Plant Hormones and Growth Regulators

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all tissues in plants originate from ___

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meristematic tissue

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plant tissue mostly ___

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totipotent: can regenerate entire plant

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21 Terms

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all tissues in plants originate from ___

meristematic tissue

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plant tissue mostly ___

totipotent: can regenerate entire plant

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meristems

are regions of continuous cell division and growth located in different points of the plant. There are 3 types:

• Apical meristems

• Lateral meristems

• Intercalary meristems (only in grasses)

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branching

• Monopodial: buds do NOT degrade, and all shoots continue to grow.

• Sympodial: terminal buds degrade, and lateral buds closest to the apex become the new terminal shoot

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development and growth of plants depends on

-internal conditions: Heredity, age and position- For example, the shape of leaves tends is an inherited trait common across members of a species

-external environment: Light, wind, temperature, etc- For example, wind-blown trees in sub-alpine areas form what is known as krummholz

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development includes

- Growth: cell division and enlargement

- Differentiation: specialization of cells (via expression of genes)

- Induced changes: modifications to cells through the life of the plant (e.g. preparation for winter)

-Development requires the coordination of cell activity with the movement of resources.

-This coordination is mediated by signals (e.g. hormones,electric pulses, hydraulic impulses)

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totipotency

(cell potency) is the ability of a cell to differentiate into other cell types. Animal cells are only totipotent early in the development of the zygote (the become determined) While plant cells also become 'partially' determined -determination of some cells is reversible. For example:

• Adventitious shoots and roots

• Protoderm might divide and become part of the

ground tissue

• Tissue culturing

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plant hormones:

(AKA plant growth regulators) are substances that provide long-range signaling to coordinate growth, development and response to environmental stimuli

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Target cells

(those affected by the hormone) have receptor proteins with binding sites

• Activation of a receptor leads to an increase in calcium in the cytoplasm

• Higher calcium activates enzymes

• Enzymes can play several roles – e.g. the activation of transcription factors that regulate gene expression

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five important plant hormones

Auxins

• Gibberellin (or Gibberellic acid)

• Cytokinin

• Abscisic acid (or ABA)

• Ethylene

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Auxins

A class of hormones that play an essential role in regulating growth (e.g. lateral root growth) and responses to the environment (e.g. gravitropism)

• Auxins are produced in the shoot apical meristems and in young leaves

• Plays a key role in apical dominance: inhibits growth of axillary buds

• High levels of auxin near the apical meristem prevent branching

• Levels decrease further down – releasing axillary buds

• Pruning also releases apical dominance

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auxins cond.

also play a crucial role in:

• Plant responses to environmental cues:

- Phototropism: movement with respect to light sources

- Gravitropism: movement with respect to direction of gravitational forces

• Initiates lateral roots

• Promotes xylem differentiation

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Gibberellin

• Shared by plants and fungi!

- Discovered by Japanese physiologists when studying a disease of rice caused by the fungus Gibberella fujikuroi

• Also produced in shoot apical meristem and young leaves, as well as in the embryo

• Plays a key role in the division and elongation of cell in the internode areas.

• In addition, gibberellins promote germination of dormant seeds

• Promotes phloem differentiation

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Cytokin

• A class of plant hormones that often play a complementary role to auxins - e.g. growth of axillary buds depends on the ration of auxin to cytokinin.

• Produced in the root apical meristem, and developing fruits

• Its main function is to promote cell division(cytokinesis)

- additionally, it diverts nutrients towards areas with high concentrations.

• It also prevents senescence by reducing protein breakdown

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Abscisic acid

• Produced in leaves and ovules

• ABA levels increase as water decreases

-prompting several responses including the closure of the stomata

• It also plays a key role in preventing seed germination and promoting dormancy. Higher ABA levels in the seeds:

- allows them to survive low water conditions

- prevents premature germination

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Ethylene

• An unusual hormone which exists as a gas in normal conditions

• Produced in damaged (wounded or aged) tissue!

• One of its main roles is to induce fruit ripening

• Another key function of ethylene is to signal for leaf and flower senescence

• It is also involved in the formation of root hairs

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homeostasis

Some plant hormones have opposite effects, and their balance determines the processes that take place:

• Auxins (made in the shoot - stimulate root growth)VS. Cytokinins (made in the root - stimulate leaf/axillary bud growth)

• Gibberellins (promotes growth of stems and fruit)VS. Absicisic Acid (inhibits growth until conditions improve)

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regulation of shoot and root growth

Lower auxin levels down the stem allow for cytokinin to activate axillary buds. Lateral root formation require higher auxin concentrations - this is provided by the combination of apical meristems in ALL branches(including lateral branches)!

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Gibberellin and rosette growth form

In their vegetative state, some plants (like lettuce and thistles), form rosettes – plants with internodes so short, leaves seem to grow from the same point! When reproductive, the flower-bearing stems (scapes) elongate quickly (bolting)Bolting is usually initiated by an environmental signal(e.g. changes in day length) and it induces an increase in Gibberellin.

• Note: bolting can be induced by spraying a plant with Gibberellin

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seed development and germination

Abscisic acid plays a key role in the formation of viable seeds:

• It induces the production of hormones that will provide nitrogen and energy to the growing embryo

• It accumulates in the seed coat, preventing the seed from germinating (dormancy)

-Gibberellin promotes germination by stimulating cell division and elongation in the embryo. Other mechanisms might include:

• Promote breakdown of storage materials

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Senescence

Ethylene upregulates genes leading to an the synthesis of chlorophyllase and protease - enzymes that breakdown chlorophyll and proteins respectively.

In many cases, senescence also involves abscission -which takes place as enzymes digest cell walls at the base of the petiole!

Fruit ripening follows a similar process - ethylene promotes the synthesis of enzymes which turn starch into sugars and soften cell walls.

Ethylene produced in one fruit can induce senescence in other fruits