Endless possibilities – plant tissue culture (2)

Plant tissue culture has gained phenomenal success in applications for fast multiplication, but the possibilities are far more than that. Unlike the conventional methods, the in-vitro method involves the usage of artificial nutrient media, plant growth regulators, and well-controlled light, air, and temperatures, all together allow tuning of the inner physiology and resetting of the expression pattern of genes that are responsible for growth and development. Tissue-cultured plants grow much faster at an early stage as compared to the propagules that are subject to numerous uncertainties in the open field. More importantly, the physiological processes that normally are strictly controlled by environmental cues and inner clocks could be broken through and utilized to promote early flowering and break dormancy. It takes an average of 60 or up to 125 years for bamboo to bloom, however, artificial blooming could be achieved in a few months in a test tube. An Apple tree won’t be flowering without a long dormancy period but that could be shortened by using hormones in tissue-cultured conditions.

Another advantage is that the disease-free plantlets can be imported or exported among different countries without the prolonged and tedious phytosanitary process. The virus eradication is achieved through meristem culture or the combination of hot and cold treatment with cryopreservation, the harmful pathogens are eliminated by promoting the growth condition in favor of healthy tissues. Also because there is no messy soil involved in tubes or containers, the plants are preserved in aseptic media with stable humidity and nutrients, which allows transportations from a country to another country easily. Without worrying about the pathogens that may be destructive for native species of the importing country, plant breeders all over the world gain wider access to a broad pool of germplasm for crop improvement and diversification. A temperature country is able to grow tropical crops in a greenhouse that mimics the tropical weather. As the global climate likely to shift to warmer conditions with elevated CO2 level in the air, insects, bacteria, fungi, and virus are expected to be more active, therefore, the reliable supply of disease-free plants are particularly important.

There are two major regeneration pathways, organogenesis, and somatic embryogenesis. Organogenesis is the formation of organs that forms true-to-type due to the totipotency. Totipotency is the ability of an explant to regenerate into the mother plant. Somatic embryogenesis is different from zygotic embryogenesis as it is not the product of the fusion of two gametes. It occurred naturally on succulent plants and in tissue culture conditions, whereby it can be further categorized into direct and indirect organogenesis. The direct regeneration pathway is regenerating from the somatic cell, whereas the indirect regeneration pathway involved the induction of callus (undifferentiated cells). Furthermore, indirect regeneration showed no vascular connection between the regenerant and the explant. The somatic embryogenesis is often used to study the physiological growth and development for a better understanding of the fundamental studies.

Somatic embryos show genetic variations in the offsprings that might not be the same as the parent plants. It is least favored by agriculture as the plantlets might not be true-to-type. However, it is favored in horticulture, especially the ornamental plants showed variegated traits that would bring up the price of the plants. As a result of somaclonal variation, the variegation that has market values is even protected by patent, which similar to that of genetically engineered crops. Somatic embryogenesis is preferred by crop breeders who are searching for genetic variation with better traits. Also, the rate of regenerated plants via somatic embryogenesis is much higher than that of micropropagation, which made it less labor-intensive to propagate the plants on a large scale. Often, somatic embryos are used as the starting material for genetic transformation as well.

Plants with medicinal properties are beneficial to our health. However, the concentration of the active compound in a plant is relatively low and unstable when it exposed to the air. Generally, one ton of leaves gives only one gram of the active compound after the extraction, purification, and chemical stabilization. Grow lots of plants in a field to produces huge biomass for medicinal value is inefficiency at a high cost with low yields. The plant tissue culture method propagate medicinal plants at a faster rate. Moreover, the plant cells with high concentrates can be used to induce callus or somatic embryos in plant tissue culture for suspension cell culture. The callus or somatic embryos are cultivated in agitated liquid media added with elicitors and precursors to stimulate and increase the production of secondary metabolites. Subsequently, the suspension will be assessed by the pharmacological department for the extraction and isolation of the active compounds.

Large-scale production of medicinal compounds or valuable secondary metabolites by plant cell cultures from roots can be easily induced by culture explant with media fortified with auxins. It is particularly useful with plants that accumulate active phytochemicals only in their roots. Traditionally methods are time and cost consumption to plant large quantities in the field and wait for years before harvesting the root. By bringing the production into a plant cell bioreactor, there will be no waiting and digging process. The production can be optimized by varying the abiotic stresses, the addition of elicitors like plant growth regulators, and supplementation of building materials as known as precursors to the callus or embryo cultures. The production of ginsenosides in hairy root cultures of American Ginseng has been a successful example.

The somatic embryo from tissue culture is useful in the conservation of germplasm. Plant tissue culture bank is known as ­ex situ short-term conservation, but a monthly subculture is still required to be scheduled to make sure the viability of the in vitro plants. Cryopreservation provides long-term conversation as compared to any other conversation. The fundamental theory or concept of cryopreservation is similar to the cryopreservation of the sperm and ovum of humans. The most common explants used in cryopreservation is meristematic tissues and somatic embryos, but somatic embryos are preferred. Because somatic embryogenesis is a regenerants that have a bipolar property in which it can grow the root and shoot at the same time. In contrast, the regenerant of organogenesis grows only shoot or root at a time, depending on the supplementations. The explants are usually dehydrated using sugar alcohols and frozen by using liquid nitrogen. Theoretically, the viability can be maintained in a nitrogen tank for a very long time.

Encapsulation, that encapsulates plant material in calcium alginate in cryopreservation allows the production of synthetic seeds, which can be sowed directly into the field to germinate into a seedling. This technique was specially designed for the plants that are sterile without seed production in their lifetime. Apart from this, parasexual hybridization which generates a wide hybridization brings a great breakthrough in crop improvement. The protoplast culture generated from callus culture by removing the cell wall using cellulase, hemicellulase, and pectinase. The protoplast culture can be used to induce haploid (half set of the genetic content, n), triploids (three sets of genetic content, 3n), or polyploid (n>3). Furthermore, it can be applied to induce somatic hybrids or cytoplasmic hybrid that involved the fusion of two protoplasts of different species.

Plant improvement is remarkably important in today’s agriculture which is facing increasing challenges. Plant biotechnologies continue to bring novel solutions for sustainable agriculture and better environmental protection. Plant tissue culture is a great tool in plant scientific studies without the need to wait for the plant to grow into a mature tree. Together with molecular biology techniques, gene function assessment, computer science, artificially intelligence, and robotic scale processing, the possibilities are endless. (previous reading, https://plantbiotechs.com/agricultural-biotechnology-plant-tissue-culture/)

Further readings: