Understanding Natural and Artificial Cloning in Plants

Asexual reproduction is a form of cloning and it results in offspring produced by mitosis known as clones.

Clones are usually genetically identical to both the parent organism and to each other.

Natural cloning

Vegetative propagation, or natural cloning, occurs in many species of flowering plants.

A structure forms which develops into a fully differentiated new plant, which is genetically identical to the parent.

The new plant may be propagated from the stem, leaf, bud, or root of the parent, depending on the type of plant, and it eventually becomes independent from its parent, for example, strawberries and spider plants.

Vegetative propagation often involves perennating organs, which enables plants to survive adverse conditions.

These contain stored food from photosynthesis and can remain dormant in the soil.

They are often not only a means of asexual reproduction but also a way of surviving from one growing season to the next.

Natural plant cloning occurs in:

• Bulbs, for example, daffodils. The leaf bases swell with stored food from photosynthesis. Buds form internally which develop into new shoots and new plants in the next growing season.
• Runners, for example, a strawberry or spider plant. A lateral stem grows away from the parent plant and roots develop where the runner touches the ground. A new plant develops – the runner eventually withers away leaving the new individual independent.
• Rhizomes, for example, marram grass. A rhizome is a specialised horizontal stem running underground, often swollen with stored food. Buds develop and form new vertical shoots which become independent plants.
• Stem tubers, for example, potato. The tip of an underground stem becomes swollen with stored food to form a tuber or storage organ. Buds on the storage organ develop to produce new shoots (e.g., the ‘eyes’ on a potato).

Using natural clones in horticulture

Natural plant cloning is exploited in horticulture by farmers and gardeners to produce new plants.

Splitting up bulbs, removing young plants from runners, and cutting up rhizomes all increase plant numbers cheaply, and the new plants have exactly the same genetic characteristics as their parents.

It is also possible to take cuttings of many plants short sections of stems are taken and planted either directly in the ground (e.g., sugar cane) or in pots, for example, pelargoniums.

Rooting hormone is often applied to the base of a cutting to encourage the growth of new roots.

Propagation from cuttings has several advantages over using seeds.

• It is much easier the lime from planting to cropping is much reduced.
• It also guarantees the quality of the plants.
• By taking cuttings from good slock, the offspring will be genetically identical and will therefore crop well.

The main disadvantage is the lack of genetic variation in the offspring should any new disease or pest appear or if climate change occurs.

Many of the world’s most important food crops are propagated by cloning.

Bananas, sugar cane, sweet potatoes, and cassava are all propagated from stent cuttings or rhizomes.

Coffee and tea bushes are also propagated from stem cuttings.

Cloning sugar cane

Sugar cane is an internationally important crop used to make sugar and manufacture biofuels.

It is one of the fastest-growing crop plants in the world the stems can grow 4-5 meters in 11 months if conditions are good and it is usually propagated by cloning.

Short lengths of cane about 30cm long, with three nodes, are cut and buried in a clear field in shallow trenches, covered with a thin layer of soil.

Per hectare, 10-25 000 lengths of stem are planted.

Artificial cloning

People have propagated plants by cloning for centuries, but there is a limit to how many ‘natural’ clones you can make from one plant.

Many plant cells are totipotent; they can differentiate into all of the different types of cells in the plant.

Scientists have developed ways of using this property to produce huge numbers of identical clones from one desirable plant.

Micropropagation using tissue culture

Micropropagation is the process of making large numbers of genetically identical offspring from a single-parent plant using tissue culture techniques.

This is used to produce plants when a desirable plant:

• does not readily produce seeds
• doesn’t respond well to natural cloning
• is very rare
• has been genetically modified or selectively bred with difficulty
• is required to be ‘pathogen-free’ by growers, for example, strawberries, bananas, and potatoes.

There are a number of ways in which plants can be micropropagated.

