants can act as scavengers of reactive oxygen species that
are by-products of osmotic stresses, and cause membrane dysfunction and
cell death (Bohnert and Jensen, 1996). Th us, osmoprotectants enhance
stress tolerance of plants.
Although the synthesis pathway of compatible solutes diff er in terms of
enzymes and steps, osmoprotectants are synthesized in response to stress,
and are located in cytoplasm, chloroplasts and other cytoplasmic compart-
ments that together occupy 20% of the cell volume. Inorganic ions such as
Na+ and Cl- are often sequestered into the large vacuole, which occupies the
remaining of 80% of the cell volume (Bohnert et al. 1995, Glenn et al. 1999).
Structures of various osmoprotectants are presented in fi g. 2. Proline is one
of the most widely distributed and thereby the most studied osmolyte ac-
cumulated under stress conditions.
Proline as an osmoprotectant
Proline contains a secondary amino group and is therefore an alpha-imino
acid (the name-giving feature is the NH group), but it is nevertheless re-
ferred to as an amino acid. Proline, a highly water soluble amino acid, is ac-
cumulated in leaf tissues and shoot apical meristems of plants during water
stress (Jones et al. 1980), in leaves of many halophytic (Briens and Larher
1982), in suspension-cultured plant cells adapted to water stress (Rhodes et
al, 1986), in root apical regions growing at low water potentials (Voetberg
and Sharp 1991) and NaCl stress (Th omas et al. 1992), and in desiccating
pollen (Lansac et al. 1996). Proline accumulation is one of the common
physiological responses in higher plants exposed to drought and salinity
(Handa et al. 1986; Delauney and Verma 1993).
In higher plants, proline is synthesized in the cytosol from glutamate
and ornithine but the levels of free proline are also controlled catabolically
(fi g. 3). During stress, the glutamate pathway is the dominant one. Proline
biosynthesis through the glutamate pathway is catalysed by a bifunctional
delta-1-pyrroline-5-carboxylate synthetase (P5CS) enzyme that yields pyr-
roline-5-carboxylate (P5C) from glutamate in a two-step reaction (Hu et al.
1992). Th e activity of P5CS represents a rate-limiting step of proline bio-
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Székely Gyöngyi
synthesis, and is controlled at the level of P5CS transcription and through
feedback inhibition of P5CS by proline (Strizhov et al. 1997). Th e inter-
mediate product P5C is further reduced to proline by the 1-pyrroline-5-
carboxylate reductase (P5CR) enzyme (Delauney and Verma 1990). In the
catabolic pathway, proline is oxidized to glutamate through two steps in
the mitochondria. Th e fi rst and rate-limiting reaction of proline catabolism
is catalysed by proline dehydrogenase (PDH) that reduces proline to P5C
(Peng et al. 1996). Subsequently, P5C is oxidized to glutamate by P5C-dehy-
drogenase (P5CDH) (Deuschle et al. 2001). Reciprocal regulation of P5CS
and PDH genes plays a key role in the control of proline levels during and
after osmotic stress (Kiyosue et al. 1996). Fig. 3 shows the main pathways of
proline biosynthesis and degradation in higher plants.
Th e accumulation of proline in plants is part of a general adaptation
to adverse environmental conditions, which has been documented in re-
sponse to various types of stress, such as high salinity, drought, low temper-
ature, nutrient defi ciency, heavy metals and high acidity. Firstly, Kemble and
MacPherson (1954) reported the accumulation of proline in wilted plant
tissue of rye grass. Th e level of naturally accumulated proline in Arabidop-
sis thaliana seedlings is 1 μmol g-1 fresh weight, and there is an eight-fold
increase under salt (NaCl: 120 mM) stress. In Nicotiana tabacum (tobacco)
leaves there is a 20-fold increase of proline concentration under 200 mM
NaCl treatment, whereas in Glycine max (soybean) leaves there is a 11-fold
increase of proline content under 200 mM NaCl treatment (Delauney and
Verma 1993). In plants, proline constitutes <5% of the total pool of free
amino acids under normal conditions. After stress this level can increase to
up to 80% of the amino acid pool (Chen and Dickman 2005).
Th e transport of amino acids in plants is regulated by both internal
and external signals. Stress factors, such as desiccation or salt stress, af-
fect long distance transport and cause changes in the partitioning of car-
bon and nitrogen. Meristems, young tissues and reproductive organs re-
quire importing amino acids to maintain growth and development. Two
families of plant amino acid transporters have been described: the amino
acid polyamine and choline transport family, and the family of amino acid
transporters (ATF). Th e proline transporters (ProT) are members of the
amino acid transporter family. It has been reported that in alfalfa, proline
transport processes are important in adaptation to osmotic stress (Kishor
et al. 2005). Using suppression assays of yeast mutants, Rentsch et al. (1996)
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Biologia | Acta Scientiarum Transylvanica, 15/1, 2007
isolated and characterized eight diff erent Arabidopsis amino acid trans-
porters clones. Two of them encode specifi c proline transporters (ProT),
and are distantly related to the amino acid permease gene family. ProT1 is
expressed in all organs of the plants, with highest levels detected in the fl o-
ral stalk. Th is is in accordance with the evidence that proline synthesis and
degradation plays an important role in fl owering and seed set (Verbrug-
gen et al. 1996). Transcripts of ProT2 are observed throughout the plant,
but their levels are elevated by water or salt stress (Rentsch et al. 1996).
