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Conservation of plant genetic resources by cryopreservation

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Abstract

Cryopreservation is a perfect method for long-term conservation of plant genetic resources, using very low temperature (liquid nitrogen, -196°C). This method has been recognized as a practical and efficient tool for the long-term storage of germplasm. Cryopreservation methods may provide the conditions for unlimited conservation of biological materials by reducing metabolic rates. During the cryopreservation all biochemical activities significantly reduced and biological deterioration are stopped. Conservation and subsequent sustainable use of genetic resources are essential to meet the demand for future food security. Several techniques have been developed yet to minimize the damaging effects of desiccation and freezing, ensuring high recovery of plant materials. Cellular division of germplasm is normally repressed after exposure to LN. In addition, metabolic and most physical processes are stopped at this temperature. Thus, plants can be stored for very long time and the problems such as genetic instability and the risk of loose accessions due to contamination or human error during subculture overcome. Techniques like cryopreservation collect and conserve plant genetic resources, especially plants with limited seed storage capability. There is only limited number of plants that cryopreservation techniques are used for their germplasm conservation, mainly because the techniques need to be adapted for each species. Therefore, continued efforts are needed in cryopreservation techniques to develop protocols for a wider range of plants. Formation of ice crystal during cryopreservation is detrimental to cellular structure integrity and causes physical damage to the cells. Air-drying, freeze dehydration, osmotic dehydration, addition of penetrating and non-penetrating cryoprotective substances, and hardening metabolism or combinations of these processes are cryogenic strategies. Nowadays, conservation of plant germplasm has altered from slow cooling to vitrification. However, the availability or developments of simple, reliable and cost-effective strategies and the subsequent regeneration of the plants are basic requirements for germplasm conservation.
778
AJCS 5(6):778-800 (2011) ISSN:1835-2707
Review
article
Conservation of plant genetic resources by cryopreservation
B.
Kaviani
Department of Horticultural Science, Rasht Branch, Islamic Azad University, Rasht,
Iran
*Corresponding author: b.kaviani@yahoo.com
Abstract
Cryopreservation is a perfect method for long-term conservation of plant genetic resources, using very
low
temperature
(liquid
nitrogen, -196°C). This method has been recognized as a practical and efficient tool for the long-term storage of germplasm.
Cryopreservation methods may provide the conditions for unlimited conservation of biological materials by reducing metabolic
rates. During the cryopreservation all biochemical activities significantly reduced
and
biological
deterioration are stopped.
Conservation and subsequent sustainable use of genetic resources are essential to meet the demand for future food security. Several
techniques have been developed yet to minimize the damaging effects of desiccation and freezing, ensuring high recovery of plant
materials. Cellular division of germplasm is normally repressed after exposure to LN. In addition, metabolic and most physical
processes are stopped at this temperature. Thus, plants can be stored for very long time and the problems such as genetic instability
and the risk of loose accessions due to contamination or human error during subculture overcome. Techniques like cryopreservation
collect and conserve plant genetic resources, especially plants with limited seed storage capability. There is only limited number of
plants that cryopreservation techniques are used for their germplasm conservation, mainly because the techniques need to be
adapted for each species. Therefore, continued efforts are needed in cryopreservation techniques to develop protocols for a wider
range of plants. Formation of ice crystal during cryopreservation is detrimental to cellular structure integrity and causes physical
damage to the cells. Air-drying, freeze dehydration, osmotic dehydration, addition of penetrating and non-penetrating
cryoprotective substances, and hardening metabolism or combinations of these processes are cryogenic strategies. Nowadays,
conservation of plant germplasm has altered from slow cooling to vitrification. However, the availability or developments of
simple, reliable and cost-effective strategies and the subsequent regeneration of the plants are basic requirements for germplasm
conservation.
Keywords: Cryoprotectant; cryoprotection; encapsulation-dehydration; germplasm collection; in vitro culture; plant genetic
resources; vitrification; genetic stability.
Abbreviations: ABA-abscises acid; AFLP-amplified fragment length
polymorphism;
DMSO-dimethylsulfoxide;
ED-
encapsulation- dehydration; LN-liquid nitrogen; MS-Murashige and Skoog; PCR-polymerase chain reaction; PEG-polyethylene
glycol; PVS-plant vitrification solution; RAPD-random amplified polymorphic DNA; RFLP-restriction fragment length
polymorphism; SSR-simple sequence repeats.
Introduction
Conservation of plant genetic resources is necessary for
food security and agro-biodiversity. Genetic diversity
provides options to develop through selection and breeding
of new and more productive crops, resistant to biological
and environmental stresses (Rao, 2004). For more food, it
will be necessary to make better use of a broader range of
genetic diversity across the glob. Many plant species are
now in danger of becoming extinct (Panis and Lambardi,
2005). More than fifteen million hectares of tropical forests
are vanished each year (Rao, 2004). Their preservation is
essential for plant breeding programs. Biodiversity provides
a source of compounds to the medical, food and crop
protection industries (Panis and Lambardi, 2005).
Genetically uniform modern varieties are being replaced
with highly diverse local cultivars and landraces of
traditional agro-ecosystems. Deforestation, urbanization,
pollution, habitat destruction, fragmentation and
degradation, spread of invasive alien species, climate
change, changing life styles, globalization, market
economies, over-grazing and changes in land-use pattern
are contributing indirectly to the loss of diversity (Pitman
and Jorgensen, 2002; Rao, 2004). These reductions are a
threat for food security in the long term. Genebanks were
established in many countries for conservation of plants
(Rao, 2004). Advances in biotechnology, especially in the
area of in vitro culture techniques and molecular biology
provide some important tools for improved conservation
and management of plant genetic resources (Ramanatha
Rao and Riley, 1994; Withers, 1995). Conservation of plant
genetic resources can be carried out either in the natural
habitats (in situ) or outside (ex situ). Ex situ conservation is
generally used to safeguard populations, in danger of
destruction, replacement or deterioration. An approach to ex
situ conservation includes methods like seed storage in seed
banks, field gene banks, botanical gardens, DNA and pollen
storage (Rao, 2004). Among these, seed storage is the most
convenient method of long-term conservation for plant
genetic resources. This involves desiccation of seeds to low
moisture contents and storage at low temperatures. Seeds of
some species, especially a large number of important
tropical and
sub-tropical
tree
species, are recalcitrant or
intermediate, i.e. they cannot stand desiccation below a
779
relatively high critical water content value (10-12% or 20%
of fresh weight) (King and Roberts, 1980; Hong et al.,
1996) and cold storage without losing viability (Berjak and
Pammenter, 1997). Intermediate seeds can be stored by
partial drying, although for shorter periods compared to
orthodox seeds. Conservation of recalcitrant seeds under
humid conditions can be carried out only for short periods,
due
to
germination onset, fungal attack or viability loss
(Engelmann and Engels, 2002). The second groups of
plants, which are not feasible for seed banking, are
vegetatively propagated species (Gonzalez-Benito et al.,
2004). They are usually highly heterozygous and, in some
cases do not produce seeds, such as banana, sweet potato,
sugarcane, cassava, yam, potato and taro (Withers and
Engelmann, 1997; Gonzalez-Benito et al., 2004). These
species are usually conserved in field gene banks. Field
collections loose germplasm (genetic erosion) because of
pests, plaque attacks, diseases and adverse weather
conditions and their maintenance is labor-intensive and
expensive (Panis and Lambardi, 2005). In vitro culture is a
feasible alternative for genetic conservation of plants where
the seed banking is not possible (Henshaw, 1975). In vitro
culture not only provides a method for clonal propagation
and safe exchange of plant material but also used for
medium-term germplasm conservation (Withers and
Engelmann, 1997; Rao, 2004). Several in vitro techniques
have been developed for storage of vegetatively propagated
and recalcitrant seed producing species (Engelmann and
Engels, 2002). In general, they fall under two categories: (1)
slow growth procedures, where germplasm accessions are
kept as sterile plant tissues or plantlets on nutrient media,
which provide short- and medium storage options, and (2)
cryopreservation, where plant materials are stored in LN for
long-term storage (Engelmann and Engels, 2002).
Cryopreservation technique is based on the removal of all
freezable water from tissues by physical or osmotic
dehydration, followed by ultra-rapid freezing.
Cryopreservation includes classical and new techniques.
Classical cryopreservation techniques have been developed
in the 70-80s. They comprise a cryoprotective treatment
followed by slow freezing (Kartha, 1985). The most
common cryoprotective substances are dimethylsulfoxide
(DMSO), polyethylene glycol (PEG), sucrose, sorbitol and
mannitol. These substances have the osmotic actions;
however some of them such as DMSO can enter to cells and
protect cellular integrity during cryopreservation
(Rajasekharan, 2006). Classical cryopreservation methods
are mainly used for freezing undifferentiated cultures such
as cell suspensions and calluses (Kartha and Englemann,
1994). For freezing of differentiated tissues and organs such
as seed, embryonic axes, shoot tips and zygotic and somatic
embryos, new techniques include encapsulation-dehydration
(ED), vitrification, encapsulation-vitrification, desiccation,
pre-growth, pregrowth-desiccation and droplet freezing
have been developed (Englemann, 1997). Many of these
techniques have been reported for conservation of plants
germplasm (Englemann, 2000). For successful
cryopreservation, many factors such as source-plant status,
starting materials, personnel, culture conditions,
pretreatment conditions, cryopreservation methods,
cryogenic facilities, regimes and post-thawing are involved
(Reinhoud et al., 2000; Reed et al., 2004). Cryopreservation
methods include both cryogenic (cryoprotectant and low
temperature treatments) and non-cryoprotectant (pre- and
post-storage culture) components (Reed et al., 2005). The
success of a protocol depends on the tolerance and
sensitivity of plant germplasm to the stresses (Reed et al.,
2005). Most plant species needs to be conserved at three
broad levels; ecosystem level (in situ), genotype level (ex
situ) and gene level (molecular) (Ganeshan, 2006).
Cryopreservation and DNA storage may provide long-term
storage capabilities. Cryopreservation may be supplemented
by DNA storage systems for long-term storage. DNA banks
provide novel
options
for
gene banks (Ganeshan, 2006).
This
technique
needs
to have further studies to establish as a
practical conservation strategy (Shikhamany, 2006).
In vitro culture or slow growth of plant germplasm
Slow growth methods allow plant material to be held for a
few years under tissue culture conditions with periodic sub-
culturing. In the other word, in vitro culture includes some
techniques involving the growth under sterile conditions
and constant environmental factors of plant germplasm on
artificial culture media. Explants
are
mostly
shoot, leaf,
flower pieces, immature embryos, hypocotyls fragments or
cotyledons (Paunesca, 2009). Generally, younger and more
rapidly growing tissues are suitable. The criteria for a
proper
quality
explants
are normal, true-to-type donor plant,
vigorous and disease free (Fay, 1992). As a rule, fragile
tissues including meristems, immature embryos, cotyledons
and hypocotyls requires less exposure to sterilizing agents
than seeds or lignified organs (Paunesca, 2009). Explants
may be obtained from seedlings grown from sterilized
seeds. In vitro conservation techniques, using slow growth
storage, have been developed for a wide range of species,
including temperate woody plants, fruit trees, horticultural
and numerous tropical species (Shikhamany, 2006). In vitro
storage based on slow growth techniques has been pointed
out as alternative strategies for conservation of genetic
resources of plants. In particular, it is useful where the seed
banking is not possible, such as vegetatively propagated
plants, recalcitrant seed species, and plants with
unavailable or non-viable seeds due to damage of grazing or
diseases, and large and fleshy seeds. Some species
conserved at in vitro conditions are Allium spp., Cocos
nucifera, Theobroma cocoa, Vitis, Prunus, Citrus spp.,
Saccharum, Solanum spp., Musa spp., Colocasia
esculentum, Manihot spp., and Ipomaea batatas (Henshaw,
1975; Zapartan and Deliu, 1994; Withers, 1995; Ashmore,
1997; Withers and Engelmann, 1997; Engelmann and
Engels, 2002; Gonzalez-Benito et al., 2004; Paunesca and
Holobiuc, 2005; Sarasan et al., 2006). Some clonal crops
are stored in slow-growth medium-term storage as in vitro
cultures for germplasm conservation (Ashmore, 1997;
Benson, 1999). In vitro storage of Zoysia was successful at
21°C for 2 years (Jarret, 1989). Lolium multiflorum Lam.
can be kept
in
vitro
at 2-4°C with yearly subculture (Dale,
1980). A broad range of Cynodon in vitro germplasm
remained healthy in storage at 4°C from 4 months to more
than 1 year (Aynalem et al., 2002). However, few long-term
management options are available for clonal plant
germplasm resources (Reed et al., 2005). The development
of cryopreservation techniques provides the option for long-
term backup of active collections that might otherwise be at
risk (Reed et al., 2005). About 37 600 accessions are
conserved by slow growth methods in gene banks,
worldwide (FAO, 1996). In vitro culture provides a method
for clonal propagation and short- and medium-term
germplasm conservation (Ashmore, 1997). For medium-
term conservation, the aim is the reduction of growth,
which increases intervals between subcultures. These
methods enable extending the subculture periods from
12
months
up to 4 years for many species (Ashmore, 1997). In
780
in vitro conservation, the material can be maintained in a
pathogen-tested state and cultures are not subjected to
environmental stresses (Withers and Engelmann, 1997).
There are several methods, by which slow growth can be
maintained. In most cases, a low temperature, often in
combination with low light intensity or even darkness, is
used to limit growth. Temperature in the range of 0-5°C are
employed with cold tolerant species, but for tropical
species, which are generally sensitive to cold, temperatures
between 15°C and 2C are used (Withers and Engelmann,
1997). A standard storage treatment for Pyrus communis
and many other species is 4°C with a 16-h photoperiod for
12 to 18 months (Bell and Reed, 2002). It is also possible to
limit growth by altering the culture medium, mainly by
reducing the sugar, mannitol or mineral elements
concentration, application of abscisic acid (ABA), and
reduction of oxygen level,
available
to
cultures. This
normally done by covering explants with a layer of liquid
medium or mineral oil, or by placing them in controlled
atmosphere (Withers and Engelmann, 1997; Engelmann and
Engels, 2002). The humidity should be between 40-50%
(Paunesca, 2009). Artificial seeds (beads) were first
introduced in the 1970s as a novel analogue to the plant
seeds, suitable for medium-term storage (Redenbauch et al.,
1988). Artificial seeds are produced by encapsulating a
plant material in a culture medium containing sodium
alginate and then a culture medium containing CaCl2. Plant
materials can grow in the proper culture media.
Encapsulation is also used for direct protection during
dehydration and thawing in cryopreservation (Saiprasad,
2001). Totally, there are three methods for reducing in vitro
growth rates, including physical (reduced temperature and
light conditions), chemical (using growth retardants), and a
combination of the two (Engelmann and Engels, 2002).
Regeneration and successful propagation of genetically
stable seedlings from cultures are prerequisites for any in
vitro conservation efforts (Ashmore, 1997). Genetically,
organized cultures such as shoots are used for slow growth
storage, since undifferentiated tissues such as callus are
more vulnerable to somaclonal variation (Ashmore, 1997).
Advantages of in vitro storage include the sterile
preservation of materials, no risk of infections by insects or
damage through inauspicious weather conditions, less work
needed for collections, and the varieties are available all
year round (Schäfer-Menuhr, 1996). Disadvantages are that
growth retardants change plant morphology and can induce
DNA methylation (Harding, 1994), and somaclonal
variation (Kumar, 1994). In vitro storage based on reduced
growth conditions is still labor intensive and there is always
the risk of losing accessions due to contamination or human
error. Moreover, in vitro material of some species is subject
to mutations, whose frequency increased during in vitro
culture.
Cryopreservation
Cryopreservation is a part of biotechnology. Biotechnology
plays an important role in international plant conservation
programs and in preservation of the world's genetic
resources (Bajaj, 1995; Benson, 1999). Advances in
biotechnology provide new methods for plant genetic
resources and evaluation (Paunesca, 2009).
Cryopreservation, developed during the last 25 years, is an
important and the most valuable method for long-term
conservation of biological materials. The main advantages
in cryopreservation are simplicity and the applicability to a
wide range of genotypes (Engelmann, 2004). This can be
achieved using different procedures, such as pre-growth,
desiccation, pregrowth-desiccation, ED, vitrification,
encapsulation-vitrification and droplet-freezing (Engelm-
ann, 2004). Cryopreservation involves storage of plant
material (such as seed,
shoot
tip,
zygotic and somatic
embryos and pollen) at ultra-low temperatures in LN (-
196°C) or its vapor phase (-150°C). To avoid the genetic
alterations that may occur in long tissue cultures storage,
cryopreservation has been developed (Martin et al., 1998).
At this temperature, cell division, metabolic, and
biochemical activities remain suspended and the material
can be stored without changes and deterioration for long
time. Walters et al. (2009) proposed that this assumption,
based on extrapolations of temperature-reaction kinetic
relationships, is not completely supported by accumulating
evidence that dried seeds can deteriorate during cryogenic
storage. After 30 years of cryogenic storage, seeds of some
species exhibited quantitatively lower viability and vigor. In
cryopreservation method, subcultures are not required and
somaclonal variation is reduced. Advantages of
cryopreservation are that germplasm can be kept for
theoretically indefinite time with low costs and little space.
Besides its use for the conservation of genetic resources,
cryopreservation can also be applied for the
safe
storage
of
plant tissues with specific characteristics. Different types of
plant cell, tissues and organs can be cryopreserved.
Cryopreservation is the most suitable long-term storage
method for genetic resources of vegetatively maintained
crops (Kaczmarczyk et al., 2008). For vegetatively
propagated species, the best organs are shoot apices excised
from in vitro plants. Shoot apices or meristems cultures are
suitable because of virus-free plant production, clonal
propagation, improving health status, easier recovery and
less mutation (Scowcroft, 1984). Seed and field collections
have been the only proper for the long-term germplasm
conservation of woody species, while a large number of
forest angiosperms have recalcitrant seeds with a very
limited period of conservability. The species, which are
mainly vegetatively propagated, require the conservation of
huge number of accessions (Panis and Lambardi, 2005).
The storage of this huge number needs large areas of land
and high running costs. Preservation of plant germplasm is
part of any plant breeding program. The most efficient and
economical way of germplasm storage is the form of seeds.
However, this kind of storage is not always feasible because
1) some seeds deteriorate due to invasion of pathogens and
insects, 2) some plants do not produce seeds and they are
propagated vegetatively, 3) some seeds are very
heterozygous thus, not proper for maintaining true-to-type
genotype, 4) seeds remain viable for a limited time, and 5)
clonally propagated crops such as fruit, nut, and many root
and tuber vegetables cannot be stored as seed (Chang and
Reed, 2001; Bekheet et al., 2007). Cryopreservation offers a
good method for conservation of the species, especially
woody plant germplasm (Panis and Lambardi, 2005).
Cryostorage of seeds in LN was initially developed for the
conservation of genetic resources of agriculturally
important species (Rajasekharan, 2006). The development
of simple cryostorage protocols for orthodox seeds has
allowed cryopreservation of a large number of species at
low cost, significantly reducing seed deterioration in storage
(Stanwood, 1987). Only a few reports are available on the
application of cryopreservation on seeds of wild and
endangered species and medicinal plants (Rajasekharan,
2006). New cryobiological studies of
plant
materials
has
made cryopreservation a realistic tool for long-term storage,
for tropical species, which are not intrinsically tolerant to
781
low temperature and desiccation, has been less extensively
investigated (Rajasekharan, 2006). Cryopreservation has
been applied to more than 80 plant species (Zhao et al.,
2005). Number of species, which can be cryopreserved has
rapidly increased over the last several years because of the
new techniques and progress of cryopreservation research
(Rajasekharan, 2006). The vitrification/one-step freezing
and ED methods have been applied to an increasing number
of species (Panis and Lambardi, 2005). A new method,
named encapsulation- vitrification is noteworthy (Sakai,
2000). The new techniques have produced high levels of
post-thaw and minor modifications (Rajasekharan, 2006). In
cryopreservation, information recording such as type and
size of explants, pretreatment and the correct type and
concentration of cryoprotectants, explants water content,
cryopreservation method, rate of freezing and thawing,
thawing method, recovery medium and incubation
conditions is very important (Reed, 2001; Gonlez-Benito
et al., 2004; Bekheet et al., 2007). All germplasm requires
safe storage because even exotic germplasm without
obvious economic merit may contain genes or alleles that
may be needed as new disease, insect, environmental, or
crop production problems arise (Westwood, 1989). It is
important to record also the recovery percentage after a
short conservation period. A major concern is the genetic
stability of conserved material.
Cryopreservation damage, ultra structural changes and
cryoprotection
Most plant cells have plenty of water and they are sensitive
to freeze. Water content is the single most important factor
affecting the ability of germplasm to be stored in LN
(Stanwood, 1985). Optimal germplasm water content must
be determined. Death or loss of viability will be occurred
during the cryopreservation, when the germplasm water
content is too much. As the water content decreases, the
interaction between water and solutes become stronger, and
the system deviates from ideal behavior. On removal of
more water, the solution becomes so concentrated that it
becomes viscous and has the properties of a glass. At very
low water contents, all the remaining water is tightly
associated with macromolecular surfaces (bound water) and
its mobility is reduced (Vertucci, 1990). Cells must be
dehydrated to avoid ice crystal formation (Mazur, 1984).