One protocol, based on work done at the Royal Botanic Garden at Kew, uses sodium dichloroisocyanurate, the sterilizing tablets used to make emergency drinking water and babies’ bottles safe.

This keeps the plant tissues sterile without being in a sterile lab so it is extremely useful for scientists in the field working with rare and endangered plant material and also for use in schools.

Other protocols are more suited to industrial micropropagation where large sterilizing units are available.

The basic principles of micropropagation and tissue culture are as follows:

• Take a small sample of tissue from the plant you want to clone the meristem tissue from shoot tips and axial buds is often dissected out in sterile conditions to avoid contamination by fungi and bacteria. This tissue is usually virus-free.
• The sample is sterilized, usually by immersing it in sterilizing agents such as bleach, ethanol, or sodium dichloroisocyanurate. The latter does not need to be rinsed off which means the tissue is more likely to remain sterile. The material removed from the plant is called the explant.
• The explain is placed in a sterile culture medium containing a balance of plant hormones (including auxins and cytokinins) which stimulate mitosis. The cells proliferate, forming a mass of identical cells known as a callus.
• The callus is divided up and individual cells or clumps from the callus are transferred to a new culture medium containing a different mixture of hormones and nutrients which stimulates the development of tiny, genetically identical plantlets.
• The plantlets are potted into compost where they grow into small plants.
• The young plants are planted out to grow and produce a crop. The scale of micropropagation is increasing. It now takes place in bioreactors, effectively making artificial embryo plants to be packaged in artificial seeds.

Advantages and disadvantages of micropropagation

The number of common plants that are largely produced by micropropagation is growing constantly and includes potatoes, sugar cane, bananas, cassava, strawberries, grapes, chrysanthemums, Douglas firs, and orchids.

Here are some of the points both for and against this process

Arguments for micropropagation

• Micropropagation allows for the rapid production of large numbers of plants with known genetic make-up which will yield good crops.
• Culturing meristem tissue produces disease-free plants.
• It makes it possible to produce viable numbers of plants after genetic modification of plant cells.
• It provides a way of producing very large numbers of new plants which are seedless and therefore sterile to meet consumer tastes (e.g., bananas and grapes).
• It provides a way of growing plants which are naturally relatively infertile or difficult to grow from seed (e.g., orchids).
• It provides a way of reliably increasing the numbers of rare or endangered plants.

Arguments against micropropagation

• It produces a monoculture – many plants that are genetically identical – so they are all susceptible to the same diseases or changes in growing conditions.
• Il is a relatively expensive process and requires skilled workers.
• The explants and plantlets are vulnerable to infection by moulds and other diseases during the production process.
• If the source material is infected with a virus, all of the clones will also be infected.
• In some cases, large numbers of new plants are lost during the process.

Banana and cloning

Bananas are now thought to be one of the oldest crops and possibly the first to be cloned.

A wild banana is full of hard seeds and it is virtually inedible.

A mutation made them parthenocarpic which means they produce fruit without fertile seeds which made them good to eat but also made them sterile.

Scientists therefore think that since the dawn of agriculture, people cloned bananas using natural asexual reproduction to propagate the plants producing the seedless, tasty fruit.

Sweet bananas are widely eaten in more economically developed countries, whilst plantains (cooking bananas) are a staple food in many less economically developed countries.

In the early 20th century almost all of the sweet bananas eaten were the cultivar Gros Michel.

Then fungal Panama disease wiped them out in the major banana-growing countries none of the clones had any resistance and a new cultivar took over.

Cavendish bananas, while apparently not as tasty as Gros Michel bananas, are resistant to Panama disease.

But Cavendish bananas are also clones.

Now another banana disease, Black Sigatoka, is destroying Cavendish plantations and is also spreading to other cooking varieties of bananas.

New biotechnologies for example genetic engineering and micropropagation offer hope for the future.

Genetically engineered strains of bananas with resistance genes from the original wild fruit could be micropropagated and used to restock banana plantains across the whole growing region.

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