25-1 Hormones and Plant Growth Essential Question How do hormones affect plant growth and development?
Patterns of Plant Growth • Plants follow general patterns of growth that differs among species. • Plant cells send signals to one another that indicate when to divide and when to develop into a new kind of cell. • Hormones are produced in apical meristems, in young leaves, in roots, and in growing flowers and fruits. • Plants grow in response to environmental factors such as light, moisture, temperature, and gravity.
Hormones and Target Cells • Actions of plant chemicals direct, control, and regulate plant growth. • Patterns of growth lead to distinct shapes of plants such as a thick trunk. • Hormones control division, growth, maturation, and development of cells • Portion of an organism affected by a particular hormone is its target cell.
Plant Hormones • target cell must contain a hormone receptor – a protein to which the hormone binds. • If the appropriate receptor is present, the hormone can exert an influence on the target cell by changing its metabolism, affecting its growth rate, or activating the transcription of certain genes. • Cells that do not contain receptors are unaffected by hormones.
Different kinds of cells may have different receptors for the same hormone causing it to behave in different ways • A certain hormone may stimulate growth in stem tissues but inhibit growth in root tissues.
Auxins are hormones that stimulate cell elongation. They are produced in the apical meristem and transported downward into the rest of the plant. When light hits one side of a stem, a higher concentration of auxins develops in the shaded part of the stem causing cells on the shaded side to elongate As a result, the stem bend away from the shade and toward the light. Phototropism is the tendency of a plant to grow towards the source of light. Auxins and Phototropism
Figure 25–3 Auxins and Phototropism Section 25-1 Highconcentrationof auxin Lowconcentrationof auxin Control Tipremoved Opaquecap Clearcap Opaque shiedover base
Gravitropism • Gravitropism is the response of a plant to the force of gravity. • Auxins are responsible for gravitropism by stimulating cell growth on lower sides of roots and stems thus helping the trunk to turn upright. • The effect of auxins in roots in opposite- causing the root to grow downward. • Auxins are responsible for the way that roots grow around objects in the soil such as rocks.
Auxins and Branching • Auxins also regulate cell divisions in meristems such as lateral buds. • Lateral buds are meristematic area on the side of a stem that gives rise to side branches. • The delay of growth a lateral bud is regulated by auxins
Cont’d • Because auxins move out from the apical meristems, the closer a bud is to the stem’s tip, the more its growth is inhibited. This phenomenon is called apical dominance. • If you snip off the tip of a plant, the growth inhibiting auxins are removed. • This causes the side branches begin to grow more quickly, resulting in a rounder, fuller plant.
Figure 25–5 Apical Dominance Lateral buds Auxins produced in the apical meristeminhibit the growth of lateral buds. Apical meristem removed Without the inhibiting effect of auxinsfrom the apical meristem, lateral budsproduce many branches. Section 25-1 Apical meristem
Herbicides are compounds that are toxic to plants. • They are manmade chemicals that mimic the effects of auxin by inhibiting growth.
Cytokinins • Cytokinins are plant hormones that stimulate cell division and the growth of lateral buds, and cause dormant seeds to sprout. • They also delay the aging of leaves and play an important part in the early stages of plant growth. • They produce the opposite effect of auxins • Auxins stimulate cell elongation; cytokinins inhibit it • Auxins inhibit the growth of lateral buds, cytokinins stimulate it.
Gibberellins • Gibberellins are plant hormones that produce dramatic increases in plant size, especially in stems and fruits. • They are only produced by seed tissue and are responsible for the rapid early growth of many plants. • They were discovered by a Japanese biologist studying a fungus that caused rice plants to grow unusually tall.
Ethylene • Ethylene is a auxins that stimulates fruits to ripen. • Commercial producers use this hormone to control the ripening process of fruits such as tomatoes and lemons. • They are picked before they are ripe and then treated with synthetic ethylene to cause them to ripen.