Plant Hormones and Growth Regulators

all tissues in plants originate from ___

meristematic tissue

plant tissue mostly ___

totipotent: can regenerate entire plant

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)

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

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

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)

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

plant hormones:

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

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

five important plant hormones

Auxins

• Gibberellin (or Gibberellic acid)

• Cytokinin

• Abscisic acid (or ABA)

• Ethylene

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

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

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

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

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

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

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)

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)!

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

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

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