The most damaging event during cryopreservation is the
irreversible injury caused by the formation of intracellular
ice crystals. Cryopreservation damage of biological material
can be caused by physical and biochemical events (Dumet
and Benson, 2000). Cryopreservation damages induce based
on the physical effects of rice crystal formation and the
dynamic effects of freezing rate (Dumet and Benson, 2000).
Physical effect accounts for large intracellular ice crystals,
which form during rapid cooling and cause mechanical
damages. Dynamic effect is the dehydration damage arising
from extracellular ice crystal formation. Intracellular ice
formation causes damages primarily on membranes (Li et
al., 1979; Muldrew et al., 2004). This damage can occur
during freezing with
ice
crystallization
or during thawing
with recrystallisation of ice. Ultrastructural studies on
potato shoot tips showed that the extensive damage was
visible after cryopreservation and rewarming (Golmirzaei et
al., 2000). These researchers reported cell wall rupture,
rupture of epidermis, protoplast outflow and anomalous
nucleus shape of surviving and killed explants.
Ultrastructural changes during cryopreservation are
important to understand and improve this method. The
plasma membrane has been considered to be one of the
most important determinants for survival at low
temperatures (Uemura et al., 2009). The physical damage to
the membrane is lethal because this results in the loss of its
semi-permeability, imbalance of cytoplasm components, the
intrusion of
extracellular
ice crystals, and many serious
injuries in plant cells (Uemura et al., 2009). However, high
desiccation also produces damages on cell membrane, due
to high concentration of internal solutes and protein
denaturation.
Thus,
plants must increase the cryostability of
the plasma membrane to withstand various stresses imposed
by freezing and accelerate the recovery process after
thawing (Uemura et al., 2009). Uemura et al. (2009)
reported that both lipid and protein compositions of the
plasma membrane dynamically alter during cold
acclimation, which ultimately results in an increase in the
cryostability of the plasma membrane. High survival of
plant cells after cryopreservation likely requires maintaining
the intactness of the plasma membrane. To help plant cells
alive, cryoprotectants are often included in the system
(Uemura et al., 2009). There are many studies
demonstrating that some of the cryoprotectants increase
stability of intactness of the plasma membrane through their
direct interactions or alterations of water distribution
inside/outside cells (Uemura et al., 2009). In their case,
plant cells must keep their plasma membrane active and
functional for survival. Panta et al. (2009) revealed that
freezing resistant genotype of potato have significant higher
regeneration rates after cryopreservation and that linoleic
acid content is positively correlated with tolerance towards
cryopreservation. Also, it was demonstrated that the edition
of putrescine to the preculture medium can improve the
response of cryopreservation of potato accessions that
originally show very low recovery rate (Panta et al., 2009).
Zhang et al. (2009) showed the changes of total soluble
proteins and calcium in pollen of Prunus mume after
cryopreservation. One of the best ways to prevent ice
crystal formation at LN without damage to membrane and
an extreme reduction in cellular water is vitrification, i.e.
non-crystalline solidification of water (Panis and Lambardi,
2005). In the other word, vitrification (the production of an
amorphous glassy state) circumvents the injurious problems
associated with ice formation (Benson, 2004). Two
requirements should be performed for vitrification of a
solution: 1. enough concentration of the solution, and 2.
enough cooling rate of the solution (Panis and Lambardi,
2005). For a solution to be vitrified at high cooling rates, a
reduction in water content to at least 20-30% is required.
Xu et al. (2009) showed ultrastructural changes during the
application of a vitrification protocol of embryogenic cells
of Musa spp. The results showed that the control cells
contained a lot of organelles, a regular nucleolus envelope
and intact plasma membrane. After treatment with 25% of
PVS2, some changes could be observed, such as smaller
vacuoles, more phenol compounds, swollen organelles and
appearance of many lipid bodies. Also, multi-vesicular
membranous structures with vesicles appeared between the
plasma membrane and the cell wall. Moreover, plasmolysis
remained limited. Only after dehydration with 100% PVS2,
plasmolysis became more severe. Nucleus envelopes in
these cells were malformed. Though the cytoplasm and the
nucleus became more electron-dense and a lot of
heterochromatin appeared, the plasma membrane was still
intact. The ultrastructure of cells after freezing, thawing and
unloading was similar to that of those cells after
dehydration. After 1-2 weeks' of post-thaw recovery, the
ultrastructure of surviving cells was similar to that of the
782
control cells (Xu et al., 2009). A second method for
desiccation is using a flow box with a defined air flow,
temperature and humidity or alternatively desiccation over
various saturated salt solutions (Zamecnik et al., 2009).
Following cryogenic strategies result in more concentrated
intracellular solutes which most of them associated with cell
volume reduction: 1. in air drying or air desiccation,
samples are dried by a flow of
sterile
air
under the laminar
airflow cabinet. However, there is no control of temperature
and air humidity, which both influence strongly the
evaporation rate. More reproducible is the air-drying
method that uses a closed vial containing a fixed amount of
silica gel (Uragami et al., 1990). Studies of Rajasekharan
(2006) on conservation of tropical horticultural species
showed being better dehydration of germplasm with silica
gel than under the laminar air flow. 2. In freeze dehydration,
a controlled temperature decrease will also cause cells to
dehydrate. During slow cooling, crystallization is initiated
in the extracellular spaces. Since, only a proportion of water
that contributes to the extracellular solution undergoes
transition into ice, the solution becomes more concentrated
and hypertonic to the cell. Thus, cellular water will leave
the protoplast. Traditional cryopreservation often
uses
a
slow cooling to avoid intracellular ice formation, a common
cause of lethal cell damage (Thinh et al., 1999; Lambardi et
al., 2000). Equipment is costly, and the method is not
effective for low temperature
sensitive
species
(Pennycooke
and Towill, 2000). Nowadays, methods to plant germplasm
cryopreservation involving direct plunging into LN have
been explored, and vitrification procedures proved to be the
most promising among them (Lambardi et al., 2000;
Tsukazaki et al., 2000). Generally, freezing rates of 0.5 to
2°C/min, and prefreezing temperatures of -30 to -40°C are
used. 3. In osmotic dehydration, non-penetrating
cryoprotective substances like sugars, sugar alcohols and
high molecular weight additives like PEG are applied to the
plant tissues. 4. In penetrating and non-penetrating
cryoprotective substances, DMSO, glycerol and some
amino acids like proline are penetrating substances. DMSO
is the best because of its rapid penetration into the cells.
DMSO droplet method improved results in potato when
applied with alternating temperature precultured
(Kryszczuk et al., 2006). Non-penetrating substances are
sugars, sugar alcohols and PEG. 5. Hardening is increasing
plant ability to environmental stress. Hardening requires a
change in metabolism of the cultures triggered by
environmental parameters like reduction in temperature and
shortening of day length, also, osmotic changes and ABA.
Hardening can result in an increase of sugars, proteins,
glycerol, proline and glycine betaine, which will act in the
increase of osmotic value of the cell solutes. Cold pre-
cultures of germplasm before cryopreservation improve
results for woody (Niino and Sakai, 1992) and herbal
species (Reed et al., 2003; Keller, 2005), which are able to
cold-acclimate to low temperature. Some sensitive species
which are not able to acclimate to the cold temperatures,
cryopreservation could be improved after exposure of
germplasm to low temperature (Leunufna and Keller, 2005).
Low temperature precultures were successfully used for
potato germplasm before cryopreservation using
encapsulation-vitrification and droplet-vitrification methods
(Hirai and Sakai, 2000; Halmagyi et al., 2005; Kryszczuk et
al., 2006; Kaczmarczyk et al., 2008). Studies of Reed
(1990) on Pyrus showed that regrowth of meristems ranged
from 0% to 51% for plants grown at 25°C and 5% to 95%
for cold-hardening plants. Cold-hardening significantly
improved the recovery rates of all species tested.
Induction of tolerance to dehydration
Dehydration causes some chemical and mechanical
damages to the most cells. Sensitivity to dehydration varies
among species (Takagi, 2000; Rajasekharan, 2006). The
success of a cryopreservation method depends on the
tolerance and sensitivity of plant germplasm to the stresses
of the cryopreservation method (Reed et al., 2005).
Understanding the degree of sensitivity to dehydration is the
first step in accomplishing an optimized method
(Rajasekharan, 2006). The ability of a germplasm to tolerate
dehydration can be achieved by various pretreatments such
as sucrose and cold acclimation. Critical phase of the
various cryopreservation methods
is the
pretreatment phase
(Rajasekharan, 2006). A number of mechanisms contribute
to desiccation tolerance such as: 1. intracellular physical
characteristics like reduction of the degree of vacuolation,
amount and nature of insoluble reserves accumulated
(Farrant et al., 1997), reaction of cytoskeleton
(microtubules
and
microfilaments)
(Sargent et al., 1981),
and conformation of DNA (Ambika, 2006), 2. intracellular
de-differentiation, which minimize surface areas of
membranes and cytoskeleton (Ambika, 2006), 3. "switching
off" of metabolism (Vertucci and Leopold, 1986), 4.
accumulation of protective molecules like some proteins,
such as late embryogenic abundant proteins (LEAs) and
dehydrines (Kermode, 1990), and sucrose or
oligosaccharides (Koster and Leopold, 1988), and 5. the
presence and operation of repair mechanisms during
rehydration (Ambika, 2006). Synthesis of some proteins
associates with the peak in ABA levels (Kermode, 1990;
Oliver and Bewley, 1997; Kermode, 1997). Results of the
study of Carpentier et al. (2009) suggested that the
maintenance of an osmoprotective intracellular sucrose
concentration, the enhanced expression of particular genes
of the energy-conserving glycolysis and the conservation of
the cell wall integrity may be essential to maintain
homeostasis and to survive dehydration. These researchers
observed a genotype specific expression of certain proteins
(isoforms)
involved
in
energy metabolism and ABA- and
salt stress- responsive proteins. Carpentier et al. (2009)
revealed that twenty eight proteins were correlated to
general osmotic stress and fifty nine proteins were
exclusively correlated to the sucrose treatment. In choosing
cryoprotective treatments or methods, it is important to take
into account the origin and physiological status of the
germplasm (e.g., temperate or tropical, dormant or active),
tolerance to abiotic stresses (e.g., cold and desiccation) as
well as operational, technical and practical factors (Reed et
al., 2005). The key for successful cryopreservation is shifted
from freezing tolerance to dehydration tolerance. Chemical
cryoprotective substances like sugars, amino acids, DMSO,
glycerol, etc, can induce this tolerance. Sugars, especially
sucrose can maintain the liquid crystalline state of the
membrane bilayers and
stabilize proteins
under frozen
conditions (Crowe et al., 1984; Kendall et al., 1993).
