Encyclopedia of applied plant sciences/ 1, [A - Gro]

1674 Pages·2003·35.548 MB·English

by  Denis J Murphy, Brian G Murray, Brian Thomas

A
ABIOTIC STRESSES
Contents
Cold Stress
Free Radicals, Oxidative Stress and Antioxidants
Mechanical Stress and Wind Damage
Cold Stress
subtropical plant species (paprika (Capsicum frutes-
cens), potato (Solanum tuberosum), tomato (Lyco-
JSutkaandGGaliba,AgriculturalResearchInstitute
persicon esculentum), etc.) which are cultivated in
ofthe Hungarian AcademyofSciences,Martonva´sa´r,
Hungary temperate regions are classified as chill tolerant but
freezingsusceptible.Theherbaceousorwoodyplants
Copyright2003,ElsevierLtd.AllRightsReserved.
characterized as freezing tolerant originated mostly
from the temperate climates. There are substantial
differences among the plant species classified as
Introduction freezing tolerant. Most of the herbaceous plants
survive only moderate freezing, between (cid:1)71C and
Apartfromtheavailabilityofwater,lowtemperature
(cid:1)301C. However, there are some much hardier
(chilling and frost) is the most important environ-
woody species, which can tolerate temperatures
mental factor that limits the productivity and
below (cid:1)801C. The effect of cold on plants is not
geographical distribution of plants in large areas of
onlydetermined bythemagnitudeofthedropofthe
the world. Cold temperatures can affect the devel-
temperature,butistoalargeextentdependentupon
opment of plants in almost each phase, from
the season, developmental stage, and for how long
germination through to seed set. The sensitivity of
the low temperature persists.
a plant species in a particular environment is
determined by the limit to which its metabolic
processes continue to function under low tempera-
ture stress or the point at which it may suffer Cold Acclimation of Plants
permanent injuries that finally bring about death.
To achieve the full genetic potential of freezing
Thecoldtoleranceofaplantspeciesisevolutionarily
tolerance the plants must have time to adapt (i.e.,
determined and depends on the climate of the
becomehardened,oracclimated)graduallytothelow
area the plant originated from. According to their
temperatures. Under natural conditions the cold
cold tolerance plants are divided into three cate-
hardening (acclimation) takes place in autumn when
gories: the temperature gradually decreases to 01C over
several weeks. Cold hardening or acclimation can be
1. Chillsusceptible:damagedbytemperaturesbelow
defined as a nonheritable modification of structures
121C.
and functions as a response to cold which minimizes
2. Chill tolerant but freezing susceptible; able to
damage and improves the fitness of an individual
acclimate to temperatures below 121C but unable
plant. The significance of the temperatures and/or
to survive freezing.
daylength (photoperiod) decreases from autumn to
3. Freezing tolerant (frost tolerant); able to acclima-
winter. A cold acclimation or freezing tolerance
tize to survive temperatures significantly below
inductor has been recognized since the nineteenth
freezing.
century; nonacclimated rye (Secale cereale), for
As shown in Table 1, tropical plants like banana instance, is killed by freezing at about (cid:1)51C, but
(Musa sapientum), papaya (Carica papaya), etc. afteraperiodofexposuretolownonfreezingtemper-
belong to the chill susceptible category. Several ature can survive freezing down to about (cid:1)301C.
2 ABIOTICSTRESSES/ColdStress
Table1 Examplesofcold-sensitive(tender)andfreezing-tolerant(hardy)plants,includingsomeimportantcropspecies
Family Species Rangeoftemperaturecausinginjury(1C)
Musaceae Musaspp.(banana) þ10 þ12
Lauraceae Perseaspp.(avocado) þ6 þ8
Caricaceae Perseaspp.(avocado) þ4 þ10
Poaceae Oryzasativa(rice) þ12 þ15
Zeamays(maize/corn) þ2 þ12
Avenasativa(oat) (cid:1)5 (cid:1)10
Hordeumvulgare(barley) (cid:1)7 (cid:1)12
Triticumaestivum(breadwheat) (cid:1)9 (cid:1)18
Secalecereale(rye) (cid:1)15 (cid:1)30
Solanaceae Lycopersiconesculentum(tomato) þ2 þ5
Capsicumannuum(paprika/pepper) (cid:1)2 þ4
Solanumtuberosum(potato) (cid:1)2.5 0
Solanumacaule(wildpotatospecies) (cid:1)6 (cid:1)8.5
Brassicaceae Arabidopsisthaliana (cid:1)9 (cid:1)14
Rutaceae Citrusspp.(orangeandlemon) (cid:1)2.2 (cid:1)10
Myrtaceae Eucalyptusspp.(eucalyptus) (cid:1)8 (cid:1)16
Cupressaceae Juniperusspp.(juniper) (cid:1)25 (cid:1)45
Pinaceae Pinusspp.(pines) (cid:1)20 (cid:1)60
Rosaceae Prunusspp.(plum) (cid:1)20 (cid:1)80
Table2 Climaticprogramforfreezingtestincereals
Time(days) Daytemperature(1C) Nighttemperature(1C) Daylength(h) Function
3 20 20 16 Germination
7 15 10 12 Growth
14 10 5 12 Growthandhardening
14 5 0 8 Hardening
7 2 (cid:1)2 8 Hardening
2 (cid:1)4 (cid:1)4 Dark Secondphaseofhardening
1or2 Between–10and–16 Dark Freezinga
2 Between0and þ2 Dark Thawing
14 16 15 12 Recovery
1 16 15 16 Evaluation
aFreezingtemperaturedependsonthegenotype.
