Plant Cell Growth

Growth is the self-multiplication of living material, the protoplasm it self. Growth is an increase in size (volume or length) duw to cell divisions and subsequent enlargement. It is an increase in dry weight or bulk of an organism associated with development.


Development is defined as an ordered change or progress, often towards a higher, more ordered or more complex state. Development may take place with growth and growth may take place with development.



Population growth is the increase in the total mass of cells making the population, or the increase in the total number of cells of the population. Individual growth is the increase in mass of the individual cell.

WHY GROWTH OCCURS?



Growth is expressed as the division of a cell to form two cells and the enlargement of the newly divided cells. When we say that cells double in all their constituents and then divide in half, we are obviously describing the average case. When we observe individual cells, however, we find deviations from the average. In some instances, the growth rate is constant rather than accelerating.

Mass of cells doubles and halves only in an average way and not in an exact way. Cells divide when they are ready and they are ready only when they have completed certain preparations for division.

CRITICAL MASS THEORY:-


In 1908 the German biologist Richard von Hertwig proposed the theory of the critical mass. In Hertwig's view a growing cell eventually reaches a size at which the ratio between the masses represented by cytoplasm on the one hand and the cell nucleus on the other becomes limiting. When that limiting ratio is reached, he proposed, some instability sets which triggers cell division. The nucleus-cytoplasm relation theory, as hertwig called it, does not hold up very well when mass alone is considered to be the factor that triggers cell division. Many exceptions to this hypothesis have accumulated that, although the division of cells usually parallels, an increase in cytoplasmic mass, this increase alone is not generally accepted as the fundamental mechanism
controlling cell cycle.

AGE OF CELLS:-



Cells taken from old cultures are often referred to as old cells, The age of a single cell cannot be greater than the time between two divisions. A cell, when first formed by cell division, although often called a young cell, contains materials both genetic and protoplasmic, which pre-existed in the parent cell. Age of the cell depend on its life spam. The life spam begins when the daughter cell is completely separated from the parental cell, this is called inception. It ends when this daughter cell itself divide later to give new daughter cells, this is known as termination. The period between inception and termination is called the generation time. This is actually the visible generation time that is observed. We can define young population of cells as the one, which is still actively growing and contains fewer cells that it is possible to obtain from that culture under tha same conditions, but old population is defined as that one which has reached the maximum number of cells under similar conditions. There are three types of following generation times :-

1. The visible generation time which is the period between inception and termination. It depends on division of cell wall.

2. Nuclear generation time depends on division of the nucleus. It starts from the moment the nucleus divides to the complete ability to divide again in the new daughter cells.

Generation time is given the symbol t.

SYNCHRONOUS AND SYNCHRONIZED GROWTH:-

Synchronous growth cultures are produced without metabolic shock. But synchronized culture are two ones in which we produce step-wise growth, the synchrony is induced by physiological shock. The principal means of doing synchronous growth are as follows :

1. Separation by physical means of cells in the population that are all at the same stage of the cellular life   cycle. Ex. by filteration, centrifugation etc.

2. Attachment of growing cells to a membrane, from which one product of cell division is shed. i.e., age selection. 

BALANCED AND UNBALANCED GROWTH:-

Exponential growth, whether in batch or continuous cultures, is the balanced growth. That is all cellular constituents are manufactured at constant rates relative to each other. If nutrient levels or other environmental conditions change, unbalanced growth results because the rates of synthesis of cell components vary relative to one another until a new balanced stage is reached.

Unbalanced growth occurs when a bacterial population is shifted down from a rich medium to a poor one. The organism may previously have been able to obtain many cell components directly from the medium. When shifted to a nutritionally inadequate medium, they need time to make the enzyme required for the biosynthesis of unavailable nutrients. Consequently cell division and DNA replication continue after the shift-down, but net protein and RNA synthesis slow.

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Tissue Culture Techniques To Increase Genetic Variability

The haploid plantlets (n chromosomes) are treated with colchicine to produce fertilie homozygous lines, called double haploid lines. The haplodiplodisation technique could give immediately new elite genotypes, hybrid parents after in vitro propagation, as asparagus supermales (MM) or useful genetic material to establish gene mapping.

PROTOPLAST CULTURE & MICROPROPAGATION :



Protoplasts are the smallest unit able to regenerate a whole plant. Therefore protoplats cultures can serve to enlarge genetic variability by introducing somaclonal variations. However, the main interest of protoplasts, is their capacity to fuse and to produce hybrids or cybrids. Naked protoplasts can accept without rejection external elements: nuclei, cytoplasmic organelles, liposomes, containing genetic information.

The hybridization programme is hampered in some cases because of sexual incompatibility, Synthetic production of hybrids has, however becomes possible through the novel technique of protoplast culture and their fusion. The isolation culture, and fusion of protoplast are one of the most fascinating fields of research, through still in developing stage. The protoplast culture technique can be suitably used for microinjection and other genetic engineering experiments. The technique are important, specially because of their far-reaching effects on crop improvement by somatic hybridization and cell modification. The protoplast culture can be regenerated into an entire plant. The discovery of enzymes which could separate the cells for isolating protoplasts and exploring the possibilities of genetic engineering.

