Plant Control Systems and Defenses

Plant Hormones

Plant hormones control growth, development, and reproduction.
In the same way that animals have chemical substances that control growth and development, so too do plants. A plant growth regulator (PGR) is a chemical compound that signals growth and development in plants. Also called a hormone, PGRs regulate plant functions, such as flowering, seed production, plant metabolism, and ripening of fruit. A phytohormone is a chemical substance in plants that signals growth and development.

Plant and animal hormones have some similarities and some differences. In both plants and animals, a low concentration of a hormone is a signal for growth and reproduction. In animals, hormones are produced in glands and then transported throughout the body. Unlike animals that have circulatory systems, however, plants have no full-organism transport system. Therefore, some plant hormones are found only where they are locally used, while others move through xylem and phloem (transport mechanisms in vascular plants) linked to the water transport system. Furthermore, phytohormones do not work in the same manner as animal hormones. Phytohormones enable the plant to respond to stressful environmental conditions, such as extreme heat, cold, drought, or a change in water supply.

Functions of Plant Hormones

Plant hormones signal plants to undergo changes such as seed germination or winter dormancy.
Throughout a plant's life, hormones trigger all key events: growth spurts, flowering, seed or spore development, and death.

Hormones begin working in a plant at the point of seed germination (when the organism begins to grow from the seed). Seeds remain dormant, with most physical functions suspended except the basics required for survival, until the specific conditions to germinate are present. These conditions include air and soil temperature, moisture, and nutrition. Additionally, seeds need signals from hormones in both the plant seed and soil bacteria to end dormancy and begin the germination process. Sending down roots and thrusting up shoots are processes initiated by hormonal signals.

The role hormones play is most evident in situations involving seasonal and environmental changes. In autumn, for example, deciduous trees undergo preparation for winter when the amount of sunlight is reduced, temperatures drop, and water moves less actively through soil. Hormones signal the need for leaves to cease producing chlorophyll, which causes the leaves to change color, and at the same time, essential elements are drawn to parenchyma cells to be stored for the winter. The leaves drop off, and the tree enters a period of dormancy over the winter, a state also signaled by hormones. When spring comes, snowmelt or rainfall provides abundant water, and temperatures rise. At that time, hormones signal the growth of leaf buds or flowers on trees, including the release of essential elements stored over the winter and needed to support new leaf growth. By the time a tree is in full leaf mode, hormones are signaling periods of new vertical or lateral growth as well as seed and fruit development.

Environmental adaptations are also hormonally driven. When drought occurs, the lack of sufficient water triggers hormonal changes within plants. Roots grow deeper or broader into soil in search of more water. Fruit and seed development slow down because these processes require too much energy and resources. Additionally, hormones signal the stomata on leaves to close and reduce water loss. Like the adjustments made for winter conditions, plants may go into a dormant state during serious drought to ensure survival. Phytohormones also allow plants to adapt and thrive in their environments. Mesquite trees, for example, have taproots that may reach over 30 meters below the surface to locate underground water resources. Other plants, such as barrel cactus and other succulents, take in as much water as possible during wet periods and store that water in roots, leaves, or stems. Each of these adaptations results from hormones driving the processes to keep plants alive and thriving.

Leaf Abscission

Hormones signal leaf-color changes in deciduous trees. (1) The leaf changes color as chlorophyll production slows down and other chemical pigments account for the change from green to yellow, orange, or red. (2) The leaf and petiole (small branch that attaches the leaf to the branch) break off at the abscission zone where the separation layer occurs. (3) The protective layer also seals the tree with a leaf scar. The axillary bud may or may not develop into a new shoot when spring arrives depending on hormonal growth signals in the tree.