The capability of plants to harden is genetically diseases; these are known as snow molds since they
determined, and individuals possessing higher freez- developunderthesnow.Amongthesefactorsfreezing
ing tolerance can be selected. In plants low positive tolerance proved to be the most important, because
temperature or freezing is the commonest inductive an intact plant can better withstand the other
environmental factor for acclimation to cold. How- damaging agents. One serious problem that plant
ever, drought or the plant hormone abscisic acid breeders encounter in developing more hardy culti-
(ABA) can substitute for cold at least in part. In vars is the fact that very severe winters producing
woody species shortening daylength causing dor- differential winter kill occur only erratically and
mancy is often also a trigger for acclimation. infrequently in most locations. This necessitates the
Farmers will grow winter cereals in preference to development of artificial testing procedures to help
spring ones provided they can rely on the survival of progress in the breeding programs.
theplantsoverwinter.Thispreferenceislargelybased The degree of freezing tolerance is highly depen-
on the superior yielding ability (the difference being dent on the hardening conditions. Under controlled
roughly30%)ofwinter-plantedcereals.Thecapacity environmental conditions temperatures slightly
for overwintering in the temperate zone depends on above 01C and photoperiods of 8–21h are consid-
several factors including low positive and freezing ered to be optimal for cold hardening of winter
temperatures, waterlogging or ice encasement (both cereals (barley (Hordeum vulgare), wheat (Triticum
of which cause anaerobic stress), and wind (which aestivum), rye, etc.). Table 2 shows one of the
enhances any tendency for shoot dehydration). In climatic hardening programs used at the Martonva´-
addition, overwintering plants can be attacked by sa´r(Hungary)plantresearchstation.Thisprogramis
ABIOTICSTRESSES/ColdStress 3
temperature on cell membranes. Freezing, however,
Cheyenne CS/Ch 5A
120
often acts indirectly, damaging the cells by dehydra-
CS/Ch 7A CS
tion.Astemperaturesdropbelow01C,iceformation
100
isgenerallyinitiatedinthecellularspaces,dueinpart
to the extracellular fluid having a higher freezing
%) 80 point (lower solute concentration) than the intracel-
al ( lular fluid. Ice nuclei may form spontaneously
v 60
urvi (homogenous nucleation), or around nonaqueous
S matter (heterogeneous nucleation). The probability
40
of homogenous nucleation is very low until the
temperature drops to (cid:1)401C. Heterogeneous nu-
20
cleators include some biological debris or inorganic
material. In addition, bacteria commonly found on
0
0 20 40 60 80 100 plants (such as Pseudomonas syringiae and Erwinia
Cold treatment (days) herbicola)produceaproteinabletonucleatefreezing
Figure 1 Time-dependent changes in frost tolerance during at temperatures as high as (cid:1)21C. Because the water
coldhardeningat þ21C.Freezingtolerancewasestablishedas potentialoftheextracellulariceislessthanthewater
thepercentageofsurvival.Plantmaterial:ChineseSpring(CS),
potential of liquid water within the cells, ice
spring wheat; Cheyenne (Ch), winter wheat; CS/Ch 5A and
formationresultsinadropinwaterpotentialoutside
CS/Ch7A,chromosomesubstitutionlines.
the cell. Consequently, there is movement of un-
frozenwaterdownthewaterpotentialgradientfrom
based on the 30-year average autumn temperature
inside the cell to the intercellular spaces. At
characteristic for this area. There are two main
equilibrium, the extent of dehydration is a direct
characteristics of the program:
consequence of temperature: with progressively
1. Thetemperaturegraduallydecreasesfrom þ151C greater falls in temperature below freezing, the less
to (cid:1)21C over 6 weeks. intracellular liquid water remains, the more strongly
collapsed are the cells, and the lower is the
2. At the end of the program the plants are kept at
(cid:1)41C for 2 days in the dark. intracellular water potential. At (cid:1)101C, more than
90% of the osmotically active water typically moves
It is considered that keeping the plants at freezing out of the cells and the osmotic potential of the
temperatures just below 01C further increases their remaining fluid is greater than 5osm.
freezing tolerance. That phenomenon is known as
the second phase of hardening.
Membranes
Cold resistance is determined by exposing the
plantstoatemperaturelowenoughtocausedamage The targets of freezing-induced dehydration are the
anddeterminingeitherthetemperaturewhichcauses membranes, particularly the plasma membrane. This
death of 50% of the plants (LT50) or the percentage undergoes phase separation which destabilizes it,
of plants surviving. Since cold injury damages cell causing it to break up. A relationship between the
plasma membranes, injury may also be measured by phase transition of membrane lipids and the chilling
electrical conductivity of the tissues or leakage of sensitivity of plants was first postulated in the early
various compounds from the cells. Vital staining or 1970s by Raison and Lyons. They proposed that the
comparing the capacity of the plants for regrowth formationofgel(orsolidphase)bylipidsinbiological
afterarecoveryperiodarealsowidelyusedmethods. membranes at nonfreezing chilling temperatures
The cold hardiness of plants changes in character- induces damage to the same plant tissues that can
istic ways with increase in the duration of the lead to the death of the plant. Dehydration in the
hardening process. In hardy wheats hardiness starts absence of cold has the same effect, confirming that
to increase after 2–3 weeks and rises to a maximum damage is a consequence of the freezing-induced
whichusuallypersistsforafewweeks,dependingon desiccation. Freeze-fracture electron microscopic stu-
the hardening conditions and the cultivar. Hardiness diesrevealthatthemembranephaseseparationwhich
later falls (Figure 1). hasbeendemonstratedtooccurconstitutesaremoval
of protein particles from areas of the membrane.
