35.6:
Plant Tissue Culture
Traditionally, farmers breed crops over many generations to produce desired qualities, such as higher yields, larger seeds, sweeter fruit, and greater disease resistance. New crop varieties can be created by grafting or cross-pollinating the parent plants. However, these methods are time-consuming and non-specific, with a high chance of eliminating desired traits.
Alternatively, the use of in vitro techniques known as plant tissue culture can rapidly produce large numbers of plant clones with desired qualities.
This technique relies on ‘totipotency’, which means that a part of a plant can give rise to an entire plant when provided with the appropriate growth regulators and environment.
Depending on the source of the plant part, or explant, tissue culture techniques can be distinguished into several types.
For example, in callus culture, the tissue explants placed on growth medium undergo rapid cell division and form a mass of undifferentiated cells called a callus.
Plant hormones, such as auxins and cytokinins, regulate several aspects of plant growth and help the callus differentiate into different plant parts. Shoots can be induced from a callus with a high concentration of cytokinins relative to auxins, while roots can be formed when the ratio is reversed. At an equal hormone concentration, both root and shoot form to regenerate the young plant.
Viruses present a major concern in agriculture because they can cause serious plant diseases. Meristem culture is particularly useful for establishing virus-free plants. This method relies on the rapidly dividing shoot tip as the explant of choice. Such shoot tips are generally free of viruses.
Plant tissue culture not only enables the cloning of desirable plants, but also facilitates genetic modification. One kind of modification involves introducing beneficial genes that might enhance the health, size, or overall yield of the plant. Such modifications are inherited by subsequent generations and produce the desirable traits without the need of additional tissue culture.
For example, genetically modified, or GM, corn with an herbicide tolerance gene has been widely adopted by farmers in the US. This modification allows crops to be sprayed with herbicide, killing competitor weeds and leaving the crop unaffected.
35.6:
Plant Tissue Culture
Plant tissue culture is widely used in both primary and applied science. Applications range from plant development studies to functional gene studies, crop improvement, commercial micropropagation, virus elimination, and conservation of rare species.
Plant tissue culture depends on the ability of plant tissue to give rise to an entire new plant when provided with a growth medium and appropriate environment. This ability of plant cells or tissues is termed ‘totipotency.’
The fundamental steps of plant tissue culture are fourfold:
- Select a healthy parent plant (explant).
- Eliminate any microbial contamination from any exposed explant surfaces.
- Inoculation the explant in an adequate culture medium.
- Incubation of the explant in a controlled environment with appropriate temperature, humidity, air quality, and illumination.
There are also four different types of plant tissue culture, which may be chosen based upon the goals of the culture, or plant species:
- cell culture (such as gametic cells, cell suspension, and protoplast culture).
- tissue culture (callus and differentiated tissues).
- organ culture (any organs such as roots, shoots, and anthers).
One of the popular applications of plant tissue culture is the in vitro clonal propagation – also known as micropropagation. Plant tissue culture, in this case, can aid in the reproduction of plants that have problems with seed germination (recalcitrant plants), or have short-seed viability. Although micropropagation can be applied to any species, it is recommended for commercially essential plants or those at risk of extinction. For example, micropropagation is widely used for the cultivation of orchids such as Paphiopedilum delenatii – a species native to the Himalayas. These orchids are traditionally propagated through seeds. However, it takes about 2-3 years for mature orchids to produce these. Therefore, plant tissue culture has become an ideal method to protect this species from extinction and achieve commercial viability.
Micropropagation can be carried out in three different ways:
- using an apical or axillary bud.
- using the adventitious bud.
- through the formation of a somatic embryo, using somatic parent plant cells.
The success of each of the techniques depends on the genetic background, culture media, and incubation conditions.