In nature, auxin influences plants throughout their lifetimes. Auxin establishes the apical-basal polarity of seed embryos, induces vascular tissue to differentiate, mediates phototropism, promotes formation of adventitious roots, and stimulates fruit development. Many of auxin's effects are of practical importance to humans. For example, embryos within developing seeds produce auxin that stimulates flower ovaries to mature into fruits. This explains why some types of seedless fruits are produced with the aid of auxin applications. Auxin also retards premature fruit drop, explaining why apple or pear growers spray trees with auxin. This hormone also fosters development of adventitious roots—those developing from stems. Houseplant stem cuttings will more readily develop roots if you dip the stem ends into commercial rooting compounds, which contain auxin. Gardeners know that if they remove the topmost portion of a plant shoot, nearby lateral buds will begin to grow and produce new branches, allowing plants to become bushier. Such decapitation works by disrupting the flow of auxin from the shoot apex. Auxin produced by intact shoot tips inhibits lateral bud growth, a process known as
apical dominance. There is evidence that auxin acts indirectly to inhibit lateral bud outgrowth, but this regulation is complex and not completely understood. However, auxin's role in phototropism has been elucidated by a series of elegant experiments.
A typical known cytokinin is
Zeatin (see attachment). Zeatin promotes cell division, influences cell specialization and plant aging, activates secondary meristem development, promotes adventitious root growth, and promotes shoot development on callus.
Like auxins, the plant hormones known as cytokinins play varied and important roles in plants. The name of these hormones reflects their major effect: an increase in the rate of plant cytokinesis or cell division. Root tips are the major sites of cytokinin production, but shoots and seeds also make this plant hormone. Cytokinins are transported in the xylem to meristems, seeds, leaves, and fruit, where they stimulate cell division. At shoot and root tips, cytokinins influence meristem size, stem cell activity, and vascular tissue development. Cytokinins are also involved in root and shoot growth and branching, the production of flowers and seeds, and leaf senescence (aging).
In the laboratory, cytokinin and auxin are essential to cloning plants. This involves a process known as plant tissue culture, which is used commercially to produce thousands of identical plants having the same desirable characteristics. Plant tissue culture begins with pieces of stem, leaf, or root that have been removed from a plant. Their surfaces are sterilized to prevent growth of microbes (step 1), and the cleaned plant pieces are then placed into dishes containing nutrients (minerals, vitamins, and sugar) and various proportions of auxin and cytokinin. If the proportions of auxin and cytokinin are about the same (1:1), plant cells undergo division, forming a mass of white tissue known as a callus (step 2). If the callus is then transferred to a new dish containing the same nutrients but with auxin-to-cytokinin proportions greater than 10:1, the callus will form roots (step 3). Auxin-to-cytokinin proportions of less than 10:1 cause the callus to develop green shoots (step 4). Thus, by altering the ratios of auxin and cytokinin, entire plants can be regenerated from a callus. A single callus can be divided into many pieces and each piece treated with these hormones, thereby producing many hundreds of identical new plants.
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