Break-up of the membrane is mediated by local
Damage Caused by Freezing
changes in phase from lamellar to nonlamellar,
The natures of chill and freezing stresses are includinghexagonalII.Theprecisesequenceofevents
different. Chill stress is a direct effect of low causing this is unclear. Mostlikely the loss of protein
4 ABIOTICSTRESSES/ColdStress
particles from areas of membrane could leave the radicals, or hydrogen peroxide) as agents causing
nonlamellar lipids (such as phosphatidylethanol- damage to the proteins, nucleic acids, and mem-
amine)inthisareawithoutstabileinteractions.Other branes.Theactivationofoxygenbythephotosystems
forms of membrane damage also can occur, e.g., in the presence of excessive light is probably the
expansion-induced lysis; or if intracellular ice forms majorsiteofproductionoffreeradicalsinleavesbut
adhesion with cell walls and membranes it can cause other electron transport systems, including those on
cellrupture.Thus,akeypartofcoldacclimationisto the mitochondria or plasmalemma, may also con-
stabilize membranes against freezing induced injury. tribute especially in nonphotosynthetic tissues. The
Acclimation of bacteria, fungi, protozoa, plants, development of the symptoms of chilling injury is
and animals to temperatures below their respective frequently coincident with peroxidation of fatty
normal growth temperatures generally results in acids. In this way lipid peroxidation would alter the
changes in the unsaturation of fatty acids in physical properties of membrane lipids, thereby
membrane lipids. The extent of unsaturation has a inhibit the function of membrane-bound proteins
considerable effect on the fluidity of membrane contributing to the development of visual symptoms
lipids.Whenorganismsareexposed tolow tempera- ofinjurycausedbycoldtemperatures.Therefore,itis
tures,thefluidityoftheirmembranelipidsdecreases. clear that the capability of the plants to enhance the
Such exposure enhances the expression of genes for free radical scavenging capacity by increasing the
fattyaciddesaturases,whichintroducedoublebonds endogenous level of antioxidants e.g.: carotenoids,
intothefattyacylchainsofmembranelipids,thereby tocopherol, ascorbate, superoxidedismutase, gluta-
compensating for the decrease in membrane fluidity. thione,etc.,isanimportantpartoftheplant’sdefense
As a result, the original physical properties of the mechanism under cold stress conditions.
membranes are restored and can support the func-
tions of the membrane associated proteins and their
Genetics of Freezing Tolerance
complexes. This hypothesis was proved experimen-
tally by the inactivation of individual genes for fatty Winter hardiness, including frost tolerance, has long
acid desaturases in the cyanobacterium Synecho- been regarded as being under complex multigenic
cystis. In higher plants, genetic manipulation of control.However,itwouldstillbepossibleformajor
glycerol-3-phosphate acyltransferase and cyanobac- differences in cold adaptation between species or
terial D9 desaturase has allowed modulation of the cultivars to depend on allelic differences in a small
level of unsaturation of phosphatidylglycerol in number of genes as suggested in cold acclimation in
thylakoid membranes. Characterization of the pea (Pisum sativum) and Solanum. Since common
photosyntheticfunctionsofsuchgeneticallymodified wheat is a hexaploid its vital genes are replicated.
strains of cyanobacteria and higher plants has This has permitted the development of a series of
revealed that polyunsaturated fatty acids are essen- chromosome-substitutionlines.Bycomparingchromo-
tial for the protection of the photosynthetic machin- some-substitution lineswith theparental lines it was
ery against photoinhibiton at low temperatures. possible to determine which chromosomes carry the
The importance of plant plasma membrane lipid locusforfreezingtolerance.Theanalysisofsubstitu-
composition in freezing tolerance was demonstrated tion lines showed that at least 10 of the 21 pairs of
by Steponkus and his colleagues using liposomes to chromosomes are involved in the control of frost
modify the membrane lipid composition of proto- tolerance and winter hardiness (Figure 1). However,
plast isolated from nonacclimated rye leaves. They major genes influencing frost resistance (Fr) and
enriched the plasma membrane from acclimated rye vernalization requirement (Vrn) were localized on
leaves, or specific saturated or unsaturated phospha- the long arm of chromosomes 5A and 5D. Of
tidylcholine species. When the lipids from the particular importance for adaptation to autumn
acclimated membrane or unsaturated phosphatidyl- showingarethegenes forvernalization requirement.