Temperate species that naturally accumulate sugars and
protective proteins during seasonal cold accumulation are
better able to withstand stresses incurred during
cryopreservation as compared to desiccation-sensitive
germplasm from tropical species (Reed et al., 2005).
Accumulation of sugars increases the stability of
membranes under conditions of severe dehydration
(Rajasekharan, 2006). Sugars replace the water normally
associated with membrane surface, thereby maintaining
lipid bilayer (Hoekstra et al., 1991; Crowe et al., 1992).
Vitrification of the aqueous phase by sucrose or certain
783
oligosaccharides leads to glassy state. The presence of
glasses, because of their high viscosity, reduce the
deleterious effects of deranged metabolism, protecting
macromolecules against denaturation and preventing or
minimizing liquid crystalline to gel phase transitions in the
membrane lipid bilayer (Koster and Leopold, 1988).
Progressive increase of the sucrose concentration reduces
the toxic effect of high sucrose concentrations, for example
the use of media containing 0.3 M, then 0.4 M, then 0.5 M,
and finally 0.75 M of sucrose instead of directly placing
explants in medium with 0.75 M sucrose. Tolerance to
dehydration can also be induced by adaptive metabolism.
Cold acclimation in nature often leads to the accumulation
of proteins like heat shock proteins, cold regulated proteins,
dehydrines, sugars, polyamines and other compounds that
can protect cell components during dehydration (Neven et
al., 1992; Steponkus et al., 1998). Cold acclimation also
alters membrane composition, thereby increasing
dehydration tolerance (Sugawara and Steponkus, 1990;
Steponkus et al., 1992). Cold-acclimation treatments or
treatments that simulate the biochemical base of cold
acclimation have been used with great success to increase
the
cryopreservation
survival
of temperate and subtropical
plant germplasm (Reed and Yu, 1995; Chang and Reed,
1999; Chang
et
al.,
2000). Chang et al. (2000) reported
successful of both temperate and subtropical grasses that
were cold acclimated for 4 weeks. Exposure to
cryoprotectants with lower concentrations than vitrification
solution minimizes the damage (Rajasekharan, 2006). Some
plant genes are induced by high sugar concentrations
(Koch, 1996). Studies of Volk et al. (2009) on Arabidopsis
shoot tips revealed that cryoprotectant treatments induce
gene expression and critical pathways may include those
involved in lipid transport and osmoregulation. Alterations
in membrane composition, influencing both their flexibility
and permeability, are reported (Ramon et al., 2002). Three
main categories of seed storage behavior are recognized
(Ellis et al., 1990): 1. recalcitrant seeds that cannot
withstand dehydration. These are shed at moisture contents
more than 50%. Recalcitrant seeds are sensitive to low
temperatures
and
must
be kept under high relative humidity
conditions. A number of economically important tropical
and subtropical crops such as tea, litchi, mango, rubber and
forest and horticultural species have recalcitrant seeds
(Ambika, 2006). Properties of water in recalcitrant seed
tissue are important to know the response to loss
of
water
(Pammenter and Berjack, 2000). 2. Orthodox seeds that
survive long-term dry storage. Seeds of most common
agricultural and horticultural species such as Allium cepa,
Glycine max, Cucumis sativus, Citrus lemon, Capsicum,
Arachis, Amaranthus, Melia azedarach, Zea mays, and
Hibiscus esculentus are tolerant
to
dehydration
and
exposure to LN. For these species, critical factor that ensure
the survival is seed
moisture
content
(Rajasekharan, 2006).
Orthodox seeds can be dried to low moisture contents less
than 50%
without
losing
viability. They can be kept
successfully for many years at ambient temperatures. 3.
Intermediate seeds such as neem, coffee and macrophylla
that can withstand dehydration to a certain extent but have
reduced longevity (Ellis et al., 1990). These seeds survive
drying to moderately low moisture contents (8-10%) but are
often injured by low temperatures (Ellis, 1991). Seed
moisture content of 4-10% has proved to be better for safe
storage of several wild species (Iriondo et al., 1992).
Cryopreservation methods
There are several methods of cryopreservation (Fig. 1). The
advantages and disadvantage of each method should be
considered. However, other factors such as personnel,
available facilities, and type of plant species could influence
the selection of the method (Reed, 2001). Cryopreservation
methods are different and include the older classic methods
based on freeze-induced dehydration of cells as well as
newer methods based on vitrification (Engelmann, 2000).
New cryopreservation methods include ED, vitrification,
encapsulation-vitrification, desiccation, pregrowth,
pregrowth-desiccation and droplet freezing (Engelmann,
1997). These methods have been reported for successful use
for many cells, tissues and organs of plant species
(Engelmann, 2000). The new methods do not need
expensive equipment. Cryopreservation is well established
for vegetatively propagated species. However, it is much
less advanced for recalcitrant seed species in order to some
of their characteristics, including their very high sensitivity
to desiccation, structural complexity and heterogeneity.
Probably, the two first questions to answer are the method
to use and the number of replicates required. Various
germplasm usually respond differently to the same method,
thus researchers must either modify the method for each
germplasm or store more explants to compensate for low
recovery. Reed (2009) found that standard protocols can be
applied to many plants with few if any changes. Screening
of groups of plants in a genus shows that many protocols
are easily applied to large groups of plants. The protocol to
use can be chosen from those developed for similar plants
or several standard protocols can be tested (Reed, 2009).
Once a protocol is chosen, some critical points can be
adjusted to improve the plant response as needed. Each of
cryopreservation methods has some basic steps that can be
modified to make them effective for many types of plants
(Reed, 2009). The overall process for cryopreservation
consists of three phase; 1. conditioning of the stock plants,
2. cryogenic conditions including solution application,
cooling and warming rates, and 3. recovery processes (Zhao
et al., 2005). Cryopreservation methods are as follows (Fig.
1). Desiccation method (air drying) Desiccation is the
simplest method and consists of hydrating explants and
freezing them rapidly by direct immersion in LN. Explants
are dried by a flow of sterile air under the laminar airflow
cabinet. However, there is no control of temperature and air
humidity (Panis et al., 2001). Air drying using a closed vial
containing
silica
gel
is more reproducible (Panis et al.,
2001). The method is mainly applied to most common
agricultural and horticultural species, orthodox seeds,
zygotic embryos, embryogenic axes and pollen grain
(Uragami et
al.,
1990;
Engelmann, 2004). Some of orthodox
seeds are very resistance to drying below 3% moisture
content, without any damage and reduction of viability
(Uragami et al., 1990; Panis et al., 2001; Engelmann, 2004).
When water was removed from the cell, it led to solute
effect such as pH changes, increasing electrolyte
concentrations, protein denaturation, membrane phase
transition and macromolecular interactions and then damage
of the cell (Dumet and Benson, 2000). Changrum et al.
(1999) suggested that the rate of water loss among different
tissues of various species and even among tissues is
variable. Thus, drying may not be necessarily beneficial for
cryopreservation, if uneven distribution of water results in
different freezing responses among cells in the same tissue.
Cells with high water content may be predisposed to the
volumetric changes of cells would lead to considerable
784
physical stress within the tissue (Grout, 1995; Reinhoud et
al., 2000; Dumet and Benson, 2000).
Slow freezing (classical method)
In this method (also called two-step freezing and slow
controlled freezing), tissues are cooled slowly at a
controlled rate (usually 0.1- 4°C/min) down to about -40°C
followed by rapid immersion of samples in LN. Slow
freezing is carried out using a programmable freezing
apparatus. Among the various cryopreservation techniques,
slow-freezing method seems to be more common (Zhao et
al., 2005). This method combines the application of
penetrating cryoprotective substances such as DMSO and
controlled freeze dehydration, often preceded by cold or
sugar hardening or osmotic dehydration. The process of
cold acclimation is a multiple trait with complex physical
and biochemical change, which alters membrane
composition; thereby increasing dehydration tolerance
(Sugawara and Steponkus, 1990; Hannah et al., 2005). Cold
pre-cultures in tropical and subtropical specie, which are
also not able to cold acclimation, have shown improved
cryopreservation results (Chang et al., 2000; Leunufna and
Keller, 2005). A typical characteristic in cold-acclimated
plants is the increased concentration of soluble sugars
(Levitt, 1972). Soluble sugars are known to have important
function in osmoprotection, cryoprotectant, and
metabolization of other protective substances during
cryopreservation (Hincha, 1990; Hitmi et al., 1999). In
addition, they have hormone-like functions as primary
messengers in signal transduction (Rolland et al., 2002).
Extracellular ice formation, during freeze-induced
dehydration withdraws free liquid water molecules through
an osmotic gradient from the cytoplasm to intercellular
spaces, where it crystallizes (Benson, 2004). Because of
dehydration the cellular concentration of solute rises and
becomes too high to nucleate to ice crystals during cooling.
This means that it solidifies without crystallization. This
status is called the "glassy state". In this state the water
molecules are amorphous and lack an organized structure
but possess the mechanical and physical properties of a
solid (Taylor et al., 2004). Many chemical solutes such as
DMSO, sucrose and PVS2 decrease the free water content
in cells (Sakai et al., 1990). DMSO and glycerol are cell
wall, membrane penetrable, and increase cellular osmolality
to avoid ice formation (Benson, 2008). Sucrose can
penetrate the cell wall, but not the plasma membrane (Tao
and Li, 1986). When cells are frozen, sucrose is
concentrated in the cell wall space and protects protoplasts
from freeze-induced dehydration. It can form a buffer layer
between cell wall and the protoplast to protect the outer
surface of the plasma membrane (Tao and Li, 1986).
Alteration in essential amino acid was found during cold
acclimation. Highest concentrations were found in
asparagines, glutamine, glycine and proline during cold
acclimation of potato shoot tips (Stewart and Larher, 1980).
