Lecture 14: Control Systems in Plants

1. Plant Responses to the Environment

•         At every stage in the life of a plant, sensitivity to the environment and coordination of responses are evident.

•         One part of a plant can send signals to other parts.

•         Plants can sense gravity and the direction of light.

•         A plant’s morphology and physiology are constantly tuned to its variable surroundings by complex interactions between environmental stimuli and internal signals

•         At the organismal level, plants and animals respond to environmental stimuli by very different means

•         Animals, being mobile, respond mainly by behavioral mechanisms, moving toward positive stimuli and away from negative stimuli.

•         Rooted in one location for life, a plant generally responds to environmental cues by adjusting its pattern of growth and development.

•         Plants of the same species vary in body form much more than do animals of the same species.

•         At the cellular level, plants and all other eukaryotes are surprisingly similar in their signaling mechanisms.

•         All organisms, including plants, have the ability to receive specific environmental and internal signals and respond to them in ways that enhance survival and reproductive success.

•         Like animals, plants have cellular receptors that they use to detect important changes in their environment.

•         These changes may be an increase in the concentration of a growth hormone, an injury from a caterpillar munching on leaves, or a decrease in day length as winter approaches.

•         In order for an internal or external stimulus to elicit a physiological response, certain cells in the organism must possess an appropriate receptor, a molecule that is sensitive to and affected by the specific stimulus.

•         Upon receiving a stimulus, a receptor initiates a specific series of biochemical steps, a signal transduction pathway.

•         This couples reception of the stimulus to the response of the organism.

•         Plants are sensitive to a wide range of internal and external stimuli, and each of these initiates a specific signal transduction pathway.

2. Signal transduction pathways link internal and environmental signals to cellular responses

•         Signals, whether internal or external, are first detected by receptors, proteins that change shape in response to a specific stimulus.

•         The receptor for greening in plants is called a phytochrome, which consists of a light-absorbing pigment attached to a specific protein.

•         Unlike many receptors, which are in the plasma membrane, this phytochrome is in the cytoplasm.

•         Ultimately, a signal-transduction pathway leads to the regulation of one or more cellular activities.

•         In most cases, these responses to stimulation involve the increased activity of certain enzymes.

•         This occurs through two mechanisms: stimulating transcription of mRNA for the enzyme or by activating existing enzyme molecules (post-translational modification).

•         In transcriptional regulation, transcription factors bind directly to specific regions of DNA and control the transcription of specific genes.

•         In the case of phytochrome-induced greening, several transcription factors are activated by phosphorylation, some through the cyclic GMP pathway, and others through the Ca2+-calmodulin pathway.

•         Some of the activated transcription factors increase transcription of specific genes, others deactivate negative transcription factors which decrease transcription.

3.  Plant hormones help coordinate growth, development, and responses to environmental stimuli

•         Found in all multicellular organisms, hormones are chemical signals that are produced in one part of the body, transported to other parts, bind to specific receptors, and trigger responses in targets cells and tissues.

•         Only minute quantities of hormones are necessary to induce substantial change in an organism.

•         Often the response of a plant is governed by the interaction of two or more hormones.

•         In general, plant hormones control plant growth and development by affecting the division, elongation, and differentiation of cells.

•         Some hormones also mediate shorter-term physiological responses of plants to environmental stimuli.

•         Each hormone has multiple effects, depending on its site of action, its concentration, and the developmental stage of the plant.

•         include auxin, cytokinins, gibberellins, abscisic acid, ethylene, and brassinosteroids.

•         Many molecules that function in plant defense against pathogens are probably plant hormones as well.

•         Plant hormones tend to be relatively small molecules that are transported from cell to cell across cells walls, a pathway that blocks the movement of large molecules.

•         Plant hormones are produced at very low concentrations.

•         Signal transduction pathways amplify the hormonal signal many fold and connect it to a cell’s specific responses.

•         These include altering the expression of genes, by affecting the activity of existing enzymes, or changing the properties of membranes.