Plant Hormones and their Functions

Plant Hormone Hormone Function
Abscisic acid Seed and bud dormancy, water-loss regulation, seed-storage protein synthesis, closes stomata during water stress
Auxin Phototropism, plant stem and root growth geotropism, apical dominance, vascular differentiation, cell elongation; stimulates release of ethylene
Brassinosteroids Regulation of cell division, promote flowering
Cytokinins Delay aging, activate dormant buds, affect cell division
Ethylene Fruit ripening, promotes leaf abscission, triple response in seedlings
Florigen Stimulates flowering and seed production
Gibberellins Promote cell elongation, affect pollen development, promote fruit growth, regulate sex identification
Jasmonic acid Seed germination, root growth, seed-storage protein synthesis
Salicylic acid Activates genes involved in plant defenses
Strigolactones Stimulate seed germination, control apical dominance, attract mycorrhizae

While many hormones have their individual roles in plant processes, most work together to ensure a healthy plant life, such as jasmonic acid and abscisic acid both stimulating seed-storage protein synthesis.

Auxin

Auxin influences plant cell growth and cell expansion.
Auxin is a plant growth hormone that controls root growth, bud formation, and fruit maturity. As light shines on a plant, auxin moves in the opposite direction and signals cell elongation on the shaded side. This elongation pushes the stem toward the light. Auxin loosens cell walls, which enables cells to elongate. Auxin was first identified in the late 1800s when English naturalist Charles Darwin and his son performed experiments to study how grass seedlings bent toward the sun. If the tip of a blade of grass was intact and available to sunlight, the blade bent toward the sun. They suggested that a substance in the grass affected this outcome. Today, scientists have demonstrated that auxin is responsible for a plant's ability to move toward light.

Auxin travels within a plant from the apical tip (the top of the stem) to the base. The hormone is produced in the tip and encourages the growth of new branches. In addition, auxin also defines the positioning of leaves on a branch and the vascular veining within a leaf. Lateral expansion of woody stems, dictated by lateral meristems (the growth control mechanism for increasing width), is guided by auxin as well.

Auxin, sugar, and other hormones control apical dominance, the suppression of lateral bud development on growing shoots. This can stall or stop the growth of an axillary bud—a structure at the axil (angle between a leaf and the branch it attaches to) of a branch that may develop into a new branch. The apical tip of a shoot requires high amounts of sucrose, a crystalline sugar that is produced during photosynthesis. As sucrose demand increases, bud development and release is expected to increase as well. Cutting off the apical tip removes the demand for sucrose, and auxin and other hormones signal growth patterns for new buds, branches, and their leaves.

How Auxin Affects Plants

Auxin is a plant growth hormone that controls root growth, bud formation, and fruit maturity. Auxin loosens cell walls, which enables cells to elongate. As light shines on a plant, auxin moves in the opposite direction and signals cell elongation on the shaded side. This elongation pushes the stem toward the light.

Abscisic Acid

Abscisic acid is a plant's stress hormone, synthesized when the plant faces environmental stressors.
Abscisic acid (ABA) is a plant growth hormone that controls dormancy and leaf aging. It is a stress hormone synthesized when plants face environmental stressors, and it encourages dormancy in buds, underground stems, and seeds, particularly when plants are faced with drought or have become waterlogged. Deciduous trees are affected by ABA when the hormone signals cambium growth (actively dividing cells that are responsible for the secondary growth of stems and roots) to cease at winter's approach. ABA prevents water loss in plants by signaling guard cells surrounding stomata to close the portals and retain water. The hormone also encourages roots to form at the locations where the stem is cut or broken, prolongs dormancy, and induces flowering in some plants. Applications of ABA are helpful to crops because they can encourage the closure of stomata, thus reducing the need to constantly irrigate crops.