choline molecules were supplied, these significantly Vrn genes determine the needs for cold temperature
increased the freezing tolerance of the protoplasts. required for flower development (see Regulators
These experiments well demonstrated that mem- of Growth: Jasmonates). Recent studies indicated
brane lipid composition is an important factor in that the Vrn1–Fr1 interval on chromosome 5A of
hardening to frost. wheat has a major effect on freezing tolerance
(Figure 2). Conservation of gene order (synteny) in
TriticumiswellknownandthisistruefortheVrn1–
OxidativeStress
Fr1 interval studied in barley, rye, and Triticum
Chilling and cold injury is mediated, in part, by monococcum, as well. For example a map of
oxygen free radicals (singlet oxygen, superoxide quantitative trait loci (QTL) in barley has resulted
ABIOTICSTRESSES/ColdStress 5
(A) (B)
Genetic map Physical map
Distance (cM) Marker FL
Centromere Centromere
Xpsr911
C-band
0.56 (5AL-10)
42
ABA2,Xpsr2021
0.64 (5AL-8)
9
Fr 1
2 0.67 (5AL-15)
Vrn-A1, Xpsr426, Fr 1
0.68 (5AL-6)
Xwg644, Xcd504
Vrn-A1, Xwg644
46
Xpsr426, Xcd504
0.78 (5AL-17)
Xpsr805 (Embp) 0.82 (5AL-20)
5
4 Q 0.87 (5AL-23) Q
Xpsr370
Translocation Xpsr370
Telomere Telomere
Figure2 (A)Geneticand(B)cytogeneticallybasedphysicalmapsofthelongarmofwheatchromosome5A.Fractionlengths(FL)
anddeletionstocknumbersareindicatedontheleftofthephysicalmap.Markernamesareindicatedontherightofthechromosomes.
in the identification of a 21-cM region on chromo- low temperature in Solanum commersonii, a plant
some 7 that has a major role in freezing tolerance. thatcoldacclimates,butnotinS.tuberosum,aplant
This region accounted for 32% of the variance in that does not cold acclimate. Moreover, the exogen-
LT values and 39–79% of the variance in winter ous application of ABA increased the freezing
50
field survival. Chromosome 7 of barley (renamed toleranceofS.commersoniiplantsatwarmtempera-
recently as 5H) is homologous to the group 5 tures.Soitwashypothesizedthatcoldacclimationis
chromosomes of wheat. The 21-cM freezing toler- activated through the action of ABA. Subsequent
ance interval in barley contains the Xwg644 and studies extended this observation establishing that
Xcdo504 molecular markers that are also linked to ABA levels increase, at least transiently, in a diverse
the Vrn1–Fr1 interval in wheat. group of plant species during cold acclimation and
The mechanism whereby the Vrn1–Fr1 interval that exogenous application of ABA at warm non-
affects freezing tolerance remains to be deter- acclimating temperaturesenhances thefreezing toler-
mined. Regulatory genes affect cold-induced ABA, ance of several plant species that cold acclimate.
water-soluble carbohydrates, fructan and proline Experiments with plants carrying mutations in ABA
accumulation, more than do genes regulating the synthesis (aba1) or the ability to respond to ABA
cold-induced expression of proteins in various (abi1) showed less freezing tolerance than the wild-
functions that were mapped in this region. Thus the type. These results support the hypothesis that ABA
possibilityisraisedthattheVrn1–Fr1intervalaffects has a key role in activating cold acclimation. Recent
frost tolerance through encoding several proteins results, emerged from gene regulation studies on
involved in regulating the expression of cold-induci- Arabidopsis and the experimental fact that during
ble genes that have roles in freezing tolerance. cold acclimation the increase in ABA levels peaks at
24h and returns to essentially the normal level after
2 days (yet freezing tolerance increases for about
Cold-Induced Metabolic Responses 1 week and remains elevated for at least 3 weeks)
adds a question mark to this hypothesis. Using only
Role ofABAin ColdAcclimation
ABAashardeningagenttheleveloffreezingtolerance
ABA is considered as a stress hormone, emphasizing is lower than can be achieved by cold treatment. We
itsroleintheadaptationofplantstodifferentabiotic may elucidate this controversy approaching this
stress conditions such as drought, salinity, heat, and question from gene regulation point of view. It is
cold.Atthebeginningofthe1980sitwasdiscovered well established that some genes are strictly cold-
that ABA levels increase transiently in response to inducible (such as barley COR14b), some are ABA
6 ABIOTICSTRESSES/ColdStress
proteins. So, the key unresolved question about these
COLD proteins is whether or not their antifreeze property
operates in vivo. Most likely they do not prevent
freezingordeferittolowertemperatures,andinstead
DEHYDRATION INJURY
itissuggestedthattheyhelpcontrolthesitesatwhich
ice forms and grows and the rate of growth. Also,
Cold and
Membrane cold-responsive genes encoding various signal trans-
dehydration
fluidity duction and regulatory proteins have been identified
regulated proteins Reactive oxigene
species and free including a mitogen-activated protein (MAP) kinase,
Osmoregulation
radicals calmodulin-related proteins and 14-3-3 proteins.
These proteins might contribute to freezing tolerance
COLD
ACCLIMATION by controlling the expression of freezing tolerance
Compatible solutes Antioxidants
(HARDENING) genesorbyregulatingtheactivityofproteinsinvolved
in freezing tolerance. Whether these cold-responsive
genes actually have important roles in freezing
ABA-inducable genes
tolerance, however, remains to be determined.