Th e plant proline transporters transport proline, but no other amino acids
(Kishor et al. 2005).
Th e role of proline accumulation during stress conditions
Proline has been proposed to act as an important compatible osmolyte and
osmoprotective compound, which is involved as possible protein folding
chaperone in osmotic adjustment and protection of cellular structures,
proteins, and membranes during osmotic stress in bacteria, fungi, inver-
tebrates and plants (Csonka 1981, 1989, Terao et al. 2003). Proline over-
production is correlated with protection of plants from UV damage and
heavy metal stress through reduction of lipid peroxidation (Mehta and
Gaur 1999). Antioxidant function for proline is suggested by reduction of
free radical levels shown in proline accumulating transgenic tobacco plants
(Hong et al. 2000). Proline is thought to play a role in the stabilization of
cellular redox potential and scavenging reactive oxygen species, too (Sirip-
ornadulsil et al. 2002).
Although a great number of physiological studies suggest that proline is
an important molecule in multiple stress protecting mechanisms, it is also
obvious that proline accumulation does not represent an exclusive condi-
tion for mounting stress tolerance. Th us, several salt and cold hypersensi-
tive Arabidopsis mutants accumulate proline at high levels but without any
apparent benefi cial eff ect on stress tolerance (Liu and Zhu 1997).
Another noticeable eff ect of proline on living organisms is the induc-
tion of apoptosis. Apoptosis is a genetically controlled type of programmed
cell death characterized by distinct morphological and biochemical chang-
es, including cell shrinkage, chromatin condensation, DNA fragmentation,
and membrane externalisation of phosphatidylserine on the cell surface. In
human tumour cell lines overexpression of PDH or P5C treatment caused
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Székely Gyöngyi
apoptosis (Deuschle et al., 2004). Further on, defects in HsP5CDH (Homo
sapiens P5CDH) result in type 2 hyperprolinemia, with patients experienc-
ing seizures and diff erent degrees of mental retardation. Spraying Arabidop-
sis leaves with Pro (20 mM) or its degradation product P5C, led to necrotic
lesions, a phenotype comparable to leaf spot disease or ozone induced le-
sions. Th e damaged tissue accumulated phenolic compounds, callose and
H2O
2 (hydrogen peroxide), as indicated by autofl uorescence or 3-diamino-
benzidine (DAB) staining (Deuschle et al. 2004). Chen and Dickman (2005)
reported the ability of proline to inhibit ROS-mediated apoptosis in the
fungal pathogen Colletotrichum trifolii, thus providing important insights
into the mechanism of proline-mediated survival during oxidative stress.
Accordingly, in many species, synthesis, transport, and degradation of Pro
must be tightly regulated to prevent cell death.
Proline is not just a by-product of stress defence, but a chemically ac-
tive compound with a capital role in the physiology of stress protection. In
addition, other positive roles of proline have been proposed, which include
stabilization of proteins by quenching ROS, regulation of the cytosolic pH,
and regulation of NAD/NADH ratio (Chen and Dickman 2005).
Proline plays a role in seed set of some dicotyledonous plant species,
attracting insects, as pollinator visitors. Th e attraction of insects is accom-
plished by productions of rich fl oral nectar that is secreted into the fl oral
tube at the base of the ovary. Nectar is an aqueous combination of sugars
and amino acids. Recently, Carter et al. (2006) reported the amino acid con-
tent examination of ornamental tobacco nectar and found that it is rich (2
mM) in proline. Further investigations of the same authors demonstrated
that two soybean species showed production of proline rich fl oral nectar.
All these data proved that some plants off er proline rich nectars as a mech-
anism to attract visiting pollinators.
To characterize the function of genes controlling proline metabolism
and clarify the physiological role of proline in various stress responses,
both biosynthetic and catabolic pathways have been modifi ed in transgenic
plants. Plants accumulating high levels of proline were obtained by overex-
pression of P5CS in transgenic plants (Kishor et al. 1995). Down-regulation
of proline biosynthesis by antisense inhibition of P5CS expression is re-
ported to reduce free proline content in transgenic Arabidopsis plants and
to cause developmental alterations and elevated stress sensitivity (Nanjo et
al., 1999). In some experiments, increased level of free proline is claimed
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Biologia | Acta Scientiarum Transylvanica, 15/1, 2007
Ta
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lin
e d
ehyd
rog
enas
e co
nfe
rred
sal
t an
d f
reez
ing
to
ler-
ance
.
