Plant Responses to Internal and External Signals: Signal Transduction and Plant Hormones

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Ch. 39 Plant Responses to Internal and External Signals

Introduction

Plants constantly perceive and respond to a variety of internal and external signals. These responses are mediated by complex signaling pathways and plant hormones, which regulate growth, development, and adaptation to environmental changes.

Signal Transduction Pathways in Plants

Overview of Signal Transduction

Signal transduction is the process by which a cell converts an external or internal signal into a specific cellular response. In plants, this often involves the perception of environmental stimuli or hormones, leading to changes in gene expression and cellular activity.

Reception: A receptor protein detects a signal (such as a hormone or environmental stimulus) at the cell surface or within the cell.
Transduction: The signal is relayed by proteins and second messengers, amplifying and transmitting the signal through the cytoplasm.
Response: Activation of cellular responses, such as changes in gene expression, enzyme activity, or cell behavior.

Example: The de-etiolation (greening) response in plants is triggered by light, which activates a signal transduction pathway involving the photoreceptor phytochrome.

Signal Transduction Example: Role of Phytochrome in De-etiolation Response

Phytochromes are light-sensitive proteins that regulate plant development in response to red and far-red light. The de-etiolation response is a classic example of phytochrome-mediated signal transduction.

Light activates phytochrome, which initiates a signaling cascade.
Second messengers such as Ca2+ and cyclic GMP (cGMP) are produced.
These messengers activate protein kinases, leading to changes in gene expression and the greening of the plant.

Equation:

Example: Potato plants exposed to light undergo de-etiolation, developing green leaves and stems.

Plant Hormones

Definition and Function

Plant hormones are signaling molecules produced in small quantities in one part of the plant and transported to other parts, where they bind to specific receptors and trigger physiological responses. Plant biologists also refer to these as plant growth regulators.

Secreting cell: Produces the hormone.
Target cell: Receives the hormone and initiates a response.
Hormones travel through the plant via the vascular system or cell-to-cell movement.

Example: Auxin produced in the shoot tip is transported downward, influencing cell elongation and growth.

Major Plant Hormones and Their Functions

Several classes of plant hormones regulate growth, development, and responses to environmental stimuli. Each hormone has distinct roles and effects.

Plant Hormone	Major Responses
Auxin	Stimulates cell elongation; regulates branching and organ bending.
Cytokinins	Stimulate plant cell division; promote later bud growth; slow organ death.
Gibberellins	Promote stem elongation; help seeds break dormancy and use stored reserves.
Brassinosteroids	Chemically similar to sex hormones of animals; induce cell elongation and division.
Abscisic acid	Promotes stomatal closure in response to drought; promotes seed dormancy.
Strigolactones	Regulate apical dominance, seed germination, and mycorrhizal associations.
Ethylene	Mediates fruit ripening.

Auxin: Discovery and Function

Auxin is a key plant hormone involved in cell elongation and directional growth. The classic experiment by Fritz Went (1926) demonstrated that a growth-promoting chemical (auxin) diffuses from the tip of a plant shoot and causes bending toward light (phototropism).

Auxin is produced in the shoot tip and moves downward.
Uneven distribution of auxin causes cells on the shaded side to elongate more, resulting in bending toward light.

Example: When a plant is exposed to light from one direction, auxin accumulates on the shaded side, causing the stem to bend toward the light source.

Gibberellins: Role in Growth and Development

Gibberellins are plant hormones that promote stem elongation, seed germination, and fruit development. They are essential for breaking seed dormancy and mobilizing stored nutrients during germination.

Gibberellins stimulate the transition from vegetative to flowering growth (bolting).
They promote the growth of larger fruits, as seen in gibberellin-treated grapevines.
During seed germination, gibberellins trigger the breakdown of starch into sugars for the growing embryo.

Example: Application of gibberellins to grapevines results in larger grapes compared to untreated vines.

Equation:

Plant Hormones and Growth Regulation

Hormonal Control of Growth

Plant hormones coordinate the control of cell division, elongation, and differentiation, influencing overall plant form and adaptation.

Auxin regulates cell elongation and directional growth.
Cytokinins promote cell division and delay aging.
Gibberellins stimulate stem elongation and seed germination.
Abscisic acid induces dormancy and stress responses.
Ethylene mediates fruit ripening and response to mechanical stress.

Example: Roots exhibit positive gravitropism (growth toward gravity), while stems show negative gravitropism (growth away from gravity), both regulated by hormonal signaling.

Summary Table: Major Plant Hormones and Their Functions

Hormone	Main Function
Auxin	Cell elongation, phototropism, gravitropism
Cytokinins	Cell division, delay of senescence
Gibberellins	Stem elongation, seed germination, fruit growth
Abscisic acid	Stomatal closure, seed dormancy
Ethylene	Fruit ripening, response to stress

Additional info:

Phototropism is the growth of a plant toward or away from light, primarily regulated by auxin.
Gravitropism is the growth response to gravity, with roots showing positive gravitropism and stems showing negative gravitropism.
De-etiolation is the process by which a plant develops chlorophyll and becomes green upon exposure to light.