BISC 101 Lecture Notes - Lecture 24: Cytokinin, Auxin, Signal Transduction

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BISC 101 – Lecture 24 – Plant Responses to Internal and External Signals
Signal Transduction
Signal transduction pathways link signal reception to response
Plants have cellular receptors that detect changes in their environment
For a stimulus to elicit a response, certain cells must have an appropriate receptor
Stimulation of the receptor initiates a specific signal transduction pathway
oEx: A potato left growing in darkness produces shoots that look unhealthy and
lacks elongated roots
oEtiolation: Morphological adaptations for growing in darkness
After exposure to light, a potato undergoes changes called detiolation,
oDetolliation: Process in which shoots and roots grow normally
A potato’s response to light is cell-signal processing, in which the stages are:
1. Reception
2. Transduction
3. Response
Reception, Transduction and Response
Receptors: Proteins that change in response to specific stimuli
oInternal and external signals are detected by shape match receptors, which
are proteins that change in response to specific stimuli
Two basic types of signals
1. Cytokines: Small, intracellular signaling proteins
Cytokines bind to specific receptors on the surfaces of specific cells
Binding of cytokine to its receptor sends a signal to the nucleus,
telling certain genes to either turn on or turn off
Since cytokines can only signal cells that are expressing the proper
receptor on their surfaces, the actions of cytokines tend to be more
specific than the actions of hormones
2. Hormones: Intracellular signaling molecules and are composed of either
proteins, modified lipids, or modified amino acids
Hormones that are made of lipid are able to cross cell membranes
directly, without any need for receptors
They then usually go directly to the nucleus and bind to an inactive
form of a transcription factor called a ‘nuclear receptor
Binding of a hormone to a nuclear receptor for that specific hormone
will convert the nuclear receptor into an active transcription factor
that can activate transcription of a specific gene
Since some hormones can cross cell membranes without the help of
specific receptors, the actions of hormones tend to be more general
and wide spread than the actions of cytokines
Second Messengers: Small molecules or ions that transfer and amplify signals from
receptors to proteins to cause responses
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Response: A signal transduction pathway leads to regulation of one or more cellular
activities
oThese responses to stimulation involve increased activity of enzymes
oThis can occur by transcriptional regulation or post-translational
modification
Transcriptional Regulation: Specific transcription factors bind directly to specific
regions of DNA and control transcription of genes
oPositive Transcription Factors (Activators): Proteins that increase the
transcription of specific genes
oNegative Transcription Factors (Repressors): Proteins that decrease the
transcription of specific genes
Post-Translational Modification: Involves processing of existing proteins in the
signal response
oIncludes proteolytic cleavage of existing protein by breaking down into
smaller polypeptides (involves hydrolysis of peptide bonds) or addition of
a modifying group (i.e., phosphorylation)
Plant Hormones
Plant hormones help coordinate growth, development, and responses to stimuli
Hormones: Chemical signals that coordinate different parts of an organism
Any response resulting in curvature of organs toward or away from a stimulus is
called a tropism
oTropism: A growth response
oEx: Phototropism refers to a plant’s response to light and gravitropism
refers to a plant’s response to gravity
Tropisms are often caused by hormones
In general, hormones control plant growth and development by affecting cell
division, cell elongation, and cell differentiation
Plant hormones are produced in very low concentrations, but a very small amount
can greatly affect growth and development of a plant organ
Auxin
Auxin: A class of plant hormone that promotes cell elongation in stems and roots
Auxin is also involved in:
oRoot formation and branching
oAffecting secondary growth by inducing cell division in the vascular
cambium and influencing differentiation of secondary xylem
oGravitropism and phototropism
Auxins only cause cell elongation at LOW concentrations
At high concentrations, auxins induce the production of ethylene
o Ethylene: Gas that promotes plant senescence (dormancy or death)
o Due to this effect at high concentrations, it does explain why auxins can be
used as herbicides
The role of auxin in cell elongation:
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