The first direct evidence for a cold-induced gene
havingaroleinfreezingtolerancecameasrecentlyas
ABA 1996. The COR15a gene encodes a 15-kDa poly-
(ABSCISIC ACID) peptide that is targeted to the chloroplast. The
constitutiveexpressionofCOR15ainnonacclimated
transgenic Arabidopsis plants increases the freezing
Figure3 Themetabolicchangesthattriggercoldacclimation.
tolerance of both chloroplasts frozen in situ and
isolated leaf protoplasts frozen in vitro. In many
responsive (Arabidopsis RAB18 and LTI65), and
cases, the gene or protein itself is known to be cold
others expressed are in response of both stimuli
responsive in a number of species, indicating at least
(Arabidopsis COR78). It is likely that to achieve full
a partial uniformity of response to cold amongst
cold hardiness the coordinated effect of ABA-
unrelated plants. Several cold-inducible genes are
dependent and ABA-independent regulatory path-
responsive to a variety of stresses, not just cold, or
ways is required (Figure 3).
are also expressed during seed development in the
nonstressed plant, such as late embryogenesis abun-
Role ofCold-InducibleGenes in ColdAcclimation
dant (LEA) genes. Thus, though their expression is
In 1985 Guy with his colleagues established that often related to low cytoplasmic water content, they
changes in gene expression occur with cold acclima- are not uniquely expressed in stressed plants.
tion.Sincethan,considerableefforthasbeendirected Speculatively, the differences between tolerant and
atdeterminingthenatureofcold-induciblegenesand nontolerant species may be attributed to differences
establishing whether they have roles in freezing in the controls of stress-related gene expression,
tolerance. This has resulted in the identification of rather than differences in the types of functional
manygenesthatareinducedduringcoldacclimation. genes present in the genome.
A large number of these genes encode proteins with
Signal Transduction Pathways
knownenzymeactivitiesthatpotentiallycontributeto
freezing tolerance. For instance, the Arabidopsis Theprerequisitefortheplantstoacclimatetocoldfor
FAD8geneencodesafattyaciddesaturasethatmight surviving the approaching winter is the ability to
contribute to freezing tolerance by altering lipid sense the declining temperatures at the onset of
composition.Othergenesencodingmolecularchaper- winter. It is still an open question how plants sense
ons including a spinach (Spinacia oleracea) hsp70 cold. At least in the cyanobacterium Synechocystis it
geneandanoilseedrape(Brassicanapus)hsp90gene was demonstrated that membrane fluidity regulates
might contribute to freezing tolerance by stabilizing theexpressionoflowtemperature-inducible genes.It
proteins against freeze-induced denaturation. A canbepostulatedthatthecoldsignalisfirstsensedby
notablefeatureofcoldacclimationistheinvolvement the plasma membrane by becoming more rigid. The
of extracellular proteins. Several of the proteins cold signal is than transmitted to the Ca2þ channels
extracted from the leaf apoplast of rye and related via rearrangement of actin and tubulin cytoskeleton.
cereals have antifreeze activity in vitro. However, Opening of the Ca2þ channels allows the frequently
these so-called ‘‘antifreeze proteins’’ have strong observedcold-triggeredCa2þ influxfromthevacuole
sequence similarity to certain pathogenesis-related into the cytoplasm. The Ca2þ influx activates the
ABIOTICSTRESSES/ColdStress 7
calcium-dependentproteinkinaseswhichinturnwill photosystem II complex. Higher plants accumulate
phosphorylateotherproteinsincludingothertypesof betaine in response to both salt and cold stress.
kinases. The kinases will activate the transcriptional Enhancement of either chilling or freezing tolerance
factors which control the expression of stress- by genetic engineering of betaine accumulation in
inducible genes. The significance of the transcript- plantshasbeendemonstrated.Cholineoxidaseofthe
ional factors in cold acclimation was demonstrated soil bacterium Arthobacter globiformis converts
by overexpression of the cold-related transcription choline to betaine. Transformation of A. thaliana
factor CBF1 in Arabidopsis. This caused enhanced and rice with cloned choline oxidase genes enabled
expression of several cold-responsive genes and the plants to accumulate betaine and enhanced its
conferred freezing tolerance on plants growing in a tolerance to cold stress.
warm noncold-acclimating environment.
Carbohydrates Carbohydrate changes are of parti-
Osmotically ActiveSolutes
cular importance because of their direct relationship
The increased osmolarity of the cellular solute is an with such physiological processes as photosynthesis,
effective mechanism for avoiding intracellular ice translocation, and respiration. Among the soluble
formation and cellular dehydration. In most, if not carbohydrates, sucrose and fructans have some
all, hardy plants, cell solutes concentrate during the potential role in adaptation to cold stress. Sucrose
process of hardening. Numerous reports point out can act in water replacement to maintain membrane
the accumulation of various osmotically active phospholipids in the liquid-crystalline phase and to
solutes, called compatible solutes, such as sucrose, preventstructuralchangesinsolubleproteins.Nextto
fructan, quaternary ammonium compounds, and theroleasaplantcarbohydratereserve,fructansmay
proline during hardening. They may function as have other functions, including involvement in
cryoprotectants and many of these compounds may droughtandfrosttoleranceinmanycereals,including
also stabilize membranes. wheatandbarley.Studiesonoverwinteringwheatand
rye cultivars have shown that cold hardiness is
Proline Prolineaccumulationhasoftenbeenshown strongly correlated with the capacity to increase
tooccurinplantsasaconsequenceofenvironmental soluble carbohydrate pools in field conditions. When
stress.Thephysiologicalsignificanceofthisaccumu- thecarbohydratereserveisdepleted(inMarch–April)
lation is assumed to be associated with the ability of coldhardinessdropsconsiderablyaswell.Association
proline to act as osmoregulator, as protective agent between frost resistance and water-soluble carbo-
for cytoplasmic enzymes and for membranes, or as hydrate content has been reported in barley and
storagecompound forpoststress growth.Significant wheat. QTL controlling traits associated with winter
positive correlations between proline level and frost hardiness(fieldsurvival,LT ,vernalizationresponse,
50
tolerance have been found in a broad spectrum of and fructan and sucrose content) have been mapped
plants. In breeding programs for frost tolerance in to chromosome 7(5H) of barley and to 5A of wheat.