Most change was in concentration of proline (Stewart and
Larher, 1980). It is known that soluble nitrogen content
increases in plants grown at lower temperatures (Levitt,
1972). Cooling speed is a main factor of the slow-freezing
method of cryopreservation (Benson et al., 1996; Chang and
Reed, 2000). In general, the slower cooling rates producing
the higher survival rates (Chang and Reed, 2000). Slow
freezing, combined with pretreatment by either cold
acclimation or
ABA
has
proven to be effective for Pyrus
germplasm (Bell and Reed, 2002). A comparison of slow
freezing and vitrification methods using 28 Pyrus genotypes
found that regrowth following slow freezing (0.1°C/min)
was 61%, while after vitrification was 43% (Luo et al.,
1995). Cryoprotectants are added to the freezing mixtures to
maintain membrane integrity and increase osmotic potential
of the external medium. Classical cryopreservation methods
have been successfully applied to undifferentiated culture
systems such as cell suspensions and calluses (Kartha and
Engelmann, 1994). Fukai (1990) has applied a controlled-
rate-freezing method for Chrysanthemum morifolium and
other Chrysanthemum species. Tobacco suspension cells
were successfully cryopreserved by a vitrification method
combined with an encapsulation technique. However, the
vitrification method was less effective than simplified slow
pre-freezing method (Kobayashi et al., 2006). In potato,
survival rate was increased by slow-freezing method as
compared with that of the basic cryopreservation method of
vitrification alone (Zhao et al., 2005).
Pre-culture and pre-culture/dehydration
Preculture or pregrowth involves preculturing the
germplasm on a medium supplemented with cryoprotectants
such as sucrose or glucose before exposure to LN. This
method is proper for zygotic and somatic embryos of some
species (Dumet et al., 1993; Engelmann, 1997a). Also, this
simple cryopreservation protocol was successfully applied
to highly proliferating meristems of banana (Panis et al.,
1996). Meristem cultures
are
grown
for 2 weeks on
proliferating medium supplemented with 0.4 M sucrose.
Then, surviving meristem clumps are excised, transferred to
cryovials and rapidly frozen. Post thaw regeneration rates
vary between 0 and 69% depending on the cultivar.
Uragami et al. (1990) obtained 63% survival after cooling in
LN, in asparagus nodal explants that had previously been
precultured for 2 days on 0.7 M sucrose and subsequently
desiccated to 20% water content with silica gel. A
cryopreservation process using dehydration was performed
for seeds of lily [Lilium ledebourii (Baker) Bioss.] (Kaviani
et al., 2009). Survival after freezing was nil for control
seeds and 75% for seeds pretreated with sucrose and
dehydration. In Pyrus, a 0.75 M sucrose preculture and 4
h
dehydration
(20% residual water) produced 80% recovery
(Scottez et al., 1992). Precultured shoot tips of some species
on medium containing high sucrose concentration have
been reported to show improved survival of cryopreserved
shoot tips (Matsumoto et al., 1995; Takagi et al., 1997;
Zhao et al., 2005). However, in some other species, the use
of very high sucrose concentration (up to 1 M) was toxic for
shoot tip survival (Martinez and Revilla, 1999). Sucrose is
an important pregrowth additive for most cryopreservation
method, which enhance desiccation tolerance during
cryopreservation. Among different types of sugars (fructose,
glucose, sorbitol, and sucrose) used as somatic agents in
preculture medium, sucrose was the best for the survival of
cryopreserved date palm tissue culture (Bekheet et al.,
2007). The highest percentage of survival (80%) was
observed with 1 M sucrose (Bekheet et al., 2007). Studies
on cryopreservation of asparagus (Uragami et al., 1990),
carrot (Dereuddre et al., 1991) and date palm (Bagniol and
Engelmann, 1992) tissues reveal that survival rates after
cryopreservation could be increased by preculturing the
tissues on media containing high concentration of sugar
(Bekheet et al., 2007). Pretreatment of stock plants with
cold acclimation or ABA is very important for
cryopreservation of many pear genotypes (Bell and Reed,
2002). Alternating-temperature (22°C for 8-12 h/-1°C for
12-16 h) cold acclimation for 2 to 15 weeks significantly
785
increase regrowth and recovery remains high for shoots
with up to 15 weeks of cold acclimation (Bell and Reed,
2002). Constant temperature acclimation is less effective
(Chang and Reed, 2000). Preculture conditions (low
temperature and ABA pretreatments) influenced cold
hardiness and improved the recovery of cryopreserved
Pyrus cordata and several other plant shoot tips (Reed,
1993; Vandenbussche and De Proft, 1998). The optimal
treatment for recovery was a 3 weeks culture on ABA
followed by 2 weeks of low temperature and shoot tips
growth increased from zero to 70% (Chang and Reed,
2001). ABA is an important stress hormone produced
during cold acclimation (Chang and Reed, 2001). Chen and
Gusta (1983) suggested that increased ABA concentration
in cells trigger cold acclimation and expression of low
temperature-responsive genes. Some plants species need
long tolerate temperature treatments, some do not respond
to low temperatures, and others do not tolerate low
temperature (Chang and Reed, 1997).
Encapsulation-dehydration
In the encapsulation-based techniques, the production of
high-quality "synthetic seeds" is required. Although the
technique was initially proposed to encapsulate single
somatic embryos inside an artificial seed coat (Murashige,
1977), today various other explants such as shoot tips, nodal
segments, bulblets, and even callus samples are used to
produce synthetic seeds (Lambardi et al., 2006). The ED
method requires cryogenic storage in the total absence of
ice (Benson et al., 1996). The encapsulation method
developed by Redenbauch et al. (1991) is still most widely
used to produce synthetic seeds. This method has been
developed for apices of numerous species from tropical
origin like cassava and sugarcane and of temperate origin
like pear, apple, grape and eucalyptus (Dereuddre, 1992;
Engelmann, 1997; Sakai, 2004). The method involves the
incubation of explants in a Na-alginate solution (1-5%, 3%
being the most used) and their subsequent release
(immersed in a drop of alginate) into a complexity agent
(50-100 mM CaCl2 solution) where bead hardening occurs
in 20-30 min. There are two main types of new
cryopreservation techniques, ED and vitrification.
Combinations of them have also been used. The ED
method
is
based on the artificial seed technology. This method was
developed by Fabre and Dereuddre (1990) for Solanum and
consists of the inclusion of apices (explants) in alginate
beads (artificial seeds). Alginate beads can contain mineral
salts and organics. The procedure is continued by
subsequent culture in a highly concentrated sucrose solution
(0.7-1.5 M), followed by physical dehydration or air drying
to a moisture content of 20-30% and direct immersion in
LN. Culture of explants on sucrose enriched medium (0.3-
0.7%) prior to encapsulation, usually improves survival
after desiccation and freezing. In the other hand, the
presence of a nutritive matrix (the bead) surrounding the
explants can promote its regrowth after thawing. ED
method is simple, but more handling of alginate beads is
required and some species do not tolerate the high sucrose
concentrations employed. Physical desiccation is carried out
either with silica gel or in the air flow of the laminar flow
cabinet (Paulet et al., 1993). Studies of Reed et al. (2005) on
Cynodon revealed that the ED cryopreservation protocol
was most effective, especially when combined with a 1- to 4
week's cold-acclimation period and dehydration to 19 to
23% moisture, before exposure to LN. The moisture content
of Lilium ledebourii (Baker) Bioss. seeds before exposure to
LN were 15-20% (Kaviani et al., 2009). Lily [Lilium
ledebourii (Baker) Bioss.], Persian lilac (Melia azedarach
L.), and tea (Camellia sinensis L.) were evaluated for long-
term storage in LN. Encapsulation within alginate beads
was shown to be beneficial in all studied species (Kaviani et
al., 2010). Some secondary metabolites were applied as
cryoprotectant. Embryonic axes of Melia azedarach L.
encapsulated into calcium alginate beads with sucrose (0.75
M) and different concentration of salicylic acid were
subjected to cryopreservation. Salicylic acid significantly
enhanced the percentage of viability of encapsulated
embryonic axes (Bernard et al.,
2002;
Kaviani,
2007). The
ED method has been shown to be an effective approach to
Citrus somatic embryo cryopreservation. 100% survival
were obtained after the beads containing somatic embryos
had been pretreated on media with high sucrose
concentration, dehydrated below 25% moisture content and
direct immersed in LN (Duran-Vila, 1995).
Vitrification
For the last ten years, new plant cryopreservation methods
have been developed, which are based on vitrification
(Uragami et al., 1989). Vitrification (most widely applicable
plant cryopreservation method) can be defined as the
solidification of a liquid brought about not by crystallization
but by an extreme elevation in viscosity during cooling
(Fahy et al., 1984). Vitrification-based methods involve
removal of most or all freezable water by physical or
osmotic dehydration of explants, followed by ultra-rapid
freezing which results in vitrification of intracellular
solutes, i.e. formation of an amorphous glassy structure
without occurrence of ice crystals which are detrimental to
cellular structure integrity. The vitrification method is easy
to perform and often has a high recovery percentage, which
makes it widely applicable, particularly to the concentration
of plant species sensitive to low temperature (Takagi et al.,
1997; Thinh et al., 1999). Vitrification includes of putting
explants
in
a highly concentrated cryoprotective solution,
then frozen rapidly. Vitrification involves an increase in
cellular viscosity, thus it is important that plants are able to
withstand lethal osmotic and evaporative dehydration
stresses (Reed et al., 2005). This method has successfully
been applied to a broad range of plant materials from
various species, including complex organs like embryos and
shoot apices (Sakai, 1993; Huang et al., 1995; Takagi, 2000;
Vidal et al., 2005; Wang et al., 2005). In the vitrification
method, the plant material is exposed to highly concentrated
cryoprotectant solutions for variable periods of time (from
15 min up to 2 h), followed by a direct plunge into LN
(vitrification/one-step freezing). This results in both intra-
and extra-cellular vitrification. Previously, to induce
desiccation tolerance, tissues are cultured on medium with
high sucrose (0.3 M) or sorbitol (1.4 M) concentration and
subsequently transferred to a glycerol-sucrose solution,
called loading solution (2 M glycerol + 0.4 M sucrose)
(Sakai, 2000). Many reports have shown that
osmoprotection with 2 M glycerol and 0.4 M sucrose is
effective in enhancing the capacity of cells to tolerate serve
dehydration with PVS2 (Hirai and Sakai, 2003; Matsumoto
and Sakai, 2003; Kobayashi et al., 2006). A widely used
vitrification solution (mixture of penetrating and non-
penetrating cryoprotectant substances) is that developed by
Sakai et al. (1990) and named plant vitrification solution 2
(PVS2) which consists of 30% (w/v) glycerol, 15% (w/v)
ethylene glycol and 15% (w/v) DMSO in liquid medium
with 0.4 M sucrose. Cells are osmotically dehydrated by
786
PVS2 at a nonfreezing temperature (Sakai et al., 1990).
Vitrification requires less handling than ED, but the main
problem is the toxicity of the concentrated vitrification
solutions. This can be overcome by cold and sugar
hardening a loading phase and the application of the
vitrification solution at 0°C instead of at room temperature.