Nan
jo e
t al
., 1
99
9
Vig
naP
5C
SR
ice
Ove
rex
pre
ssio
n r
edu
ced
ox
idat
ive
stre
ss u
nd
er o
smo
tic
stre
ss.
Ho
ng
et
al.,
20
00
AtP
DH
Ara
bid
op
sis
An
tise
nse
pla
nts
sh
ow
ed h
yper
sen
siti
vity
to
pro
lin
e.M
ani
et a
l., 2
00
2
AtO
AT
To
bac
coO
vere
xp
ress
ion
in
crea
sed
pro
lin
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iosy
nth
esis
an
d o
smo
tole
ran
ce.
Ro
ose
ns
et a
l., 2
00
2
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naP
5C
SW
hea
tO
vere
xp
ress
ion
in
wh
eat
resu
lted
en
han
ced
pro
lin
e le
vels
an
d s
alt
tole
ran
ce.
Saw
ahel
et
al.,
20
02
Vig
naP
5C
SC
hla
my
do
-
mo
nas
Tra
nsg
enic
alg
ae h
ave
bee
n e
xp
ress
ing
Vig
na
P5
CS
an
d h
ad h
igh
er f
ree
pro
-
lin
e co
nte
nt
and
wer
e to
lera
nt
to t
ox
ic h
eav
y m
etal
s.
Sir
ipo
rnad
uls
il e
t al
.,
20
02
Vig
naP
5C
SR
ice
Ove
rex
pre
ssio
n i
n t
ran
sgen
ic r
ice
pla
nts
sh
ow
ed b
ette
r ro
ot
gro
wth
an
d b
io-
mas
s d
evel
op
men
t d
uri
ng
20
0 m
M N
aCl
trea
tmen
t.
An
oo
p e
t al
., 2
00
3
AtP
5C
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oyb
ean
Ove
rex
pre
ssio
n e
nh
ance
d h
eat
and
dro
ug
ht
stre
ss.
De
Ro
nd
e et
al.
, 20
04
OsP
5C
S 2
Ric
eO
vere
xp
ress
ion
en
han
ced
sal
t an
d c
old
str
ess
tole
ran
ce.
Hu
r et
al.
, 20
04
Vig
naP
5C
SR
ice
Str
ess
ind
uci
ble
ex
pre
ssio
n o
f P
5C
S g
ene
in r
ice
seed
lin
gs
sho
wed
hig
her
to
l-
eran
ce t
o d
rou
gh
t an
d s
alt
stre
ss.
Su
et
al.,
20
04
AtP
5C
S1
Po
tato
Tra
nsg
enic
po
tato
pla
nts
sh
ow
in
crea
se i
n p
roli
ne
leve
l an
d i
mp
rove
d s
alt
tol-
eran
ce d
uri
ng
10
0 m
M N
aCl.
Hm
ida-
Say
ari
et
al.,
20
05
19
Székely Gyöngyi
to correlate with improved stress tolerance (Hong et al. 2000), whereas in
others such correlation is not observed (Mani et al. 2002). Alternatively,
proline accumulation was achieved by inactivation of the PDH gene with
T-DNA insertion mutations and antisense inhibition of PDH transcription
(Nanjo et al. 2003). As P5CS is encoded by two closely related genes (P5CS1
and P5CS2) in Arabidopsis, so far it is unknown how conventional anti-
sense approaches aff ect the function of individual P5CS homologues. Table
1 presents some phenotypic eff ects of various transgenic plants developed
from proline biosynthetic pathway genes that provide abiotic stress toler-
ance.
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Ozmotikus stressz a növényekben
Kivonat
A szárazság, a talaj magas sótartalma valamint a hideg ozmotikus stresszt
okoz, ami limitáló tényező a növények növekedésében és a terméshozam-
ban. Az ozmotikus stresszt előidéző környezeti tényezők hatására, számos
magasabbrendű növény ozmoprotektív vegyületet halmoz fel, mint pl. a
prolint. A prolin feltehetőleg szerepet játszik a sejtek plazmamembrán in-
tegritásában, és gyökfogóként is működhet. Ugyanakkor, a prolin redukáló
anyagok forrása is lehet, átmeneti C és N forrás egyéb fehérjék bioszin-
téziséhez. A prolin homeosztázisa szorosan összefügg annak szintézisével,
lebomlásával és transzportjával. A jelen cikkben jellemezve vannak az oz-
motikus stressz által indukált egyes gének, és összegezve van a prolin felhal-
mozódásának szerepe az ozmotikus stresszt előidéző körülmények közt.