Modification of plant hormone levels and signaling as a tool in plant biotechnology.

Plant hormones are signal molecules produced within the plant, and occur in extremely low concentrations.

There are five general classes of hormones: auxins, cytokinins, gibberellins, ethylene

Plant hormones are signal molecules, present in trace quantities, that act as major regulators of plant growth and development. They are involved in a wide range of processes such as elongation, flowering, root formation and vascular differentiation. For many years, agriculturists have applied hormones to their crops to either increase the yield, or improve the quality of the commercial product. Nowadays, the knowledge of hormone biosynthesis, degradation and signaling pathways has allowed the utilization of biotechnological tools to further improve the main agricultural crops. Natural or artificial mutants, with impaired functioning of the corresponding genes, have been adopted because of their superior phenotype in specific agricultural traits. In addition, transgenic plants have been generated to regulate internal hormone levels, or their signaling pathways, resulting in some crops that have revolutionized agriculture.

A number of techniques exist for the production of GM plants. The two most commonly employed are the bacterium Agrobacterium tumefaciens, which is naturally able to transfer DNA to plants, and the ‘gene gun’, which shoots microscopic particles coated with DNA into the plant cell. Generally, individual plant cells are targeted and these are regenerated into whole GM plants using tissue culture techniques. Three aspects of this procedure have raised debate with regard to human health.

  • The use of selectable markers to identify transformed cells
  • Transfer of extraneous DNA into the plant genome (i.e. genes other than those being studied)
  • The possibility of increased mutations in GM plants compared to non-GM counterparts due to tissue culture processes used in their production and the rearrangement of DNA around the insertion site of foreign genes.

To facilitate the transformation process, a selectable marker gene conferring, for example, resistance to an antibiotic (e.g. kanamycin, which will kill a normal non-GM plant cell), is often co-transferred with the gene of interest to allow discrimination of GM tissue and regeneration of GM plants. Critics of the technology have stated that there is a risk of the spread of antibiotic resistance to the bacterial population either in the soil or in the human gut after ingestion of GM food. However, these antibiotic resistance genes were initially isolated from bacteria and are already widespread in the bacterial population. In addition, kanamycin itself has GRAS status (Generally Regarded As Safe) and has been used for over 13 years without any known problems. Studies have concluded that the probability of transmission of antibiotic resistance from plants to bacteria is extremely low and that the hazard occurring from any such transfer is, at worst, slight. Nevertheless, other selection strategies that do not rely on antibiotic resistance have been developed, and procedures to eliminate the selectable marker from the plant genome once its purpose has been fulfilled have also been designed.

The second aspect of the plant transformation procedure that has been criticized is that unnecessary DNA is transferred into the plant genome as a consequence of the engineering and transfer process. Of course, there is no reason that DNA per se should be harmful, as it is consumed by humans in all foods, but again plant technologists have responded to the criticism by designing ‘minimal cassettes’ in which only the gene of interest is transferred into the plant.

Finally, it has been claimed that GM plants carry more mutations than their untransformed counterparts as a result of the production method. Genome-wide mutations may be produced by the tissue culture process, generating so called somaclonal variation, and endogenous DNA rearrangements may occur around the integrated transgene. Theoretically, this may mean that plants may be produced with, for example, reduced levels of nutrients or increased levels of allergens or toxins. (although the alternative must also hold true, that positive traits may be expressed). Latham et al. have stated that mutations around foreign gene insertion sites have not been fully characterized in either experimental or commercialized GM plants. Consequently, these authors have proposed several recommendations involving improved molecular analysis prior to the future commercialization of GM crops

Auxins
An auxin, indole‐3‐acetic acid (IAA), was the first plant hormone identified. It is manufactured primarily in the shoot tips (in leaf primordia and young leaves), in embryos, and in parts of developing flowers and seeds. Its transport from cell to cell through the parenchyma surrounding the vascular tissues requires the expenditure of ATP energy. IAA moves in one direction only—that is, the movement is polar and, in this case, downward. Such downward movement in shoots is said to be basipetal movement, and in roots it is acropetal.

Cytokinins
Named because of their discovered role in cell division (cytokinesis), the cytokinins have a molecular structure similar to adenine. Naturally occurring zeatin, isolated first from corn ( Zea mays), is the most active of the cytokinins. Cytokinins are found in sites of active cell division in plants—for example, in root tips, seeds, fruits, and leaves. They are transported in the xylem and work in the presence of auxin to promote cell division. Differing cytokinin:auxin ratios change the nature of organogenesis. If kinetin is high and auxin low, shoots are formed; if kinetin is low and auxin high, roots are formed. Lateral bud development, which is retarded by auxin, is promoted by cytokinins. Cytokinins also delay the senescence of leaves and promote the expansion of cotyledons.

Gibberellins
The gibberellins are widespread throughout the plant kingdom, and more than 75 have been isolated, to date. Rather than giving each a specific name, the compounds are numbered—for example, GA1, GA2, and so on. Gibberellic acid three (GA3) is the most widespread and most thoroughly studied. The gibberellins are especially abundant in seeds and young shoots where they control stem elongation by stimulating both cell division and elongation (auxin stimulates only cell elongation). The gibberellins are carried by the xylem and phloem. Numerous effects have been cataloged that involve about 15 or fewer of the gibberellic acids. The greater number with no known effects apparently are precursors to the active ones.

Ethylene
Ethylene is a simple gaseous hydrocarbon produced from an amino acid and appears in most plant tissues in large amounts when they are stressed. It diffuses from its site of origin into the air and affects surrounding plants as well. Large amounts ordinarily are produced by roots, senescing flowers, ripening fruits, and the apical meristem of shoots. Auxin increases ethylene production, as does ethylene itself—small amounts of ethylene initiate copious production of still more. Ethylene stimulates the ripening of fruit and initiates abscission of fruits and leaves. In monoecious plants (those with separate male and female flowers borne on the same plant), gibberellins and ethylene concentrations determine the sex of the flowers: Flower buds exposed to high concentrations of ethylene produce carpellate flowers, while gibberellins induce staminate ones.

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