SOMATIC HYBRIDS & CYBRIDS :

The first protoplast fusion application was a cytoplasmic transfer from one genotype to another to induce male sterility (CMS) from mitochondrial origin. These male sterile hybrids are interesting to produce F1 hybrids (Brassica, Cichorium).

Since long protoplast fusion was proposed as a novel and important method for producing hybrid plants that can be obtained by sexual means. Mechanical or enzymatic removal of cell wall from a plant cell yields, a protoplast, which is bound by plasma membrane. An isolated protoplast, under suitable culture condition can regenerate entire plant. Plasmolyzing cell prior to enzymatic treatment helps to facilitate protoplast isolation. The isolated protoplast, having lost the protective cell wall, has to be protected against the osmotic shock by keeping them in the isotonic state. 13% mannitol is a good agent in this context. A variety of enzymes are available for protoplast isolation. Most common are cellulose, pectolyase and macerozyme. The viable protoplast tend to synthesize new cell wall within few hours to few days of culture divide and re-divide, produce clumps of cells which finally produce plantlets.

PRODUCTION OF VIRUS FREE PLANTS :

In 1952, Morel and Martin were successful in regenerating a virus-free dahlia plant by the excision of some meristematic domes from virus infected shoots. Semal and Lepoivre (1992) reported that a virus-free sweet potato was producing 40T/ha in china by comparison of the 20 T/ha produced before meristem culture.

Virus eradiction is dependent on several parameters. But to take advantage of the non uniform and imperfect virus distribution in the host plant body, the size of the excised meristem should be as small as possible. For Stone (1963), only tips between 0.2 and 0.5 mm most frequently produce virus free carnation plants. The explants smaller than 0.2 mm can't survive and those larger than 0.7 produce plants that still contain mottle virus.

There are various explanations have been given: absence of plasmodesm in the meristematic domes, competition between synthesis of nucleoproteins for cellular division and viral replication, inhibitor substances, absence of enzymes present only in the cells of the meristematic zones and suppression by excision of small meristematic domes. This last proposal could explain why some potato plant showing virus particles in the meristematic domes, could regenerate a virus free plant.

SHOOT TIP MICROGRAFTING :

When meristematic tip culture fails, it is possible to graft small meristematic domes on young seedlings growing in vitro. In this way, Navarro et al. (1975) eradicted all the virus diseases from spanish Citrus orchards. This technique was also very successful in eliminating virus diseases from peach tress (Mosella et al 1980).

MICROPROPAGATION TECHNIQUES:

During the micropropagation process, the genetic stability of new shoots dependent upon their origin. Axillary shoots issue from pre-existing buds and are normally true to type, indeed the meristematic cells are genetically very stable. Adventitious shoots, such as somatic embryos, are neoformed buds developed directly on some organs, or indirectly through a callus phase formed on this organ. So, if the mother plant presents a cell mosaic or chimaeric tissues, risks of genetic variation exists. It is similar in the case of an indirect regeneration, when the callus phase is too long.

PROPAGATION BY AXILLARY SHOOTING :

This technique has proved to be the most applicable and reliable method of in vitro propagation. Axillary shoot growth is stimulated by overcoming apical meristem dominance. Commercial tissue culture laboratories are now able to propagate a large number of herbaceous ornamental species and several woody plants in this way. However, the propagation of Pelargonium, Howea and a few other horicultural plants are always difficult to propogate by axillary branching.

PROPAGATION BY DIRECT OR INDIRECT ORGANOGENESIS :

Adventitious shoots could arise directly from the tissue of explants without callus formation. Several plants of tha family gesneriaceae (Saintpaulia, Streptocarpus) regenerate directly buds on leaf explants, likewise Lilium regenerate on scales. However, more often, like for Ficus lyrata, adventitious buds appear on callus. While coffee, cocoa trees and many conifers are produced by somatic embryogenesis developed on callus or cell suspensions.

IMPROVEMENT OF AXILLARY BRANCHING :

The cost of micropropagated plantlets is also an important limitation of the techniques. In New zealend, where they produce 2-3 million micropropagated radiata pine per annum, the relative cost of micropropagated planting stock had dropped from 13,8 times the cost of seedlings in 1988 to 6,9 by 1993 (Smith, 1997). To reduce manpower costs, several improvements have been proposed.. The more simple method was in vitro layering developed by Wang(1977) to clone PVX-free potato plants.The first plantlets placed on the medium in a horizontal position developed axillary shoots. They are harvested by cutting one centimeter above the medium surface, at 3 weeks intervals. A similar technique called "hedging system" by Aitken christie and Jones (1987) was later used to produce Pinus radiata. Since 1988, Duhem was producing vary large quantities of Eucalyptus plantlets in petri dishes without anti-gibberellin but in complete darkness. Transfers from one petri to another is made by a simple squashing.

SOMATIC EMBRYOGENESIS PROPAGATION :

For genetically stable species, somatic embryogenesis offers a very fast scaling-up system, especially when it's possible to produce embryos in bioreactors. Only a few model plants are successfully produce by such technology: carrot, celery. Other applications remian at the experimental stage: coffee, oil, palms, conifers, Euphorbia pulcherrima and several other horticultural species. 

