Plant Hormones - Student Notes - PDF

Plant Hormones Vinicius Azevedo Plant sensory systems respond to a wide variety of environmental stimuli What plants can sense? Plants can sense and respond to information about light, gravity, pressure, and wounds. Plants have the equivalent of a sense of smell and can perceive certain airborne mol...

Plant Hormones Vinicius Azevedo Plant sensory systems respond to a wide variety of environmental stimuli What plants can sense? Plants can sense and respond to information about light, gravity, pressure, and wounds. Plants have the equivalent of a sense of smell and can perceive certain airborne molecules. Plants have a sense of taste because their roots can detect nutrients in the soil. How do plants gather, process and respond to stimuli? 1. Sensory cells receive an external signal and change it into an intracellular signal 2. Sensory cells then send a signal to target cells in other parts of the plant body 3. Target cells receive the signal and change their activity to produce an appropriate response How does information from an activated sensory cell get to a target cell? Usually, a hormone is transported to the target cells, where it causes a physiological response. Hormones can be hydrophobic or hydrophilic, They can be gases, amino-acid derivatives, or peptides (proteins). How are they transported? ü By specialized transport proteins in cell membranes ü In xylem sap ü In phloem sap ü By simple diffusion Plant cells may receive several hormones at once ü It is common for different hormones to interact with one another and modulate the cell’s response What hormones control? In general, hormones control plant growth and development by affecting cell division, cell elongation, cell differentiation, and cell death. Plant hormones are produced in very low concentrations, which can greatly affect the growth and development of a plant organ. Tropism Responses observed in plants ü It is a growth response. ü Any response resulting in curvature of organs toward or away from a stimulus. ü Positive tropism: growth towards the stimulus. ü Negative tropism: growth away from the stimulus. ü Types of tropism: o Phototropism: response to light (usually positive). Phototropism Gravity o Gravitropism: response to gravity (stem presents negative tropism while roots present positive tropism). o Thigmotropism: response to mechanical disturbance (can be positive or negative) Gravitropism Thigmotropism Responses observed in plants Dormancy ü Inhibition of plant growth, including inhibiting seed germination. Germination ü Development of plant from a seed or spore after a period of dormancy Flowering ü The process of producing flowers. Responses observed in plants Fruit maturation and ripening ü The process of fruits becoming more palatable (sweeter) and falling from the plant stem. Senescence ü Programmed death of plant cells or organs or an entire plant. Leaf abscission ü Process of leaf falling. Main Plant Hormones Auxin Cytokinins Abscisic acid Gibberellins Ethylene Auxin The term auxin refers to a class of plant hormones that promotes cell elongation in stems and roots. o According to the acid growth hypothesis, auxin stimulates proton pumps in the plasma membrane. o The proton (H+) pumps lower the pH in the cell wall, activating expansins, which are enzymes that break hydrogen bonds between cellulose fibres, thus loosening the cell wall. o With the cellulose loosened, the cell is free to absorb more water, causing it to swell and elongate Auxin Auxin is also involved in: üRoot formation and branching. üSecondary growth by inducing cell division in the vascular cambium and influencing differentiation of secondary xylem. üGravitropism and phototropism. Cytokinins Cytokinins stimulate cytokinesis (cell division which divides the cytoplasm of a parental cell into two daughter cells) Control of Cell Division and Differentiation üCytokinins are produced in actively growing tissues such as roots, embryos, and fruits. üCytokinins regulate growth by activating genes that keep the cell cycle going, stimulating cell division. üCytokinins work together with auxin to control cell division and differentiation (cytokinins alone have no effect!) Cytokinins Control of Apical Dominance üCytokinins, auxin, and other factors interact in the control of apical dominance, a terminal/apical bud’s ability to suppress the development of axillary/lateral buds so that the plant can grow vertically üIf the terminal bud is removed, plants become bushier as lateral buds grow and develop into new shoots Anti-Aging Effects üCytokinins delay the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and assembling nutrients from surrounding tissues o If removed leaves from plant are dipped in cytokinin solution, they stay green much longer o Also, cytokinins slow deterioration of leaves on intact plants (can be used to spray on cut flowers to keep fresh) Gibberellins Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination. Stem Elongation üGibberellins stimulate the growth of leaves and stems. üIn stems, they stimulate cell elongation and cell division. Fruit Growth üIn many plants, both auxin and gibberellins must be present for fruit to set. o For example, gibberellins are used in the spraying of Thompson seedless grapes. Gibberellins Germination üAfter water is taken up by a dry seed (imbibition), gibberellins are released from the embryo signals to allow for seed germination. o The embryo releases gibberellin to the aleurone (outer layer of the seed coat). o The aleurone layer secretes alpha-amylase (digestive enzyme) to hydrolyze sugars in the endosperm…the released sugars are then used by the growing embryo. o Sugars are then absorbed by the scutellum (cotyledon), which initiates the embryo to grow. Embryo Abscisic acid Abscisic acid (ABA) slows growth (so antagonistic to plant growth hormones like auxin) Seed dormancy üABA inhibits plant growth (including inhibiting seed germination) o Adding ABA to the aleurone layer decreases alpha-amylase levels üIn some seeds, dormancy is broken when ABA is removed by heavy rain, light, or prolonged cold Drought tolerance üABA is the primary internal signal that enables plants to withstand drought üPlant begins to wilt à ABA accumulates in leaves à rapid closing of stomata à reduces transpiration (prevents further water loss) ABA inhibits both H+ATPases and inwarddirected