Vitrification method eliminates the need for controlled slow
freezing and permit tissues to be cryopreserved by direct
transfer to LN (Kohmura et al., 1992). The treatment of
vitrification solution subjects cells to osmotic stress, making
it very likely for some constituents to enter into cells and
resulting in toxicity (Matsumoto et al., 1994). Thus, careful
vitrification solution exposure is critical. Damage to plants
because of exposure to vitrification solution may be due to
chemical toxicity or osmotic stress (Sakai, 2000).
Therefore, delaying permeation of some components of
vitrification solution but allowing time for adequate
dehydration is critical (Charoensub et al., 1999; Tsukazaki
et al., 2000). Direct exposure of germplasm to PVS2
reduces viability
and
is
toxic. The stepwise increase in
PVS2 concentration reduces this toxic effect (Kobayashi et
al., 2006). Temperate species that accumulate sugars and
protective proteins during cold season are better able to
withstand as compared to tropical species (Chang et al.,
2000; Reed et al., 2005). Improving the tolerance of tropical
and warm-temperature plant germplasm to vitrification has
been implemented through the use of high-sucrose
pretreatments (Benson, 2004; Reed et al., 2004; Sakai,
2004). Substituting sucrose for cold-acclimation treatments
may be advantageous for species that are cold sensitive
(Niino and Sakai, 1992; Chang and Reed, 2000). The
traditional cryogenic technique (slow-cooling) and
innovative (vitrification/one-step freezing) methods have all
been successfully applied to Citrus cryopreservation (Wang
and Deng, 2004; De Carlo and Lambardi, 2005). Wang et
al. (2005) showed that the recovery percentage of six
cultivars' shoot tips of Carica papaya L. after vitrification
was between 48% and 93%. To determine the potential of
vitrification on freezing tolerance of date palm cultures
were exposed to a PVS2 for 20-100 min. The maximum rate
of survival was obtained with cultures exposed for 80 min at
C followed by 40 min at 25°C (Bekheet et al., 2007).
Studies of Sarkar and Naik (1998) on potato showed the
shoot tips precultured on medium containing 0.3 M sucrose
plus 0.2 M mannitol, and loaded with PVS2 for 30 min
followed by 15 min incubation in 60% PVS2 and 5 min
incubation in 100% PVS2 at 0°C resulted in up to 54%
survival after vitrification.
Other methods
Some other cryopreservation methods are the DMSO
droplet, droplet-vitrification, two-step vitrification, and
encapsulation- vitrification (Fig. 1). Nowadays, DNA
storage method also is using. The droplet method can be
considered a modification of the previous methods
(Schäfer-Menuhr et al., 1997). It was developed for potato
germplasm cryopreservation and consists on treating
germplasm in drops of a 10% DMSO solution placed on
aluminium foil strips, which are rapidly immersed in LN
(Schäfer-Menuhr et al., 1994; Mix-Wagner et al., 2003).
The term 'droplet' refers to droplets of cryoprotectant on an
aluminium foil, into with explants are placed for cooling
and rewarming. The original idea of using aluminium foils
came from Kartha et al. (1982), who cryopreserved cassava
shoot tips on foils in plastic Petri dishes using a two-step
cooling method. The foils make it easier to put a large
number of germplasm at once quickly into and out of LN.
Also, aluminium is a very good heat conductor important
for quick cooling and rewarming of samples (Schäfer-
Menuhr, 1996). Schäfer-Menuhr et al. (1994) used this idea
for a fast cooling method of potato shoot tips. This method
has been applied successfully to more than 150 varieties
(Schäfer-Menuhr et al.,
1996).
Droplet
freezing was
successfully performed with Chrysanthemum (Halmagyi et
al., 2004). Halmagyi et al. (2004) showed that the droplet
method, especially when combined with vitrification has the
considerable advantage than for the other methods. In recent
years the number of species cryopresereved using the
combined droplet vitrification method with rapid cooling
and rewarming is increasing (Gonzalez- Arnao et al., 2008).
Tokatli and Akdmir (2009) showed that the optimized
droplet vitrification protocol improved the mean
regeneration rates to more than 30% than ED on Fraser
photinia. Totally, droplet vitrification is now one of the
methods that receives much attention and is moreover on a
large scale applied to germplasm collections of for example
banana, potato and garlic (Panis et al., 2009). The method is
also now under investigation to be applied in collections
from other plant species such as sweet potato, taro and
pelargonium (Panis et al., 2009). In some cases, the DMSO
droplet method after the alternating temperature preculture
is applied. New experiments using alternating temperatures
(22/8°C day/night temperature, 8 h photoperiod, 7 days)
prior to cryopreservation showed improved regeneration for
potato shoot tips (Kaczmarczyk et al., 2008). Total
concentrations of soluble sugars (glucose, fructose and
sucrose) were higher for all accessions after the alternating
temperature preculture, which could be the reason for
improved cryopreservation results. Zhao et al. (2005)
demonstrated that pretreatment of potato plants at 10°C
resulted in improvement of cryopreservation results. Some
studies developed a new alternative vitrification solutions,
modified either from the original PVS2 vitrification
solution by increasing glycerol and sucrose and/or
decreasing DMSO and PEG concentration, or from the
original PVS3 by decreasing glycerol and sucrose
concentration (Kim et al., 2009). The application of these
vitrification solutions to some species in a droplet-
vitrification method revealed that PVS3 were superior to
PVS2 (Kim
et
al.,
2009; Gonlez-Arnao et al., 2009).
More recently, protocols combining the above techniques
have been developed and named encapsulation-vitrification
method (Sakai et al., 2000). Firstly, germplasms are
encapsulated and then submitted to vitrification, without
any further physical desiccation. A modified ED
cryopreservation protocol based on the replacement of cold
acclimation with high-sucrose pretreatment was assessed
for the long-term storage of Ribes germplasm (Reed et al.,
2005). In many temperate species, incubation of germplasm
at low temperature (generally 4°C to 10°C), for periods
ranging from days to weeks, increases survival after
freezing for both types of methods, classical and
new
(Reed,
1990; Wu et al., 1999). During cold acclimation cellular
changes such as numerous smaller vacuoles, more abundant
mitochondria and rough endoplasmic reticulum and
accumulation of certain proteins
occur.
DNA
and gene
banking are applied for especial genetic stocks and all
germplasm (Shikhamany, 2006). Cryopreservation and
DNA storage may provide long-term storage capabilities
(Ganeshan, 2006). Cryopreservation may be supplemented
by DNA storage systems for long-term conservation.
DNA
storage will be initially limited to gene constructs that code
for useful characteristics such as disease or insect resistance
787
(Ganeshan, 2006).
Conservation of ornamental plants
Some of ornamental plants are in danger of becoming
extinct. Efforts are being performed for conservation of
them. The conservation of ornamental germplasm can take
advantage of innovative techniques, which allow
preservation in vitro (slow growth storage) or in LN
(cryopreservation) of plant material. Slow growth storage
refers to the techniques enabling the in vitro conservation in
aseptic conditions by reducing markedly the frequency of
periodic subculturing without affecting the viability and
regrowth. Little coordinated effort
has
been
made up to date
to conserve and protect the genetic resources of ornamental
plants. One exception is the genebank of the "Ornamental
Plant Germplasm Center" in USA (Ozudogru et al., 2010).
Seed banks are a common way of conserving plant genetic
resources. In these, orthodox and sub-orthodox seeds are
stored at temperature of either -15°C to -20°C (cold tolerant
species) or 0 to -5°C (temperate and tropical species).
However, as the seeds do not represent the genetic profile of
the mother plant, this approach cannot be used when
endangered clonal germplasm is to be preserved, e.g., that
of ancient cultivars (Ozudogru et al., 2010).
For
vegetatively-propagated
species, the conservation of clonal
germplasm is made in field (clonal collections). However,
genotypes preserved only this way run the risks of biotic
and abiotic stresses. Today, important complementary
approaches to seedbanks and clonal orchards are offered by
biotechnology with the
possibility
to
preserve in vitro (slow
growth storage) or at -196°C (cryopreservation) plant
material.
Slow growth storage of ornamental plants
In this method, depending on the species, subculturing can
be decreased to once in every several months (sometimes
even to 1-2 years) and, because of this, the technique is
considered a "medium-term conservation" method. The
most widely used approach to slow growth storage of plant
material is the coupling of a low temperature (2-5°C for
temperate species and 15-25°C for tropical species) with the
culture in the dark or low light intensity (Reed, 1992;
Lambardi and De Carlo, 2003). Low temperature and light
intensity have physiological consequences, such as the
reduction of respiration, water loss, wilting, and ethylene
production, which allow for safe conservation. In addition
to cold storage, in vitro conservation can be achieved by
modifying the medium compositions, i.e., by 1. reducing the
sugar and/or mineral concentration, 2.
using
growth
retardants (e.g., chlorocholin chloride and ABA) or
osmotically active compounds (e.g., mannitol), and 3.
covering the explants with a layer of liquid medium or
mineral oil to reduce the oxygen available to
the
plants
(Withers and Engelmann, 1997). These methods can lead to
the appearance of somaclonal variations and thus are not so
advisable when true-to-type material is required (Ashmore,
1997). To date, only a
limited
number
of reports, such as
Camellia spp. (Ballester et al., 1997), Humulus spp. (Reed
et al., 2003), Nerium oleander and Photinia fraseri (Ozden-
Tokatli et al., 2008), Splachnum ampullaceum (Mallón et
al., 2007), and Rosa (Previati et al., 2008) deal with the in
vitro conservation of ornamental plants, all based on the
cold
storage
approach. Ornamental plants are generally
stored just a few degrees above the freezing (mainly, at 4-
C) (Ozudogru et al., 2010). Storage in total dark
conditions is preferred for a better slowing down of cell
metabolism; however, storage under low light intensity
showed to be effective for shoot cultures of Camellia
(Ballester et al., 1997), and Humulus spp. (Reed et al.,
2003). In general, maximum time of conservation of shoot
cultures
in
cold
conditions ranges from some months to 1
year. As for synthetic seeds of ornamentals, they have a
shorter time of conservability, ranging from 1.5 to 9 months
(Ozudogru et al., 2010). Two exceptions are Hibiscus
moscheutos (almost 20 months of storage) (West et al.,
2006) and Splachnum ampullaceum (30
months)
(Maln
et
al., 2007).
Cryopreservation of ornamental plants
In ornamental species, one-step freezing techniques (such as
vitrification, encapsulation-vitrification and ED) are widely
preferred than slow cooling (Ozudogru et al., 2010).
Examples are Chrysanthemum grandiflora (Halmagyi et al.,
2004), Humulus spp. (Reed et al., 2003), Acer mono (Park
et al., 2005), Gentian spp. (Tanaka et al., 2004), Dianthus
caryophyllus (Halmagyi and Deliu, 2007), and Ribes spp.