winter barley, the proline levels in cold-hardened
leaves are indeed used as an early selection criterion
Applications of the Results of Cold
for frost-tolerant lines. Further support for the
Stress Research
involvement of proline in the adaptation mechanism
for frost tolerance comes from in vitro selection Theimportanceofcoldstressresearchcanbeverified
studies.Isolatedhydroxyproline-resistant(Hyp-resis- if we take account that globally there is an annual
tant) cell lines of potato, barley, and wheat possess expenditure of US$100 million to minimize frost
an increased amount of proline and of other amino damage to crops, and annual losses of US$10–100
acidsaswellasincreasedfrosttolerance.Thehighest millionormorefromfrostdamage.Testingplantsfor
increase in tolerance of regenerated plants were at their frost tolerance is standard practice in breeding
2–31C. These characteristics were shown to be programs which help to avoid the frost damage of
heritable in wheat. wintercrops.Recently,progressforwinterhardiness
improvementincerealshasbeenslowduetovarious
Glycine betaine Glycine betaine (hereafter referred reasons. As shown above, the genetic control of
to as betaine) is widely distributed in higher plants, winterhardinessisextremelycomplex.Breedershave
animals,andbacteria.Betaineprotectscellsfromsalt to work with small differences in large populations.
stress by maintaining an osmotic balance with the Most importantly, the exploitable genetic variation
environment and by stabilizing the quaternary for winter hardiness appears to be nearly exhausted.
structure of complex proteins. In photosynthetic As a striking example for these statements, a survey
systems, betaine stabilizes the oxygen-evolving of winter wheat cultivars available worldwide
8 ABIOTICSTRESSES/ColdStress
revealedthannoneofthemismuchmoreresistantto Genemapping Determination of the position of a gene
low temperatures than Kharkov 22MC, which was locusona chromosome.
released for commercial production in Canada in
Oxidative stress Accumulationofreactiveoxygenspecies
1912. Our better understanding of how plants are induced by various environmental ef-
able to cope with cold stress and the usage of the fects.
recent tissue culture and molecular techniques
Vernalization Subjection of a plant to cold tempera-
certainly will help our breeders to improve freezing
ture in order to ensure the flower
tolerance of cultivated plants.
development.
GeneticTransformation
See also: Abiotic Stresses: Free Radicals, Oxidative
Manganese superoxide dismutase from Nicotiana Stress and Antioxidants. Crop Improvement: Chromo-
plumbaginifoliawasconstituvelyexpressedinalfalfa some Engineering; Genetic Maps; Mutation Techniques.
and targeted to chloroplasts or mitochondria. The GeneticModificationofPrimaryMetabolism:Proteins.
GrowthandDevelopment:ControlofGeneExpression,
performancesoftransformantandcontrollineswere
PostTranscriptionalRegulation;ControlofGeneExpres-
compared in the field over 3 years, where the
sion, Regulation of Transcription. Photosynthesis and
transformants showed a significantly higher winter
Partitioning:Photoinhibition;PrimaryProductsofPhoto-
survival and yield.
synthesis, Sucrose and other Soluble Carbohydrates.
RegulatorsofGrowth:AbscisicAcid;Photomorphogen-
InVitroSelection
esis;Vernalization.TissueCultureandPlantBreeding:
Proline-overproducer somaclonal lines had also Somaclonal Variation.
enhanced freezing tolerance. These characters were
shown to be heritable in wheat, and in the third
Further Reading
generation increased proline content and improved
freezing tolerance of acclimated plants were corre-
Do¨rffling K, Do¨rffling H, Lesselich G, et al. (1997)
lated. Heritable improvement of frost tolerance in winter
wheat by in vitro selection of hydroxyproline-resistant
Marker-AssistedSelection prolineoverproducing mutants. Euphytica93: 1–10.
Galiba G, Kerepesi I, Va´gu´jfalvi A, et al. (2001) Mapping
ThesegregationofVrn1–Fr1intervalaffectsthefrost
of genes involved in glutathione, carbohydrate and
tolerance, followed in near-isogenic lines (NILs) by
COR14bcold-inducedproteinaccumulationduringcold
Xwg644 restriction fragment length polymorphism
hardeningin wheat.Euphytica119: 173–177.