K+ channels Ethylene Ethylene is a plant hormone in the form of a gas that is released from plant tissues (antagonistic to plant growth hormones like auxin) Plants produce this hormone in response to stresses such as drought, flooding, mechanical pressure, injury, and infection. The effects of ethylene include response to fruit ripening, mechanical stress, senescence, and leaf abscission. Fruit Ripening A burst of ethylene production in a fruit triggers the ripening process (enzymatic breakdown of cell wall softens fruit, conversion of starches/acids to sugars make them sweet) Moving air prevents ethylene accumulation…more specifically, carbon dioxide prevents ethylene production and slows the ripening of stored fruits. Ethylene Senescence Ethylene üProgrammed death of plant cells or organs or the entire plant üA burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plants Leaf Abscission üA change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls Low auxin concentrations and high ethylene concentrations are involved in abscission and senescence As auxin levels drop, the abscission zone in the leaf petiole becomes more sensitive to ethylene üIncreased ethylene sensitivity activates enzymes that weaken the walls of cells in the abscission zone üEventually, the cell walls degrade enough that the leaf falls Ethylene The Triple Response to Dark and Mechanical Stress üEthylene induces the triple response, which allows a growing shoot to avoid obstacles üThe triple response consists of a slowing of stem elongation, a thickening of the stem (making it stronger), and horizontal growth (curvature) Responses to light are critical for plant success Light cues many key events in plant growth and development There are two major classes of light receptors: blue-light photoreceptors and phytochromes Blue-light photoreceptors are stimulated by light in the blue wavelength üThey control stomatal opening, chlorophyll synthesis, and phototropism (induce production of auxins) Phytochromes are stimulated by light in the red wavelength (but turned off by far-red light) üBiologically active state of phytochrome control seed germination, shade avoidance, induction of flowering, chloroplast development (not including chlorophyll synthesis), leaf senescence and leaf abscission Phytochromes exist in two photoreversible states, with the conversion of Pr to Pfr triggering many developmental responses: ü ü ü ü Red light triggers the immediate conversion of Pr to Pfr Far-red light triggers the slow conversion of Pfr to Pr Sunlight increases the ratio of Pfr to Pr Pfr is biologically active form while Pr is biologically inactive form 660 nm 730 nm How Do Red Light and Far-Red Light Affect the Germination of Lettuce Seeds? Biologists exposed moistened lettuce seeds to flashes of light containing one of two wavelengths in sequence: red and far-red (FR). After exposure to the last flash of light, the seeds were held in the dark for several days. Light Exposure Sequence Germination (%) None (control) 9 Red 98 Red → FR 54 Red → FR → Red 100 Red → FR → Red → FR 43 Red → FR → Red → FR → Red 99 Red → FR → Red → FR → Red → FR 54 Red → FR → Red → FR → Red → FR → Red 98 SOURCE: Borthwick, H. A., S. B. Hendricks, M. W. Parker, et al. 1952. A reversible photoreaction controlling seed germination. PNAS 38: 662–666, Table 1. ‒ Pr (phytochrome red) absorbs red light ‒ Pfr (phytochrome far-red) absorbs far-red light Summary of Plant Responses to Light Blue-light Causes phototropism, stomata opening, and induces chlorophyll synthesis. Red/Far-Red light ratio Associated with germination control, chlorophyll development (not synthesis) and flowering. Photoperiod (photoperiodism) Control flowering, leaf growth and senescence. Plants responses to stimulus other than light Gravitropism üPlants can use gravity to sense which direction is up and which is down to send their roots toward the ground and their shoots and leaves up toward the Sun. üMediated by statoliths inside apical bud cells (part of shoot system) and root cap cells (part of root system) üStatoliths fall to the bottom of cells, indicating which direction is down and allowing the plant to orient its roots and shoots accordingly. Statoliths ü specialized amyloplasts (starchstoring organelles) that are denser than cytoplasm and are able to settle to the bottom of the cell, activating pressure receptors in the root cap or in the shoot system. Position of amyloplasts activates sensory proteins (namely pressure receptors) located in the plasma membrane, which initiate the gravitropic response Thigmotropism üResponse from mechanical disturbance (i.e., touch and wind) üThigmomorphogenesis refers to changes in form caused by daily mechanical disturbance in young plants: o Results in plants that are shorter in height while the stems and trunk become thicker (compared to young plants not disturbed mechanically) This response is mediated by the hormone ethylene The shorter aspect and thicker stems/trunks that are characteristic of thigmomorphogenesis are thought to be an adaptation against being blown over when growing in a windy environment Vernalization üFlowering in response to prolonged periods of low temperatures, such as those experienced in winter Plants responses to attacks by herbivores and pathogens Plants use defence systems to deter herbivory, prevent infection, and combat pathogens. Herbivory, animals eating plants, is a stress that plants face in any ecosystem. Plants counter excessive herbivory with physical defences such as thorns and chemical defences such as distasteful or toxic compounds. o For example, some plants produce an unusual amino acid, canavanine, which resembles arginine. o If an insect eats a plant containing canavanine, this unusual amino acid is incorporated into the insect’s proteins in place of arginine. o Because canavanine is different enough from arginine to adversely affect the conformation and, hence, the function of the proteins, the insect dies. Some plants even “recruit” predatory animals that help defend against specific herbivores For example, a leaf damaged by caterpillars releases volatile compounds that attract parasitoid wasps, hastening the destruction of the caterpillars Parasitoid wasps inject their eggs into their prey, including herbivorous caterpillars The eggs hatch within the caterpillars, and the larvae eat through their organic containers