In 1965, French botanist George Morel was
attempting to obtain a virus-free orchid plant when he discovered that a millimetre-long
shoot could be developed into complete plantlets by micropropagation. This was the
beginning of tissue culture. Thereafter, in the 1970s developed countries began commercial
exploitation of this technology. It entered the developing world in the 1980s. It was
earlier used to develop ornamental plants and flowering plants for export. With tree
species, the technique of tissue culture remained confined for many years to the
laboratory stage and had generally invited only academic interest. But in most developing
countries, the shortage of biomass and the ever-increasing energy requirements created the
need to explore possibilities of mass propagation of trees by tissue culture.
Tissue culture or mass cloning methods of
elite tree species is done for increasing land productivity. They are being modified or
adapted for large-scale modification.
Species are selected for tissue culture on
the following basis.
 |
Species that have regeneration problems,
specially because of poor seed set or germination (as in Anogeissus and bamboo). In these
cases, seeds collected from superior trees are used for initiating cultures. |
|
Species that vary markedly in their
desirable traits, i.e. Eucalyptus. The selected trees are marked from the variant
population for the desirable trait such as disease resistance, straight bole, higher
productivity, etc. in consultation with officials from state forest department or growers. |
|
Species where plants of any one particular
sex is of commercial importance, for example female plants of papaya and male plants of
asparagus |
In tissue culture cells, tissues, and
organs of a plant are separated. These separated cells are grown especially in containers
with a nutrient media under controlled conditions of temperature and light. The cultured
plant requires a source of energy from sugar, salts, a few vitamins, amino acids, etc.
that are provided in the nutrient media. From these cultured parts, an embryo or a shoot
bud may develop, which then grows into a whole new plantlet. Similarly, portions of organs
or tissues can be cultured in a culture media. Generally, these give rise to an
unorganized mass of cells called callus (soft tissue that forms over a cut surface).
Tissue culture plantlets have poor
photosynthesis efficiency and lack the proper mechanism to control water loss. They need
to be hardened gradually by moving them along a humidity gradient in the greenhouse. Once
these plants are in the research fields, they are evaluated under field conditions and the
data is collected every 6 months. A large number of tissue culture plants that have grown
into trees are remarkably uniform and show an increase in biomass production over the
conventionally raised plants.
Figure Tissue culture and totipotency
Application of tissue culture
Micropropagation
Rapid vegetative multiplication of valuable plant material for agriculture, horticulture,
and forestry.
Production of disease-free plants
When the apex of shoot is used for multiplication by tissue culture, we get disease free
plants because the shoot apical meristem, a group of dividing cells at the tip of a stem
or root, is free from pathogens.
Plant breeding
Tissue culture has also been successfully used in plant breeding programmes.
Production of disease- and pest-resistant plants
Plants grown from tissue culture usually pass trough callus phase and show many
variations. These show some agronomic characteristics like tolerance to pests, diseases,
etc.
Cloning
Genetically identical plants derived from an individual are called clones. Processes that
produce clones can be put under the term ‘cloning’. This includes all the
methods of vegetative propagation such as cutting, layering, and grafting. Propagation by
tissue culture also helps in producing clones. Using the shoot tip, it is possible to
obtain a large number of plantlets. This technique is used extensively in the commercial
field for micropropagation of ornamental plants like chrysanthemum, gladiolus, etc. and
also crops such as sugar cane, tapioca, and potato. Thus an unlimited number of plants
that are genetically similar or are clones can be produced in a short span of time by
tissue culture.
Large-scale propagation
To bridge the gap between research and
application, the Department of Biotechnology, Government of India sponsored the setting-up
of two pilot-scale facilities for large-scale propagation of elite planting material of
forest trees through tissue culture. One of these facilities has been established at
TERI’s 36-hectare-campus in Gual Pahari, Haryana with an annual capacity of a million
plantlets. Research at these facilities focuses exclusively on developing new protocols
for mass cloning of elite planting material, mainly of trees.
Till date, over 4 million plants have been
dispatched for field plantation from these facilities. The tissue culture raised plants
are presently being evaluated under field conditions. This is being done in tandem with
the forest departments of Haryana, Uttar Pradesh, Madhya Pradesh, Bihar, Jammu and
Kashmir, and Orissa. For initial screening for phenotypically superior trees only a few
hundred plantlets of the same are raised and tested under various agroclimatic zones. The
best clones are then mass multiplied and monitored regularly for their performance. Field
data suggest a survival percentage of more than 90% even in the harsh conditions of
Aravalis without the life-saving irrigation. At half the rotation age some of the selected
clones of Eucalyptus are showing a significant increase in productivity as compared to the
conventional seed raised progenies.
This Division contains some organisms which are free-living, others which are
Many green algae, especially the multicellular ones, have both sexual
and asexual stages in their life cycles, thus we must re-introduce the
idea of Alternation of Generations we discussed along with
meiosis. When we first discussed Alternation
of Generations, we looked at a very simple diagram in which adults
produced 1n gametes by meiosis, and those gametes joined by syngamy to
form a new 2n generation. In reality in algae and plants, there are a
few more stages in the process, thus we
now need to re-visit this cycle. The 2n generation, which in humans is
called an “adult,” in algae and plants is called a