(RFLP) marker. These lines derived from a cross
Gombos Z and Murata N (1998) Genetic engineering of
between a freezing-sensitive spring wheat and a theunsaturationofmembraneglycerolipid:effectsonthe
freezing-tolerant winter wheat. The NILs that ability of the photosynthetic machinery to tolerate
inherited the vrn1–fr1 interval from the winter temperature stress. In: Siegenthaler PA and Murata N
wheat parent were about 41C more freezing- (eds) Lipids in Photosynthesis: Structure, Function
tolerant than those carrying the spring locus and Genetics, pp. 249–262. Dordrecht: Kluwer Aca-
(Vrn1–Fr1). demic.
Kocsy G, Galiba G, and Brunold C (2001) Role of
glutathione in adaptation and signalling during chilling
List of Technical Nomenclature and cold acclimation in plants. Physiologia Plantarum
113:158–164.
Abscisic acid Aplant growth regulator. Li PH and Chen THH (1997) Plant Cold Hardiness:
(ABA) Molecular Biology, Biochemistry, and Physiology. New
York: Plenum Press.
Chilling Suboptimal temperature stress above McKersie BD and Leshem YY (1994) Stress and Stress
01C. CopinginCultivatedPlants.London:KluwerAcademic
Publishers.
Coldacclima- Nonheritable modification of structures
Pearce RM (1999) Molecular analysis of acclimation to
tion and functions by cold temperatures in
cold.PlantGrowth Regulation 29:47–76.
ordertominimizecold-inducedinjuries.
SakamotoAandMurataN(2000)Geneticengineeringof
Coldstress The effect of suboptimal temperature glycine betaine synthesis in plants: current status and
resulting in structural and functional implicationsforenhancementofstresstolerance.Journal
injuries. of Experimental Botany51: 81–88.
Steponkus PL and Web MS (1992) Freeze-induced dehy-
COR proteins Cold-regulated proteins.
dration and membrane destabilization in plants. In:
Freezing Temperature stress below01C. SomeroGN,OsmondCB,andBolisCL(eds)Waterand
ABIOTICSTRESSES/FreeRadicals,OxidativeStressandAntioxidants 9
Life:ComparativeAnalysisofWaterRelationshipsatthe Table1 Examplesoffreeradicalsandreactiveoxygenspecies
Organismic,CellularandMolecularLevel,pp.338–362. inplantcells
Berlin: Springer-Verlag. Name Formula
Thomashov MF (1999) Plant cold acclimation: freezing
tolerance genes and regulatory mechanisms. Annual Diatomicoxygen O2
Singletoxygen 1O
ReviewofPlantPhysiologyandPlantMolecularBiology 2
Superoxide O(cid:2)(cid:1)
50: 571–591. Hydroxyl OH2(cid:2)
Hydrogenperoxide H O
2 2
Transitionmetalatoms/ions Fe,Cu
Thiyl RS(cid:2)
Peroxyl,alkoxyl RO(cid:2);RO(cid:2)
2
Free Radicals, Oxidative Stress Nitricoxide NO(cid:2)
Peroxynitrite ONOO–
and Antioxidants
K SGould,University ofAuckland, Auckland,New
Zealand radicalsarecollectivelyknownasreactive(oractive)
oxygen species (ROS or AOS).
Copyright2003,ElsevierLtd.AllRightsReserved.
Free radicals can be generated in plants when a
nonradical gains or loses an electron, or receives
excitation energy from a photoactivated pigment.
Introduction Most radicals are unstable and are therefore short-
lived; half-lives in the order of a microsecond or a
Plants are continuously exposed to free radicals.
nanosecond are common. Stability is restored when
These unstable and often highly reactive molecules
two radicals meet and share their unpaired electrons
present a formidable challenge to all plants, even
in a covalent bond. However, when a free radical
underoptimalgrowingconditions.Ifleftunchecked,
reacts with an organic molecule such as an unsatu-
free radicals can cause oxidative injury by initiating
ratedfattyacidinamembranebilayer,anewradical
chain reactions that disrupt membranes, denature
results, and chain reactions are established. Such
proteins, fragment DNA, and ultimately precipitate
reactions can eventually lead to oxidative injury.
cell death. The problems are exacerbated in plants
Free radicals and ROS are generated in the
that face additional stressors such as high light,
chloroplasts, mitochondria, endoplasmic reticulum,
temperature extremes, drought, or fungal infections.
peroxisomes, glyoxysomes, plasma membrane, and
Inthesesituations,protectionfromtheeffectsoffree
apoplasmofplantcells.Theyarepresentintheroots
radicals is not a luxury – it may be critical for
and shoots of both vegetative and reproductive
survival! Fortunately, plants have evolved a sophis-
individuals. Many different types of radicals have
ticated armory of antioxidant defense, a diverse
been identified (Table 1). The oxygen-centered
assortment of enzymes, pigments, and secondary
radicals have been most extensively characterized,
metabolites that serve to scavenge or quench the
possiblybecauseofthepivotalrolesofoxygeninthe
reactive molecules before they inflict injury. Efforts
processes of respiration and photosynthesis. Indeed,
to enhance the antioxidant levels in plants have far-
diatomic oxygen is itself a free radical, which,
reachingimplicationsbothforcropproductivityand
althoughrelativelystable,istoxictoplantsathigher
human nutrition.
concentrations.However,itisbecomingincreasingly
evident that the nitrogen radicals, primarily nitric
oxideandthereactivenitrogenspeciesperoxynitrite,
Free Radicals
also contribute significantly to a plant’s oxidative
A free radical is defined as any atom or molecule load.