Cytokinin

Cytokinins affect cell division and the formation of new plant organs.
Cytokinin is a class of plant hormones that influences tissue growth. It affects cell division and the formation of new plant organs, such as roots and shoots. This hormone is synthesized in the apical meristems (plant mechanisms that control the growth of addition stems and leaves) of roots and moves upward through the xylem to other plant parts. Two important roles of cytokinins are delayed aging in plants by mobilizing nutrients to needed plant organs (phloem transport) and promoting plant repairs when leaves, branches, or stems suffer injury. Cytokinins promote femaleness in flowers—which is the dominance of female organs at seed and fruit development—and they increase plant resistance to temperature changes and infection. Cell division involves auxin in conjunction with cytokinin. If there is more auxin available than cytokinin, plants expand their root systems. When the concentration of cytokinin is greater than auxin, plants send up shoots. Applications of cytokinins to recently picked fruits, vegetables, and flower stems increases the shelf life of produce.

Gibberellin

Gibberellins encourage plant growth.
Gibberellin is one of a class of plant hormones that regulates stem length, seed germination, flowering, sexual expression, and leaf and fruit ripening. Gibberellins, also known as gibberellic acids, affect plants in a similar way as auxin. They encourage growth and, if uncontrolled, will cause plants to grow too tall to support themselves. The primary role of gibberellins is in internode growth—the area of plant stems between nodes (the locations on a plant stem that holds one or more leaves and is the location from which branches can grow). While auxin promotes branching and height growth from nodes, gibberellins signal the need to lengthen stems between nodes. Knowledge of this hormone's effects is important in agriculture, particularly where growth is advantageous. Taller stalks of sugar cane from added application of gibberellic acid, for example, bring a higher profit to a farmer than shorter stalks. Gibberellins also play a role in the germination and growth of grains, such as barley and wheat.

Commercially, gibberellins are used for developing fruit and seed production. Fruit development on plants requires the combined signals of auxin and gibberellins. In agricultural conditions, applications of gibberellins increase the size and quantity of fruit. This is common practice in some seedless grape vineyards, red delicious apple orchards, and navel orange groves. Seed production can be stimulated by the application of specific gibberellic acids. Conifers, necessary for rebuilding forests, increase seed production when sprayed with gibberellic acids. For vegetables, such as beets and cabbage, gibberellins speed up seed production time.

Ethylene

Ethylene is a gaseous hormone that stimulates ripening and aging.
Ethylene is a plant hormone that controls ripening of fruit. It is a gaseous hormone that is produced in all plant parts, from roots to fruits. Ethylene is most closely associated with ripening and aging. It speeds up the aging and decay of plant matter, or senescence, and stimulates abscission, the natural separation of flowers, fruit, or leaves from the parent plant. It also encourages fruits to ripen. Seeds, buds, and storage organs come out of dormancy because of ethylene. While ethylene allows fruit to naturally ripen on trees, shrubs, and vines, it is also used commercially to encourage artificial ripening of apples, mangoes, and bananas that are picked early for transport purposes. Ethylene also encourages the natural thinning of some plants, such as cotton, cherries, and walnuts, which enhances the growth of the remaining fruit. Ethylene is also essential for a process called the triple response, which is the reaction of a plant shoot to avoid an obstacle to growth. This triple response gives a plant shoot the ability to avoid obstacles (such as stones, cement, or thick tree roots) as it grows. A germinating seed sends roots downward and shoots upward. When a shoot comes upon, for example, a slab of cement, a mechanical stress is exerted on the shoot. The plant, however, has a mechanism to overcome such obstacles. Ethylene production stimulates a growth maneuver—the triple response—which (1) slows stem elongation, (2) thickens and strengthens the shoot, and (3) initiates shoot curvature that makes the stem grow horizontally. Horizontal growth continues until the shoot sends up a tip that finds clear passage to the surface. At this point, the stress is removed, ethylene production ceases, and the sprout begins growing vertically again.

Ethylene and Fruit Ripening

The formation and ripening of fruit and seeds involves an auxin signal to begin fruit development by cell division and cell expansion. Abscisic acid signals cause changes in cell walls, sugar production, and pigment changes. Auxin and abscisic acid are both plant hormones that are involved with fruit ripening. Ethylene, the plant hormone that controls fruit ripening as well, has recently been found to signal final ripening and senescence (decay) in strawberries.