(Johnson et al., 2007). Protocorm-like bodies of Oncidium,
precultured with high concentrations of sucrose and
glycerol, maintained their cell shape and subcellular
components and remained intact after desiccation and
freezing, while the cellular structures of protocorm-like
bodies, which were not pre-cultured, were damaged (Miao
et al., 2005).
Halmagyi
and
Pinker (2006) tested four
different sugars or sugar alcohols (i.e., sucrose, glucose,
mannitol, and sorbitol) in preculture and observed that the
tolerance to freezing of rose shoot tips was highest when
sucrose was used. Pretreating explants at 10°C or below
(cold hardening) is another approach for inducing freezing
tolerance (Ozudogru et al., 2010). For example, nodal
segments of Chrysanthemum stored at 10°C and low light
intensity for 3 weeks, shoot cultures of Photinia fraseri
stored at 4°C in darkness for 2-3 weeks (Ozden-Tokatli et
al., 2008). Also in vitro-grown gentian plants stored up to
50 days at 5°C and low light intensity (Tanaka et al., 2004),
and embryogenic callus of Aesculus hippocastamum stored
at 4°C for 5 days in darkness (Lambardi et al., 2005), then
plunged in LN. The time of loading explants in PVS2, as
well as of dehydrating beads under air flow or on silica gel,
should be carefully determined for each species. In
ornamental plants, the PVS2 treatment ranges from 5 min
(shoot tips of Chrysanthemum grandiflora) (Halmagyi et
al., 2004) to 3 h (encapsulated shoot tips of Dianthus
caryophyllus) (Halmagyi and Deliu, 2007). Lynch et al.
(1996) reported that 2 h dehydration of encapsulated shoot
tips on silica gel was sufficient to induce 25%
of
germination
of Rosa multiflora after storage in LN.
Encapsulated shoot tips of Photinia fraseri needed to be
dehydrated 8 h under the sterile air of a laminar flow hood
to achieve tolerance to ultra-rapid freezing in LN (Ozden-
Tokatli et al., 2008). Warming temperatures ranging from
20°C (Johnson et al., 2007) to 45°C (Reed et al., 2003;
Lambardi et al., 2005) have been proposed for ornamental
species (Ozudogru et al., 2010). Several orchids were
cryopreserved using the ED and vitrification methods
(Lurswijidarus and Thammasiri, 2004; Thammasiri and
Soamkul, 2007). Some orchid seeds with moisture content
lower than 14% can be conserved in LN (Pritchard, 1995;
Wang et al., 1998). Halmagyi et al. (2004) reported deep-
freezing of shoot tips of Chrysanthemum morifolium by
different technical methods: controlled- rate-freezing, ED,
788
ultra-rapid-freezing by the droplet method and vitrification.
While vitrification yielded the highest shoot regeneration
rates, the droplet method was also successful.
Cryopreservation of Lilium ledebourii (Baker) Bioss.
germplasm by encapsulation-vitrification, and ED methods
as well using sucrose and dehydration as pretreatment was
performed (Kaviani et al., 2008; Kaviani et al., 2009;
Kaviani, 2010; Kaviani et al., 2010). Cryopreservation
using sucrose and dehydration showed that survival seeds of
lily after freezing were nil for control seeds and 75% for
seeds treated with 0.75 M sucrose and dehydration for 1 h
in laminar flow (Kaviani et al., 2009). Cryopreservation of
lily seeds by ED was revealed that survival rate was
nil
for
control seeds, 22% for seeds treated with sucrose 0.6 M
sucrose and dehydration for 1 h in laminar flow and up to
50% for seeds treated with 0.6 M sucrose and dehydration
for 1 h and encapsulation (Kaviani et al., 2008). Also,
studies on cryopreservation of lily germplasm (seeds,
embryonic axes, lateral buds and bulblets) demonstrated
about 10% of cryopreserved seeds and embryonic axes
pretreated with PVS2, sucrose (0.75M) and encapsulation
were able to sprouting, while there was no survival after LN
storage of seeds and embryonic axes pretreated with PVS2
and sucrose (0.75 M). None of lateral buds and bulblets
pretreated with sucrose (0.75 M) and encapsulation-
vitrification was survival after cryopreservation (Kaviani et
al., 2010). Our studies
showed
that
the best lily germplasm
is seed and the best pretreatments for survival of lily
germplasm after cryopreservation are 0.75 M sucrose and
dehydration for 1 h (Kaviani et al., 2008; 2009; 2010).
Evaluation of genetic stability and diversity after
cryopreservation
The risk of genetic instability has always been a cause of
alteration. In theory, metabolic activities at temperatures of
LN are reduced to zero, so that after rewarming from
cryopreservation, true-to-type plants are expected (Panis et
al., 2001). Cryopreserved tissue should be genetically
identical to non-treated phenotype and can directly produce
normal plants (Dumet and Benson, 2000). A large number
of reports showing no evidence of morphological,
cytological, biochemical, or molecular alterations in plants
from storage
at
-196°C (Harding, 2004). In the
cryopreservation process, some genomic alterations may be
induced, thus the determination of genetic integrity is
necessary after storage in LN (Ashmore, 1997). The ability
to identify genetic variation is important to effective
management and use of genetic resources. Genetic
instability and somaclonal variations may be caused some
differences in genotype and phenotype profiles of
cryopreserved plants (Harding, 2004). So, viability and
genetic stability are two important factors after
cryopreservation (Anand, 2006). Many cryopreservation
methods have caused analysis of genetic stability at
different levels, but the number of chromosome and its
morphology are primary cytogenetic parameters that must
remain stable after cryopreservation (Surenciski et al.,
2007). Alteration of ploidy level is one of the most frequent
genetic variations in in vitro systems (Larkin and
Scowcroft, 1981). Chromosomal instability is also
influenced by the genotype and tissue culture conditions
(Surenciski et al., 2007). Methods for assessment of plant
genetic diversity (traditionally toward advanced) are
summarized as follows: Phenotypic or morphological
characters Biochemical methods (protein and enzyme
electrophoresis) → Molecular methods (isozyme markers
and DNA-based methods such as RFLP, PCR, RAPD,
AFLP, SSR and microsatellite) (Karp et al., 1997; Rao,
2004; Harding, 2004). Traditionally, diversity is assessed by
phenotypic characters such as flower color, growth habitat
or quantitative agronomic traits like yield potential, stress
tolerance, etc (Rao, 2004). The recovered orchid plantlets
from cryopreservation by ED technique showed normal
growth characteristics (Lurswijid and Thammasiri, 2004).
Studies of Marassi et al. (2006) on rice showed 80% of the
seedlings developed into normal plants after being
transferred to greenhouse conditions. Histological
observations showed that the origin of the plants was not
modified by cryopreservation process. Studies of Kobayashi
et al. (2005) on cryopreservation of tobacco suspension cell
cultures revealed that there were no differences in the
morphology or growth profiles between cryopreserved cell
cultures and the original cell cultures. Shoot tips of Solanum
tuberosum L. exhibited normal developmental patterns after
regeneration from cryopreservation. Cytological studies
revealed that their ploidy status was constant and
chromosomal abnormalities were not observed (Benson et
al., 1996). After phenotypic characters, biochemical
methods based on seeds protein and enzyme electrophoresis
were introduced (Rao, 2004). Proteins are useful for genetic
study because they are the primarily products of structural
genes. Even change of single amino acid can be detected in
electrophoresis (Anand, 2006). Use of biochemical methods
eliminates the environmental influence; however, their
usefulness is limited due to their inability to
detect
low
levels of variation (Rao, 2004). Molecular markers have
increasingly been used to study genetic diversity from
natural populations, plant breeding, identify redundancies in
the collections, test accession stability and integrity, and
formulate efficient sampling strategies to obtain maximum
variation for genetic resources conservation (Rao, 2004).
Molecular techniques proved useful in a number of ways to
improve the conservation and management of plant genetic
resources (Rao, 2004). Molecular markers were used as
tools to fingerprint trifoliate orange and sunflower
germplasm accessions (Fang et al., 1997; Hongtrakul et al.,
1997). Variation within species has been studied to show
geographic or ecological patterns of distribution of diversity
in many crops such as banana (Pillay et al., 2001), sorghum
(Nkongolo and Nsapato, 2003), tea (Balasaravanan et al.,
2003) and sweet potato (Gichuki et al., 2003). Molecular
markers are being used to resolve problems of taxonomy
and phylogenetic relationships to determine of genomic
homologies in devising proper breeding strategies for
appropriate conservation and gene transfer (Ramanata Rao
and Riley, 1994; Rossetto et al., 2002; Rao, 2004).
Molecular markers can be used for characterization of
germplasm, varietal identification and clonal fidelity
testing, assessment of genetic diversity, validation of
genetic identification and marker-assisted
selection
(Anand,
2006). Enzymes encoded by different alleles at one or more
gene loci are known as allozymes and isozymes,
respectively. Isozyme markers were found to be more
specific than total protein patterns (Anand, 2006). A total of
about 90 isozyme systems have been used for plant
assessment (Paunesca, 2009). The advantages of this
technique are that is relatively simple and less expensive
(Anand, 2006). The main limitations of isozyme analysis
are the reduction of number of analyzed markers and the
phenotypical, developmental and seasonal dependence of
the markers (Paunesca, 2009). Examples of the use of
molecular markers to
study
genetic
diversity have been
789
reported in plants such as flax (van Treuren et al., 2001),
barley (Lund et al., 2003), wheat (Cao et al., 1998),
sorghum (Dean et al., 1999), and grapevine (Cervera et al.,
1998). DNA-based markers have been applied after enzyme
markers (Anand, 2006). DNA- based markers are derived
from the initial template DNA and provide the best
measure
of
genetic variation (Anand, 2006). DNA- based techniques
have potential to identify polymorphisms represented by
differences in DNA sequences in nuclear and organelles and
is not modified by environmental exposure. Also, the
analysis of DNA can be carried out at any time during plant
development and it may cover the entire genome (Paunesca,
2009). These techniques analyze the variation at the DNA
level that includes all environmental influences too (Rao,
2004). The analysis can be performed at any growth stage
using any plant part and it requires only small amounts of
material (Rao, 2004). DNA-based analysis was applied to
study the genetic stability of plant tissue culture due to its
high sensitivity and accuracy in detecting every single base
change (Shuji et al. 1992). Molecular methods, e.g., RFLP
(restriction fragment length polymorphism), PCR
(polymerase chain reaction), RAPD (random amplified
polymorphic DNA), AFLP (amplified fragment length
polymorphism), SSR (simple sequence repeats) or
microsatellite were used for detecting DNA sequence
variation are based on the use of restriction enzymes that
recognize and cut specific short sequences of DNA. RFLP
analysis of mitochondrial DNA was used for assessing
polygenetic relationship in Solanum spp. (Isshiki et al.,
2003). PCR and RFLP are applied to endangered species
(Chase and Fay, 1997). Microsatellites have been deployed
to establish the genetic stability of long-term maintained
germplasm (Paunesca, 2009). Microsatellites include low
copy regions of plant genomes (Morgante et al., 2002).