capable of independent existence that contains one
or more unpaired electrons. An unpaired electron is
Sources of Reactive Oxygen and Nitrogen
one that occupies an atomic or molecular orbital by
itself. Free radicals are usually denoted by a super- Routine processes such as photosynthesis, respira-
(cid:2)
script dot after the chemical formula, such as OH tion, and nitrogen metabolism generate free radicals
for the hydroxyl radical. A related group of and ROS. These are quickly scavenged by anti-
compounds, including hydrogen peroxide, singlet oxidants, and are not usually harmful to plants.
oxygen,andozone,lackunpairedelectrons,but,like Indeed, some oxidants are beneficial at low concen-
the radicals, can participate in cellular redox reac- trations; H O , for example, is required in the
2 2
tions; these together with the oxygen containing apoplast for lignin biosynthesis, and has been
ABIOTICSTRESSES/FreeRadicals,OxidativeStressandAntioxidants 9
Life:ComparativeAnalysisofWaterRelationshipsatthe Table1 Examplesoffreeradicalsandreactiveoxygenspecies
Organismic,CellularandMolecularLevel,pp.338–362. inplantcells
Berlin: Springer-Verlag. Name Formula
Thomashov MF (1999) Plant cold acclimation: freezing
tolerance genes and regulatory mechanisms. Annual Diatomicoxygen O2
Singletoxygen 1O
ReviewofPlantPhysiologyandPlantMolecularBiology 2
Superoxide O(cid:1)(cid:2)
50: 571–591. Hydroxyl OH2(cid:1)
Hydrogenperoxide H O
2 2
Transitionmetalatoms/ions Fe,Cu
Thiyl RS(cid:1)
Peroxyl,alkoxyl RO(cid:1);RO(cid:1)
2
Free Radicals, Oxidative Stress Nitricoxide NO(cid:1)
Peroxynitrite ONOO–
and Antioxidants
K SGould,University ofAuckland, Auckland,New
Zealand radicalsarecollectivelyknownasreactive(oractive)
oxygen species (ROS or AOS).
Copyright2003,ElsevierLtd.AllRightsReserved.
Free radicals can be generated in plants when a
nonradical gains or loses an electron, or receives
excitation energy from a photoactivated pigment.
Introduction Most radicals are unstable and are therefore short-
lived; half-lives in the order of a microsecond or a
Plants are continuously exposed to free radicals.
nanosecond are common. Stability is restored when
These unstable and often highly reactive molecules
two radicals meet and share their unpaired electrons
present a formidable challenge to all plants, even
in a covalent bond. However, when a free radical
underoptimalgrowingconditions.Ifleftunchecked,
reacts with an organic molecule such as an unsatu-
free radicals can cause oxidative injury by initiating
ratedfattyacidinamembranebilayer,anewradical
chain reactions that disrupt membranes, denature
results, and chain reactions are established. Such
proteins, fragment DNA, and ultimately precipitate
reactions can eventually lead to oxidative injury.
cell death. The problems are exacerbated in plants
Free radicals and ROS are generated in the
that face additional stressors such as high light,
chloroplasts, mitochondria, endoplasmic reticulum,
temperature extremes, drought, or fungal infections.
peroxisomes, glyoxysomes, plasma membrane, and
Inthesesituations,protectionfromtheeffectsoffree
apoplasmofplantcells.Theyarepresentintheroots
radicals is not a luxury – it may be critical for
and shoots of both vegetative and reproductive
survival! Fortunately, plants have evolved a sophis-
individuals. Many different types of radicals have
ticated armory of antioxidant defense, a diverse
been identified (Table 1). The oxygen-centered
assortment of enzymes, pigments, and secondary
radicals have been most extensively characterized,
metabolites that serve to scavenge or quench the
possiblybecauseofthepivotalrolesofoxygeninthe
reactive molecules before they inflict injury. Efforts
processes of respiration and photosynthesis. Indeed,
to enhance the antioxidant levels in plants have far-
diatomic oxygen is itself a free radical, which,
reachingimplicationsbothforcropproductivityand
althoughrelativelystable,istoxictoplantsathigher
human nutrition.
concentrations.However,itisbecomingincreasingly
evident that the nitrogen radicals, primarily nitric
oxideandthereactivenitrogenspeciesperoxynitrite,
Free Radicals
also contribute significantly to a plant’s oxidative
A free radical is defined as any atom or molecule load.
capable of independent existence that contains one
or more unpaired electrons. An unpaired electron is
Sources of Reactive Oxygen and Nitrogen
one that occupies an atomic or molecular orbital by
itself. Free radicals are usually denoted by a super- Routine processes such as photosynthesis, respira-
(cid:1)
script dot after the chemical formula, such as OH tion, and nitrogen metabolism generate free radicals
for the hydroxyl radical. A related group of and ROS. These are quickly scavenged by anti-
compounds, including hydrogen peroxide, singlet oxidants, and are not usually harmful to plants.
oxygen,andozone,lackunpairedelectrons,but,like Indeed, some oxidants are beneficial at low concen-
the radicals, can participate in cellular redox reac- trations; H O , for example, is required in the
2 2
tions; these together with the oxygen containing apoplast for lignin biosynthesis, and has been