Plant tissue culture comprises a set of in vitro
techniques, methods and strategies that are part of the group of technologies
called plant biotechnology. Tissue culture has been exploited to create genetic
variability from which crop plants can be improved, to improve the state of
health of the planted material and to increase the number of desirable
germplasms available to the plant breeder. Tissue culture protocols are available
for most crop species, although continued optimization is still required for
many crops, especially cereals and woody plants. Tissue culture techniques, in
combination with molecular techniques, have been successfully used to
incorporate specific traits through gene transfer. In vitro techniques for the
culture of protoplasts, anthers, microspores, ovules and embryos have been used
to create new genetic variation in the breeding lines, often via haploid
production. Cell culture has also produced somaclonal and gametoclonal variants
with crop improvement potential. The culture of single cells and meristems can
be effectively used to eradicate pathogens from planting material and thereby dramatically
improve the yield of established cultivars. Large scale micropropagation laboratories
are providing millions of plants for the commercial ornamental market and the
agricultural, clonally propagated crop market. With selected laboratory
material typically taking one or two decades to reach the commercial market
through plant breeding, this technology can be expected to have an ever
increasing impact on crop improvement as we approach the new millenium.
Tissue culture techniques are part of a large group
of strategies and technologies, ranging through molecular genetics, recombinant
DNA studies, genome characterization, gene transfer techniques, aseptic growth
of cells, tissues, organs, and in vitro regeneration of plants, that are considered
to be plant biotechnologies. The products of plant breeding and the
fermentation industries (e.g. cheese, wine and beer), for example, have been
exploited for many centuries. We no longer have to rely on pollination and cross-fertilization
as the only ways to genetically modify plants. That the newer molecular and
cellular technologies have yet to make a broad based significant impact on crop
production is not surpris- ing since a plant breeding process of 10 to 20 years
duration is still required to refine a selected plant to the stage of cultivar
release.
The applications of various tissue culture approaches
to crop improvement are;
·
Breeding & biotechnology,
·
wide hybridization,
·
Haploidy,
·
Somaclonal variation
·
Micropropagation,
·
Synthetic seed,
·
Pathogen eradiction,
·
Germplasm preservation.
Plant Breeding and Biotechnology
Plant breeding can be conveniently separated into
two activities: manipulating genetic variability and plant evaluation.
Historically, selection of plants was made by simply harvesting the seeds from
those plants that performed best in the field. Controlled pollination of plants
led to the realization that specific crosses could result in a new generation
that performed better in the field than either of the parents or the progeny of
subsequent generations, i.e. the expression of heterosis through hybrid vigour
was observed. Because one of the two major activities in plant breeding is manipulating
genetic variability, a key prerequisite to successful plant breeding is the
availability of genetic diversity. It is in this area, creating genetic diversity
and manipulating genetic variability, that biotechnology (including tissue-culture
techniques) is having its most significant impact.
In spite of
the general lack of integration of most plant biotechnology and plant breeding programmes,
field trials of transgenic plants have recently become much more common. There
are therefore reasons to believe that we are on the verge of the revolution, in
terms of the types and genetic makeup of our crops, that has been predicted for
more than a decade.More than 50 different plant species have already been genetically
modified, either by vector dependent (e.g. Agrobacterium) or vector independent
(e.g. biolistic, micro-injection and liposome) methods. In almost all cases,
some type of tissue culture technology has been used to recover the modified
cells or tissues. In fact, tissue culture techniques have played a major role
in the development of plant genetic engineering. Tissue culture will continue
to play a key role in the genetic engineering process for the foreseeable
future, especially in efficient gene transfer and transgenic plant recovery.
Wide Hybridization
A
critical requirement for crop improvement is the introduction of new genetic
material into the cultivated lines of interest, whether via single genes,
through genetic engineering, or multiple genes, through conventional
hybridization or tissue culture techniques. During fertilization in angiosperms,
pollen grains must reach the stigma of the host plant, germinate and produce a
pollen tube. The pollen tube must penetrate the stigma and style and reach the ovule.