Microsatellite profiles in propagules of some potato
varieties were identical to those of the parental plants and
their progeny (Harding and Benson, 2001). Microsatellites
were applied to distinguish different cultivars of grapevine
(Thomas et al., 1994), and to compare landraces and
develop unique DNA profiles of soybean genotypes
(Rongwen et al., 1995). Microsatellites analysis
demonstrated high level of polymorphism within and
among Lycopersicon, which was correlated with cross-
pollinating (Alvarez et al., 2001). Tang and Knapp (2003)
performed phylogenetic analysis on cultivated and wild
germplasm accessions of Helianthus using microsatellite
loci, which revealed the possibility of multiple
domestication origin in Helianthus. As a complementary
method to molecular markers, flow cytometry is used to
detect the possible changes in ploidy levels and DNA
content (Fukai et al., 2002). In potato shoot tips using the
DMSO droplet method, genetic stability was confirmed
using morphological parameters, flow cytometry and RFLP
analysis (Schäfer-Menuhr et al., 1997). Storage of Solanum
tuberosum L. in LN for up to ten years was found to have
no adverse effect (somaclonal variation) on the regeneration
rates (Mix-Wagner et al., 2002; Keller et al., 2006). DNA
markers are extremely useful for testing clonal fidelity
(Anand, 2006), a key position in breeding strategies, genetic
engineering, genome mapping, DNA fingerprinting, genetic
variation, cultivar identification, characterization of
genotypes in plant germplasm collections, identification of
genetic contamination and quantification of genetic
drifts/shifts and taxonomic studies (Ananthanarayanan,
2006). Newer molecular techniques are permitting precise
and versatile analyses of genetic variation. But, it is difficult
to compare the different techniques and determine which
one is best and for what purpose because each method has
its advantages and disadvantages (Rao, 2004). The
appropriateness of individual marker systems varies
depending on the objective of study and the properties of
the species (Karp et al., 1997). Surenciski et al. (2007)
analyzed Cyrtopodium hatschbachii cytogenetically and
found that the cryopreserved encapsulated seeds were stable
at chromosome and phenotypic level. However, limited
condensation of the chromatin during the first stages of their
development was observed. In Swietenia macrophylla,
changes in chromatin conformation after recovery from LN
were observed, possibly due to changes in methylation
status of the DNA (Harding and Millam, 2000). Studies
confirmed the stability of the ribosomal RNA genes and the
nuclear-chloroplast DNA in potato plants regenerated from
cryopreserved shoot apices (Harding, 1991; Harding and
Benson, 2000). Xu et al. (2002) determined the variation in
chloroplast DNA
simple
sequence
repeats in wild and
cultivated soybean accessions, and indicated a higher
genetic diversity in the wild species. In apples, the
methylation status of DNA was altered after storage in LN
and the changes were accompanied by an increase in the
capacity of cryopreserved shoot tips for rooting (Hao et al.,
2001). DNA methylation patterns are stable and inherited,
resulting in the phenomenon of DNA imprinting (Shemer et
al., 1996). Methylation changes in genomic DNA after
cryopreservation were found in chrysanthemum shoots
(Martin and Gonzalez-Benito, 2006), potato (Harding,
1997), and almond leaves (Channuntapipat et al., 2003).
DNA methylation can play a role in somaclonal variation
(Kaeppler et al., 2000), but Channuntapipat et al. (2003) and
Harding (2004) suggest that these changes may not be
induced by cryopreservation but are the results of the whole
process of in vitro culture and regeneration. Studies of
Scocchi et al. (2004) on cryopreservation of apical
meristem-tips of Melia azedarach L. using
encapsulation/dehydration showed that this
cryopreservation treatment preserved genetic stability, when
it was evaluated using the electrophoresis pattern of nine
isozymes and RAPD bands. The regenerated plants
appeared morphologically similar to the control ones
(Scocchi et al., 2004). RAPD technique has been used to
study the genetic stability of cryopreserved tissue cultures
of date palm (Bekheet et al., 2007). According to RADP
analysis, plantlets derived from cryopreserved cultures were
identical to that derived from non-treated cultures and both
were similar with the field growth plants (Bekheet et
al.,
2007).
Codominant isozymic markers as well as dominant
RAPD markers are valuable tools to look for differences to
ensure genetic stability (De Loose and Gheysen, 1995).
Genetic variation within and between natural populations of
Digitalis obscura was quantified using RAPDs and the
results were used for optimizing sampling strategies for
conservation of genetic resources of the species (Nebauer et
al., 1999). RAPD analysis in Brassica oleracea
demonstrated that 14 phenotypical uniform accessions
could be reduced to 4 groups with minimal loss of genetic
variation (Phippen et al., 1997). RAPDs were used to
identify dwarf off-types arising from micropropagation of
banana cultivars and changes in genetic diversity following
regeneration of potato and wheat accessions (Damasco et
al., 1996; Del Rio et al., 1997; Börner et al., 2000). AFLP
marker analysis revealed genetic diversity within and
between some accessions such as sweet potato and Coffea
arabica (Fajardo et al., 2002; Steiger et al., 2002). AFLPs
were applied to validate taxonomic classification in wild
potato species (McGregor et al., 2002). New molecular
790
techniques detect variation at specific gene loci, permitting
precise and versatile analyses of genetic variation (Sicard et
al., 1999; Rao, 2004). PCR is too sensitive and pathogen
specific for explants health testing, which are applied for
elimination of systemic diseases for safe exchange of
germplasm (Rao, 2004).
Cryopreservation of organs and tissues of plants
All parts of plants may be conserved by cryopreservation.
Suspension or callus cultures, dormant buds, apical
meristem, embryonic axes, seeds, somatic embryos and
pollen are now stored in LN (Bell and Reed, 2002).
Suspension cells and calluses are often cryopreserved using
the classical slow-cooling method (0.5°C/min up to -40°C).
The main goal of cryopreserving these tissues is the
conservation of specific features that can be lost during in
vitro conditions (Panis and Lambardi, 2005).
Cryopreservation of cell suspensions and calluses is
reported for some species (Panis et al., 2004). Many
successful methods are based on the slow-cooling method
consisting in the pretreatment of embryogenic callus of
Citrus with cryoprotectants, mainly DMSO and sucrose
before being slowly cooled to -40°C and then immersed in
LN (De Carlo and Lambardi, 2005). The PVS2 was
developed working with embryogenic cells of Citrus spp.
(Sakai et al., 1990). Tobacco suspension cells were
successfully cryopreserved by a vitrification method
combined with an encapsulation technique. However, the
vitrification method was less effective for cryopreservation
than a simplified slow freezing method
(Kobayashi
et
al.,
2006). Pollen grains are cryopreserved for breeding
programs, distributing and exchanging germplasm among
locations, as well as for studies in some sciences such as
physiology, biotechnology and in vitro fertilization (Towill
and Waters, 2000). In Citrus, the survival of ovules to
cryopreservation has been shown to be very erratic, but the
pollen has been successfully stored in LN (De Carlo and
Lambardi, 2005). Storage of orthodox seeds in LN is an
alternative to the traditional storage at -20°C. For seed
cryopreservation, dehydration/one-step freezing methods
have been successfully applied to some species (Pence,
1995). Embryo axes and intact seeds have been shown to be
excellent explants for Citrus cryopreservation (De Carlo
and Lambardi, 2005). Meristematic tissues are the most
common explants for cryopreservation of vegetatively-
propagated species, such as fruit trees. Somaclonal variation
is lower in organized tissues like meristem than that of non-
organized tissues like callus and cell suspensions. ED and
encapsulation-vitrification methods are mainly used for
meristematic tissues (De Carlo and Lambardi, 2005; Panis
and Lambardi, 2005). Shoot tips or apical meristem are
widely used for cryopreservation of many species
(Lambardi et al., 2001). The explants above described are
mainly applied for herbaceous species. Three categories are
mainly used for
woody
plant cryopreservation; 1. shoot tips,
2. seeds or the isolated embryo axes, and 3. embryogenic
callus. Different methods have been used for the
cryopreservation of embryogenic calluses, shoot tips, ovules
and pollen, embryo axes, and seeds from a wide range of
Citrus species and cultivars (Pérez, 2000).
Recovery of cryopreserved germplasm
Long-term storage can take place in LN or in the vapor
phase. For the recovery of germplasm after cryostorage,
rapid rewarming is usually required to avoid
recrystallisation (Towill, 1991). Vials containing the
germplasm are usually immersed in a water bath at 35-
40°C. When germplasm are not included in vials, for
example in the droplet method, rewarming usually takes
place in liquid medium at room temperature (Mix-Wagner
et al., 2003). In many species, recovery of apices
cryopreserved with the new methods is direct, without
callus formation. By contrast with the classic
methods,
the
structural integrity of most cells is well preserved
(Engelmann, 1997a). Some studies have shown the
importance of the proper post-thawing culture conditions to
enhance organized growth. In many cases, selection of a
suitable growth medium for germplasm recovery is
necessary. Adjustment of growth regulator concentration
(Withers et al., 1988) or even medium salt formulation
(Pennycooke and Towill, 2001; Decruse and Seeni, 2002)
could be required for the normal development of frozen
germplasm. In some germplasm conservation centers, 20%
recovery is considered enough for long-term preservation
(Golmirzaie and Panta, 2000). Other authors consider that
survival should be higher than 40% (Reed, 2001). It is
important that those percentages be reproducible.
Conclusion
Biotechnology has created significant contributions to
improved conservation and use of plant
genetic
resources.
The rapid progress made in in vitro culture technology,
cryopreservation and molecular markers has helped in
improving the plant germplasm conservation and offer a
valuable alternative to plant
diversity
studies,
and
management of genetic resources. Cryopreservation has
proven to be an efficient long-term conservation method
for genetic resources. Nowadays, vitrification method is a
standardized method, although more studies should be
performed. Adjustments of the methods to the genebank
would be necessary to exploit all the advantages of
cryopreservation. The two most important factors that need
to be optimized are the preparation phase of tissues
towards dehydration (especially by sugar and cold
treatments) and the length of explants treatment with the
vitrification solution. Researches should move toward
standardizing
and
simplifying
the methods.
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