Various applications of plant growth regulators

THE WORD hormone is derived from Greek, meaning 'set in motion.' Plant hormones affect gene expression and transcription levels, cellular division, and growth. A large number of related chemical compounds are synthesized by humans, they are used to regulate the growth of cultivated plants, weeds, and in vitro-grown plants and plant cells; these man-made compounds are called Plant Growth Regulators or PGRs for short. Plant hormones are not nutrients, but chemicals that in small amounts promote and influence the growth, development, and differentiation of cells and tissues. The biosynthesis of plant hormones within plant tissues is often diffused and not always localized.

Commercial Applications of Plant Growth Regulators

(1) Food-growing Industry:

Plant hormones are widely used in the food-growing industry. For example:

(i) Growing cuttings:

Cuttings develop much bigger root systems if they are dipped in hormone rooting powder or planted in rooting compound containing growth hormone - making it possible to clone plants quickly and cheaply.

(ii) Producing fruit without seeds:

To do this, growth hormones are sprayed onto un-pollinated flowers to make them grow fruit without fertilization.

(iii) Ripening fruit:

Bananas are harvested before they are ripened to reduce damage to them during transport, and then sprayed with ethylene to ripen them. Hormone secretions often originate from one part of the plant and circulate through to other areas of the plant body. Ripening effects occur as hormones trigger physiological processes within the cells that make up the leaf, stem and fruit structures. Hormones involved in the ripening process include ethylene and abscisic acid. Ethylene secretions increase the rate at which fruit matures, while abscisic acid triggers a plant's dormancy period, causing plant cells and tissues to die off.

(iv) Increasing the size of fruit:

For example, grapes are sprayed with the hormone gibberellin to increase the size of the fruit.

(v) Controlling weeds:

Some synthetic weed killers which selectively kill unwanted plants contain growth substances that speed up the metabolism. As a result weeds grow so fast that their food and water supply cannot keep up, so they run out of energy and die.

(2) Medical applications:

Plant stress hormones activate cellular responses, including cell death, to diverse stress situations in plants. Researchers have found that some plant stress hormones share the ability to adversely affect human cancer cells . For example, sodium salicylate has been found to suppress proliferation of lymphoblastic leukemia, prostate, breast, and melanoma human cancer cells. Jasmonic acid, a plant stress hormone that belongs to the jasmonate family, induced death in lymphoblastic leukemia cells. Methyl jasmonate has been found to induce cell death in a number of cancer cell lines.

(3) Plant propagation:

Synthetic plant hormones or PGRs are commonly used in a number of different techniques involving plant propagation from cuttings, grafting, micropropagation, and tissue culture. The propagation of plants by cuttings of fully developed leaves, stems, or roots is performed by gardeners utilizing auxin as a rooting compound applied to the cut surface; the auxins are taken into the plant and promote root initiation.

(4) Seed dormancy:

Plant hormones affect seed dormancy by affecting different parts of the seed. Embryo dormancy is characterized by a high abscisic acid/gibberellin ratio, whereas the seed has a high ABA sensitivity and low GA sensitivity. To release the seed from this type of dormancy and initiate seed germination, an alteration in hormone biosynthesis and degradation towards a low ABA/GA ratio, along with a decrease in ABA sensitivity and an increase in GA sensitivity needs to occur.

ABA controls embryo dormancy, and GA embryo germination. Seed coat dormancy involves the mechanical restriction of the seed coat. GA releases this dormancy by increasing the embryo growth potential, and weakening the seed coat so the radical of the seedling can break through the seed coat. ABA affects testa or seed coat growth characteristics, including thickness, and effects the GA-mediated embryo growth potential.

(5) Endosperm Dormancy:

Hormones also mediate endosperm dormancy: Endosperm in most seeds is composed of living tissue that can actively respond to hormones generated by the embryo. The endosperm often acts as a barrier to seed germination, playing a part in seed coat dormancy or in the germination process. Living cells respond to and also affect the ABA/GA ratio, and mediate cellular sensitivity; GA thus increases the embryo growth potential and can promote endosperm weakening. GA also affects both ABA-independent and ABA-inhibiting processes within the endosperm.

(6) Cell Division Effects:

Plant growth and development rely on the different rates of cell division that take place within plant structures. Rates of cell division are regulated by the amounts of hormones secreted within the plant body. Specific hormone chemicals such as auxins and gibberellins affect flower and fruit growth, as well as stem elongation rates. Plant development processes such as cell division activities give rise to differentiated structures that form plant roots, stems and leaves.

(7) Seed Germination Effects:

Germination processes involve the growth and development stages that take place inside the embryo, which exists inside the seed part of a plant. Gibberellins and cytokinins are two hormone chemicals that trigger seed germination processes. Both hormones stimulate cell division activities, which result in tissue growth. When applied in synthetic chemical form, gibberellins can also interrupt a seed's natural dormancy period and cause germination processes to begin. Abscisic acid, another synthetic hormone chemical, triggers seed dormancy periods and prevents germination from occurring.http://www.technologytimes.pk/mag/2011/march11/issue01/various_applications_of_plant.php