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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Biologia | Acta Scientiarum Transylvanica, 15/1, 2007

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-

7

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)

8


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

9

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

10


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-

11

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

12


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.

13

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.

14


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-

15

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)

16


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

17

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

18


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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.

Osmotic Stress in Plants

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