The discharge of sperm within the female gametophyte triggers syngamy and the
two sperm nuclei must then fuse with their respective partners. The egg nucleus
and fusion nucleus then form a developing embryo and the nutritional endosperm,
respectively. This process can be blocked at any number of stages, resulting in
a functional barrier to hybridization and the blockage of gene transfer between
the two plants. Pre-zygotic barriers to hybridization (those occurring prior to
fertilization), such as the failure of pollen to germinate or poor pollen tube
growth, may be overcome using in vitro fertilization. Post-zygotic barriers (occurring
after fertilization), such as lack of endosperm development, may be overcome by
embryo, ovule or pod culture. Where fertilization cannot be induced by in vitro
treatments, protoplast fusion has been successful in producing the desired
hybrids. To overcome this problem we do;
·
In vitro Fertilization
·
Embryo Culture
·
Protoplast Fusion
In
vitro Fertilization
IVF has been used to facilitate both interspecific
and intergeneric crosses, to overcome physiological based self incompatibility
and to produce hybrids. A wide range of plant species has been recovered
through IVF via pollination of pistils and self and cross-pollination of ovules.
This range includes agricultural crops, such as tobacco, clover, com, rice,
cole, canola, poppy and cotton. The use of delayed pollination, distant
hydridization, pollination with abortive or irradiated pollen, and physical and
chemical treatment of the host ovary have been used to induce haploidy.
Embryo
Culture
The most common reason for post-zygotic failure of
wide hybridization is embryo abortion due to poor endosperm development. Embryo
culture has been successful in overcoming this major barrier as well as solving
the problems of low seed set, seed dormancy, slow seed germination, inducing
embryo growth in the absence of a symbiotic partner, and the production of
monoploids of barley. The breeding cycle of Iris was shortened from 2 to 3
years to a few months by employing embryo rescue technology. A similar approach
has worked with orchids and roses and is being applied to banana and Colocasia.
Interspecific and intergeneric hybrids of a number of agriculturally important crops
have been successfully produced, including cotton, barley, tomato, rice, jute, Hordeum X Secale,Triticum x Secale,Tripsacumx
lea and some Brassicas. At least seven Canadian barley cultivars (Mingo, Rodeo,
Craig, Winthrop, Lester and TB891-6) have been produced out of material
selected from doubled haploids originating through the widely used bulbosum
method of cross pol1ination and embryo rescue. Mingo, in particular, was a
breakthrough, as it was the first barley cultivar produced by this technique to
be licensed, in 1980.
Protoplast
Fusion
Protoplast
fusion has often been suggested as a means of developing unique hybrid plants
which cannot be produced by conventional sexual hybridization. Protoplasts can
be produced from many plants, including most crop species. However, while any
two plant protoplasts can be fused by chemical or physical means, production of
unique somatic hybrid plants is limited by the ability to regenerate the fused
product and sterility in the interspecific hybrids rather than the production
of protoplasts. Perhaps the best example of the use of protoplasts to improve
crop production is that of Nicotiana, where the somatic hybrid products of a
chemical fusion of protoplasts have been used to modify the alkaloid and
disease-resistant traits of commercial tobacco cultivars. Somatic hybrids were
produced by fusing protoplasts, using a calcium-polyethylene glycol treatment,
from a cell suspension of chlorophyll deficient N. rusfica with an albino mutant of N. tabacum.
Protoplast fusion should focus on four areas:
- Agriculturally
important traits.
- Achieving
combinations that can only be accomplished by protoplast fusion
- A
somatic hybrids integrated into a conventional breeding programme and
- The
extension of protoplast regeneration to a wider range of crop species.
Protoplast
fusion products are presently grown on approximately 42% of the fluecured
tobacco acreage in Ontario, Canada. This represents a value of approx.
US$199,000,000.
