11.3 Plant Hormones

11.3 Plant Hormones

A plant hormone is a chemical signal made in one region and active in small amounts, often after it moves. Five hormones carry most of the growth questions: auxin, cytokinin, gibberellin, abscisic acid, and ethylene. Each has more than one effect, and two of them often work as a pair. Learn the source the question is likely to name, the effect on cells or organs, and the mix-up attached to the name.

Auxin from the shoot tip

The main natural auxin is indole-3-acetic acid (IAA). Young leaves and the shoot tip produce it, and it moves away from that tip. In a young stem, auxin promotes cell elongation. It activates proton pumps, the cell wall becomes more acidic, wall-loosening proteins let the wall yield, and turgor stretches the cell. The region just under the tip lengthens.

Auxin from an intact tip also maintains apical dominance. The tip keeps axillary buds below it from growing out, so the main axis leads and side shoots stay short. Pinch the tip off and that auxin source is lost, so the lateral buds can elongate. Auxin is also the hormone moved during phototropism. Light from one side leaves more auxin on the shaded side of the shoot, that side elongates more, and the shoot bends toward the light.

A concentration that elongates a stem can inhibit root elongation, because root cells are more sensitive. On a cutting, auxin favors adventitious roots. Rooting and apical dominance are different outcomes of the same hormone.

Cytokinins and cell division

Cytokinins promote cell division. They are produced largely in roots and move upward in the xylem, though shoots can make them as well. Growing root tips, young fruits, and a cultured mass of cells are cytokinin settings. Cytokinins can also delay senescence. A leaf that receives cytokinin keeps chlorophyll longer and can draw nutrients from older leaves, so yellowing slows where the hormone is high.

The ratio with auxin

Organ formation in culture depends on both hormones. A high auxin-to-cytokinin ratio favors roots. A high cytokinin-to-auxin ratio favors shoots. A more even pair can keep an undifferentiated callus dividing. That ratio is why a cutting roots when auxin is relatively high, and why raising cytokinin can push a culture toward buds. On the intact plant, cytokinin from the roots can encourage axillary buds once auxin from the tip declines.

Gibberellins

Gibberellins promote stem elongation, seed germination, and bolting. In stems they lengthen internodes through both cell division and cell elongation. A plant that makes too little gibberellin, or responds weakly, stays dwarf. Bolting is the rapid rise of a flowering stem from a low rosette, as in some cabbages and other biennials, and gibberellin drives that rise.

In a cereal grain, gibberellin connects the embryo to stored food. The embryo releases gibberellin. The aleurone then secretes digestive enzymes, including amylase. Starch in the endosperm becomes sugar the embryo can use. That is the germination effect. Gibberellin can also help end some kinds of seed dormancy, which sets it against abscisic acid in the seed.

Abscisic acid

Abscisic acid (ABA) is the dormancy and drought hormone in this set. In a maturing seed it promotes dormancy, so the seed does not germinate on the parent plant or in a bad season. High ABA makes the seed wait. A later shift that lowers ABA relative to gibberellin lets germination proceed when water and temperature allow it.

In a drought, ABA rises. Guard cells lose solutes and water, turgor falls, and the stomata close. Closed pores slow water loss. A wilted plant in dry soil is an abscisic-acid scene. The name is the trap. Abscisic sounds like abscission, the dropping of leaves, but ABA is not the main hormone of leaf drop. Leaf drop belongs to ethylene, and it also follows a decline of auxin from the blade. A drought or dormancy item is not a leaf-fall item, and ABA is not the primary hormone when leaves separate from the stem.

Ethylene gas

Ethylene is a gas produced by ripening fruit, aging leaves, and stressed tissues. Because it is a gas, it spreads through air spaces and can leave one fruit and affect another. In climacteric fruits such as banana, tomato, and apple, ethylene promotes ripening: softening, a rise in sugar, and a color change. A ripe banana in a closed bag releases ethylene that ripens green fruit beside it. Ethylene is not a liquid nutrient poured on roots to feed the plant. Products that release the gas can trigger ripening, but the signal is the gas, not a mineral fertilizer.

Ethylene also promotes leaf abscission. As a leaf ages, the blade makes less auxin, and the abscission zone at the base of the petiole becomes sensitive to ethylene. Cells in that zone weaken, and the leaf drops. Name ethylene for ripening and for that separation.

Drought in the pot, fruit in the bowl

Two events can happen on the same afternoon and still use different hormones. Dry soil raises abscisic acid, guard cells lose turgor, and stomata close, so the plant keeps more of its water. A ripe banana on the counter gives off ethylene, and nearby green bananas soften and sweeten. The drought response is abscisic acid. The ripening response is ethylene. Auxin did not shut the pores, and gibberellin did not ripen the fruit.