•         According to the acid growth hypothesis, in a shoot’s region of elongation, auxin stimulates plasma membrane proton pumps, increasing the voltage across the membrane and lowering the pH in the cell wall.

•         Lowering the pH activates expansin enzymes that break the cross-links between cellulose microfibrils.

•         Increasing the voltage enhances ion uptake into the cell, which causes the osmotic uptake of water

•         Uptake of water with looser walls elongates the cell.

•         A change in the balance of ethylene and auxin controls abscission.

•         An aged leaf produces less and less auxin and this makes the cells of the abscission layer more sensitive to ethylene.

•         As the influence of ethylene prevails, the cells in the abscission layer produce enzymes that digest the cellulose and other components of cell walls. 

•         A chain reaction occurs during ripening: ethylene triggers ripening and ripening, in turn, triggers even more ethylene production - a rare example of positive feedback on physiology.

•         Because ethylene is a gas, the signal to ripen even spreads from fruit to fruit.

•         Fruits can be ripened quickly by storing the fruit in a plastic bag, accumulating ethylene gas or by enhancing ethylene levels in commercial production.

•         Alternatively, to prevent premature ripening, apples are stored in bins flushed with carbon dioxide, which prevents ethylene from accumulating and inhibits the synthesis of new ethylene. 

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4. Plant responses to light

•         Light is an especially important factor on the lives of plants.

•         In addition to being required for photosynthesis, light also cues many key events in plant growth and development.

•         These effects of light on plant morphology are what plant biologists call photomorphogenesis.

•         Light reception is also important in allowing plants to measure the passage of days and seasons.

•         Plants detect the direction, intensity, and wavelengths of light.

•         The photoreceptor responsible for these opposing effects of red and far-red light is a phytochrome.

•         It consists of a protein (a kinase) covalently bonded to a nonprotein part that functions as a chromophore, the light absorbing part of the molecule.

•         The chromophore reverts back and forth between two isomeric forms with one (Pr) absorbing red light and becoming (Pfr), and the other (Pfr) absorbing far-red light and becoming (Pr).

•         This interconversion between isomers acts as a switching mechanism that controls various light-induced events in the life of the plant.

•         Long-day and short-day plants are distinguished not by an absolute night length but by whether the critical night lengths sets a maximum (long-day plants) or minimum (short-day plants) number of hours of darkness required for flowering.

•         In both cases, the actual number of hours in the critical night length is specific to each species of plant.

•         While the critical factor is night length, the terms “long-day” and “short-day” are embedded firmly in the jargon of plant physiology.

 

5. Plant responses to other environmental variables

•         Much of the plant’s response to a water deficit helps the plant conserve water by reducing transpiration.

•         As the deficit in a leaf rises, the guard cell lose turgor and the stomata close.

•         A water deficit also stimulates increased synthesis and release of abscisic acid in a leaf, which also signals guard cells to close stomata.

•         Because cell expansion is a turgor-dependent process, a water deficit will inhibit the growth of young leaves.

•         As many plants wilt, their leaves roll into a shape that reduces transpiration by exposing less leaf surface to dry air and wind.

•         These responses also reduce photosynthesis.

 

•         Plants in flooded soils can suffocate because the soil lacks the air spaces that provide oxygen for cellular respiration in the roots.

•         Some plants are adapted to very wet habitats.

•         Mangroves, inhabitants of coastal marshes, produce aerial roots that provide access to oxygen.

•         Less specialized plants in waterlogged soils may produce ethylene in the roots causing some cortical cells to undergo apoptosis, which creates air tubes that function as “snorkels” (aerenchyma tissue – which is modified parenchyma)

•         Excessive heat can harm and eventually kill a plant by denaturing its enzymes and damaging its metabolism.

•         Transpiration helps dissipate excess heat through evaporative cooling, but at the cost of possibly causing a water deficit in many plants.

•         Closing stomata to preserve water sacrifices evaporative cooling.

•         Respiration increases faster with temperature than photosynthesis