Plant hormones are specialized chemicals produced by plants. They are the main internal factors that control growth and development. Hormones are produced in one part of the plant and transported to other parts, where they are effective in very small quantities. Depending on the target tissue, a given hormone can have different effects. Plant hormones play a fundamental role in controlling the growth and development of plants. A plant hormone is generally described as an organic compound synthesized in one part of the plant and translocated to another part, where at low concentrations it elicits a physiological response. There are five generally recognized classes of plant hormones; some classes are represented by a single compound, others by several different compounds. They are all organic compounds, they may resemble molecules found elsewhere in the structure or function of the plant, but they are not directly involved as nutrients or metabolites. Hormone Source Action Auxin apical meristem (moves downwards only), seed embryo, young leaves • Control of cell elongation • apical dominance (prevents lateral buds) • prevents abscission • continuous fruit growth • cell division in the vascular and cork cambia--formation of lateral roots from the pericycle--formation of adventitious roots from cuttingsGibberellins Roots and young leaves • Cell (stem) elongation (works on stems and leaves, but not on roots) • breaking of seed dormancy /shoots • stimulation of fruit set Cytokinins actively growing roots, embryos and fruits • Promotes cell division - signals axillary/lateral growth of shoots - prevents leaf abscission • chloroplast development • interruption of dormancy in some seeds • improvement of flowering • promotion of fruit development Abscisic acid ...... middle of paper ...... cascade which ultimately results in modified enzymatic activity, altered metabolic processes and different phenotypes. One thing plant hormones specifically control gene expression. The exact mechanisms by which hormones regulate gene expression are poorly understood. Gene expression is part of a large amplification process. This process involves the repeated transcription of DNA which gives rise to many copies of mRNA (1st amplification phase); The mRNA is processed and enters the cytoplasm where it is translated several times by ribosomes into a genetic product such as an enzyme (2nd amplification step); the enzymes are modified to become functional and capable of high catalytic activity even at low concentrations. They catalyze the production of many copies of an important cellular product (3rd amplification step). It is likely that gene regulation is affected by some enzymes after initial hormone binding. Genes can also be altered by secondary and tertiary messengers of a cellular cascade. Hormones can indirectly control gene expression through these enzymes and messengers at a number of control sites such as transcription, mRNA processing, mRNA stability, translation, and post-translation.