Several bottlenecks limit the use of this interesting technology. one of main problems is genetic stability. Therefore, despite the clonal nature of nucellar embryos, different morphological anomalies can occur among mango somatic embryos, as it was also observed in Citrus plants derived from nucellar cultures (Litz et al 1993). Another difficulty is the loss of embryogenic capacity overtime, a phenomenon observed with different species. It is also important that somatic embryogenic lines of conifers are always originated from immature embryos.

SYNSEEDS :

Another very interesting possibility of the somatic embryogenesis technology has been developed during the past 15 years by Redenbaugh and his team (1991, 1993). They were able to encapsulate somatic embryos by hydrogel coatings (sodium alginate), producing single embryo artificial seeds. To date, some improvements offer the possibility to directly plant the artificial seeds in the greenhouse on special substrates (vermiculite, sans). This methodology will provide in future a good technique to reduce the cost of transplants. 

SOMACLONAL VARIATIONS :

The production of plantlets by callus regeneration, cell suspensions, protoplast cultures could present some deviations with regard to the mother plant. This is the way to increase the genetic variability. Associated with a selective pressure (Stress to toxins, pH, salinity, cold). it's used to obtain resistant lines.Indeed after regeneration, plants can express new potentialities rarely obtained another way. Stable and profitable variants are selected and introduced in breeding programmes. In 1976, a pelargonium cv Velvet Rose was created by this technique (Reisch, 1983).

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Microbial Biopesticides, Biofungicide, Herbicides And Agricultural Antibiotics

Several biopesticides are in use today. Biopesticide products are based on natural agents such as microorganisms and fatty acid compounds. They are toxic to targeted pests (such as the European corn borer) and do not harm humans, animals, fish, birds or beneficial resistance to conventional pesticides. One of the most common microorganisms used in biologically based pesticides is the Bacillus thuringiensis, or Bt, bacterium. Several of the proteins the Bt bacterium products are lethal to individual species of insects. Using Bt bacteria in pesticide formulations can eliminate target insects without relying on chemically based pesticides. It is also possible to use pheromones in pest control. Pheromones are naturally occuring substances that insects produce to attact mates. In pest control, pheromones are used to attract insects away from crop plants. For example, pheromone-based traps were used to control fruit fly infestations in california. The European corn borer, one of the most prevelant pests, costs the united states $1.2 billion crop damage each year.

Using biotechnology, it is possible to make crop plants tolerant of specific herbicides. When the herbicide is sprayed, it will kill the weeds but have no effect on the crop plants. This lets farmers reduce the number of times herbicides have to be applied and reduces the cost of producing crops and damage to the environment.

The contributions of biotechnology to the practice of medicine through the discovery and development of a number of potent antibacterial antibiotics is widely recognized. A major effort concerned with the discovery of new structures with antibacterial activities and the chemical modification of existing natural products is still ongoing. Although these antibiotics have found widespread use as growth promoters in the animal health area, only more recently were microbial products produced primarily for animal growth or agricultural purposes, for instance monensin as a coccidiostat for poultry and a growth permittant for ruminant animals. Bacteria, fungi and virus are the most commonly researched as potential microbial biocides. The development of low cost production methods and successful formulation of products such as microbial insecticides represents the commercial cutting edge of biotechnology.

The term insecticide usually bring thoughts of poisonous, non-selective, energy-consuming chemicals that are used to kill pest insects. However, not all insecticides fit this generally accepted stereotype. Those microorganisms which often suppress and naturally control populations or pests which injure man and causes extensive damage to food and fibre crops. Formulation of living viruses, bacteria, fungi and protozoa are called microbial insecticides.

Revolutionary techniques in biotechnology offer tantilizing new prospects for the future. Biological control has a long history, reaching into the undated past when the chinease used Pharoah's ants to control pests of stored gain. Others date the start of biological control to the domestication of the cat. The introduction of an exotic species to control a pest, the classical form of biological control, is claimed to date from 1762 when the indian Mynah was brought to mauritius to control the Red Locust. Subsequent attempts at introduction ocassionally had unwanted results, the introduction of cats to control rats on Ascension Island in 1815 resulted in the extermination of virtually all the sea birds on the island. Successful control of cotton cushion scale by Australian ladybirds in california in 1888 encouraged practitioners to continue to develop biological pest control by the introduction of exotic enemies.

Haplodiplodisation

Many species are able to produce haploids through different in vitro techniques. The oldest was anther culture of androgenesis. To date, the results vary considerably from one species to another. Solanae (datura, tobacco, red pepper, eggplant, petunia), cereals (wheat, barley, rice, triticale, maize) or crucifers (soybean, cabbage) are species easy to generate by anther culture. To the contrary, tomatoes, leguminous or compositae are recalcitrant. Ovule culture, or gynogenesis was a successful technique for Gerbera, beet, courgette. However, the most common teechnique is pollination with irradiated pollen, in order to induce in vivo parthenogenesis. The oospheres developed in embryos without fertilization are saved by embryo rescue.