Haploids
Haploid plants have the gametophytic (one-half of
the normal) number of chromosomes. They are of interest to plant breeders because
they allow the expression of simple recessive genetic traits or mutated
recessive genes and because doubled haploids can be used immediately as
homozygous breeding lines. The efficiency in producing homozygous breeding
lines via doubled in vitro-produced haploids represents significant savings in
both time and cost compared with other methods. Three in vitro methods have
been used to generate haploids:
- Culture
of excised ovaries and ovules.
- The
bulbosum technique of embryo culture.
- Culture
of excised anthers and pollen.
At
least 171 plant species have been used to produce haploid plants by pollen,
microspore and anther culture. These include cereals (barley, maize, rice, rye,
triticale and wheat), forage crops (alfalfa and clover), fruits (grape and
strawberry), medicinal plants (Digitalis and Hyoscyamus),ornamentals
(Gerberaand sunflower), oil seeds (canola and rape), trees (apple, litchi, poplar
and rubber), plantation crops (cotton, sugar cane and tobacco), and vegetable
crops (asparagus, brussels sprouts, cabbage, carrot, pepper, potato, sugar
beet, sweet potato, tomato and wing bean).
Somaclonal Variation
In addition to the variants/mutants (cell lines and
plants) obtained as a result of the application of a selective agent in the
presence or absence of a mutagen, many variants have been obtained through the
tissue culture cycle itself. These somaclonal variants, which are dependent on
the natural variation in a population of cells, may be genetic or epigenetic,
and are usually observed in the regenerated plantlets. Somaclonal variation itself
does not appear to be a simple phenomenon, and may reflect pre-existing cellular
genetic differences or tissue culture induced variability. The variation may be
generated through;
- several
types of nuclear chromosomal re-arrangements and losses,
- gene
amplification or de-amplification:
- non-reciprocal
mitotic recombination events,
- transposable
element activation,
- apparent
point mutations,
- reactivation
of silent genes in multigene families,
- alterations
in maternally inherited characteristics.
Many of the changes observed in plants regenerated
in vitro have potential agricultural and horticultural significance. These
include alterations in plant pigmentation, seed yield, plant vigour and size,
leaf and flower morphology, essential oils, fruit solids, and disease tolerance
or resistance. Such variations have been observed in many crops, including
wheat, triticale, rice, oats, maize, sugar cane, alfalfa, tobacco, tomato,
potato, oilseed rape and celery. The same types of variation obtained from
somatic cells and protoplasts can also be obtained from gametic tissue. One of
the major potential benefits of somaclonal variation is the creation of additional
genetic variability in coadapted, agronomically useful cultivars, without the
need to resort to hybridization. This method could be valuable if selection is
possible in vitro, or if rapid plant screening methods are available. It is
believed that somaclonal variants can be enhanced for some characters during
culture in vitro, including resistance to disease pathotoxins and herbicides
and tolerance to environmental or chemical stress. However, at present few
cultivars of any agronomically important crop have been produced through the
exploitation of somaclonal variation.
Micropropagation
During the last 30 years it has become possible to
regenerate plantlets from explants and/or callus from all types of plants. As a
result, laboratory scale micropropagation protocols are available for a wide
range of species and at present micropropagation is the widest use of plant
tissue culture technology. Along with the impressive successes there are
several limiting factors to its use. The cost of the labour needed to transfer
tissue repeatedly between vessels and the need for asepsis can account for up
to 70% of the production costs of micropropagation. Problems of vitrification,
acclimatization and contamination can cause great losses in a tissue-culture
laboratory. Genetic variations in cultured lines, such as polyploidy,
aneuploidy and mutations, have been reported in several systems and resulted in
the loss of desirable economic traits in the tissue-cultured products.
There
are three methods used for micropropagation:
- Enhancing
axillary bud breaking,
- production
of adventitious buds,
In the latter two methods, organized
structures arise directly on the explants or indirectly from callus. Axillary bud
breaking produces the least number of plantlets, as the number of shoots
produced is controlled by the number of axillary buds cultured, but remains the
most widely used method in commercial micropropagation and produces the most
true-to-type plantlets. Adventitious budding has a greater potential for
producing plantlets, as bud primordia may be formed on any part of the
inoculum. Unfortunately, somatic embryogenesis, which has the potential of
producing the largest number of plantlets, can only presently be induced in a
few species. Nevertheless, the production of somatic embryos from cell cultures
presents opportu- nities not available to plantlets regenerated by the organogenic
routes, such as mechanization. One approach envisages the use of bioreactors
for large-scale production of somatic embryos and their delivery in the form of
seed tapes or artificial seeds. No commercial operation based on somatic
embryogenesis exists but such embryogenesis is playing an important role in
improving herbaceous dicots, herbaceous
monocots and woody plants.
Synthetic Seed
A synthetic or artificial seed has been defined as a
somatic embryo encapsulated inside a coating and is considered to be analogous
to a zygotic seed. There are several different types of synthetic seed:
- Somatic
embryos encapsulated in a water gel,
- Dried
and coated somatic embryos,
- Dried
and uncoated somatic embryos,
- Somatic
embryos suspended in a fluid carrier,
- Shoot
buds encapsulated in a water gel.
The use of synthetic seeds as an improvement
on more traditional micropropagation protocols in facultatively propagated
crops may, in the long term, have a cost saving, as the labour intensive step
of transferring plants from in vitro to soil/field conditions may be overcome.
Other applications include the maintenance of male sterile lines, the maintenance
of parental lines for hybridcrop production, and the preservation and
multiplication of elite genotypes of woody plants that have long juvenile developmental
phases. However, before the widespread application of this technology, somaclonal
variation will have to be minimized, large scale production of high quality
embryos must be perfected in the species of interest, and the protocols will
have to be made cost effective compared with existing seed or micropropagation
technologies.
Pathogen Eradication
Crop plants, especially vegetatively propagated
varieties, are generally infected with pathogens. Strawberry plants, for
example, are susceptible to over 60 viruses and mycoplasms and this often
necessitates the yearly replacement of mother plants. In many cases, although
the presence of viruses or other pathogens may not be obvious, yield or quality
may be substantially reduced as a result of the infection. In China, for example,
virus-free potatoes, produced by culture in vitro, gave higher yields than the
normal field plants, with increases up to 150%. As only about 10% of viruses
are transmitted through seeds, careful propagation from seed can eliminate most
viruses from plant material. Fortunately, the distribution of viruses in a plant
is not uniform and the apical meristems either have a very low incidence of
virus or are virus free. The excision and culture of apical meristems (the
meristem with one to three of the subjacent leaf primordia), coupled with therm
or chemtherapy, have been successfully employed to produce virus free and
gener- ally pathogen-free material for micropropagation.
Germplasm Preservation
One way of conserving germplasm, an alternative to
seed banks and especially to field collections of clonally propagated crops, is
in vitro storage under slow growth conditions (at low temperature and/or with
growth retarding compounds in the medium) or cryopreservation or as desiccated
synthetic seed. The technologies are all directed towards reducing or stopping
growth and metabolic activity. Techniques have been developed for a wide range
of plants. The most serious limitations are a lack of a common method suitable
for all species and genotypes, the high costs and the possibility of somaclonal
variation and non intentional cell type selection in the stored material (e.g.
aneuploidy due to cell division at low temperatures or non-optimal conditions
giving one cell type a selective growth advantage).
Plant tissue culture technology is playing an
increasingly important role in basic and applied studies, including crop improvement.
In modern agriculture, only about 150 plant species are extensively cultivated.
Many of these are reaching the limits of their improvement by traditional methods.
The application of tissue culture technology, as a central tool or as an
adjunct to other methods, including recombinant DNA techniques, is at the
vanguard in plant modification and improvement for agriculture, horticulture and
forestry.