THE LIVING WORLD

Unit Seven. Plant Life

 

34. Plant Reproduction and Growth

 

34.8. Plant Hormones

 

After a seed germinates, the pattern of growth and differentiation that was established in the embryo is repeated indefinitely until the plant dies. But differentiation in plants, unlike that in animals, is largely reversible. Botanists first demonstrated in the 1950s that individual differentiated cells isolated from mature individuals could give rise to entire individuals. In an experiment, shown in figure 34.10, F. C. Steward was able to induce isolated bits of phloem tissue taken from carrots to form new plants, plants that were normal in appearance and fully fertile. Regeneration of entire plants from differentiated tissue has since been carried out in many plants, including cotton, tomatoes, and cherries. These experiments clearly demonstrate that the original differentiated phloem tissue still contains cells that retain all of the genetic potential needed for the differentiation of entire plants. No information is lost during plant tissue differentiation in these cells, and no irreversible steps are taken.

 

 

Figure 34.10. How Steward regenerated a plant from differentiated tissue.

 

Once a seed has germinated, the plant’s further development depends on the activities of the meristematic tissues, which interact with the environment through hormones (discussed next). The shoot and root apical meristems give rise to all of the other cells of the adult plant. Differentiation, or the formation of specialized tissues, occurs in five stages in plants and is shown in figure 34.11. The establishment of the shoot and root apical meristems occurs at stage 2; after that point, the tissues become more and more differentiated.

 

 

Figure 34.11. Stages of plant differentiation.

As this diagram shows, the different cells and tissues in a plant all originate from the shoot and root apical meristems. It is important to remember, however, that this is showing the origin of the tissue, not the location of the tissue in the plant. For example, the vascular tissues of xylem and phloem arise from the vascular cambium, but these tissues are present throughout the plant, in the leaves, shoots, and roots.

 

The tissue regeneration experiments of Steward and many others have led to the general conclusion that some nucleated cells in differentiated plant tissue are capable of expressing their hidden genetic information when provided with suitable environmental signals. What halts the expression of genetic potential when the same kinds of cells are incorporated into normal, growing plants? As we will see, the expression of some of these genes is controlled by plant hormones.

Hormones are chemical substances produced in small (often minute) quantities in one part of an organism and then transported to another part of the organism, where they stimulate certain physiological processes and inhibit others. How they act in a particular instance is influenced both by which hormones are present and by how they affect the particular tissue that receives their message.

In animals, there are several organs, called endocrine glands, that are solely involved with hormone production (hormones are produced in other organs as well). In plants, on the other hand, all hormones are produced in tissues that are not specialized for that purpose and carry out many other functions.

At least five major kinds of hormones are found in plants: auxin, gibberellins, cytokinins, ethylene, and abscisic acid. Their chemical structures and descriptions are provided in table 34.1. Other kinds of plant hormones certainly exist but are less well understood. Hormones have multiple functions in the plant; the same hormone may work differently in different parts of the plant, at different times, and interact with other hormones in different ways. The study of plant hormones, especially how hormones produce their effects, is today an active and important field of research.

 

TABLE 34.1. FUNCTIONS OF THE MAJOR PLANT HORMONES

 

Hormone

Major Functions

Where Produced or Found in Plant

Practical Applications

Promotes stem elongation and growth; forms adventitious roots; inhibits leaf abscission; promotes cell division (with cytokinins); induces ethylene production; promotes lateral bud dormancy

Apical meristems; other immature parts of plants

Seedless fruit production; synthetic auxins act as herbicides

Promotes stem elongation; stimulates enzyme production in germinating seeds

Root and shoot tips; young leaves; seeds

Uniform seed germination for production of barley malt used in brewing; early seed production of biennial plants; increasing size of grapes by allowing more space for growth

Stimulates cell division, but only in the presence of auxin; promotes chloroplast development; delays leaf aging; promotes bud formation

Root apical meristems; immature fruits

Tissue culture and biotechnology; pruning trees and shrubs, which causes them to "fill out"

Controls leaf, flower, and fruit abscission; promotes fruit ripening

Roots, shoot apical meristems; leaf nodes; aging flowers; ripening fruits

Fruit ripening of agricultural products that are picked early to retain freshness

Controls stomatal closure; some control of seed dormancy; inhibits effects of other hormones

Leaves, fruits, root caps, seeds

Research on stress tolerance in plants, specifically drought tolerance

 

Key Learning Outcome 34.8. The development of plant tissues is controlled by the actions of hormones. They act on the plant by regulating the expression of key genes.