Plant Physiology BIOL 3460 Topic 12 Plant Hormones and Signal Transduction Plant Hormones and Signal Transduction • As sessile organisms, plants constantly adjust in response to their environment, either to take advantage of favorable conditions or to survive unfavorable ones. • Plants have evolved sophisticated sensory systems to optimize water and nutrient usage; to monitor light quantity, quality, and directionality; and to defend themselves from biotic and abiotic threats. Plant Hormones and Signal Transduction Temporal and Spatial Aspects of Signaling Signal Perception and Amplification Hormones and Plant Development Phytohormone Metabolism and Homeostasis Signal Transduction and Cell - to - Cell Communication Hormonal Signaling Pathways Plant Hormones and Signal Transduction • An environmental input that initiates one of more plant responses is referred to as a signal • The physical component that biochemically responds to that signal is called a receptor • Receptors are either proteins, or in the case of light receptors, pigments associated with proteins Plant Hormones and Signal Transduction cont’d • All signal transduction pathways typically involve the following chain of events: Signal receptor signal transduction response • In many cases, the initial response is the production of secondary messengers which are then transported to the site of action to evoke the main physiological response Temporal and Spatial Aspects of Signaling • Plants use signal transduction to coordinate both rapid and slow responses to stimuli. • Rapid response mechanisms involve electrochemical responses to transduce signals. • Processes occurring on a longer timescale (hours) often involve changes in gene expression. • Longer - term environmental responses can operate over months or years and involve modification to plant development or plant architecture over the life of the plant. Fig 12.1 • (A) Insect movements on modified leaves of a Venus flytrap ( Dionaea muscipula ) activate trigger hairs, inducing rapid closure of the leaf lobes. • milliseconds Fig 12.1 • (B) The leaves of the sundew plant Drosera anglica capture insects in a sticky fluid produced by stalked glands called tentacles, then roll up to secure the prey and begin digestion. Fig 12.1 • (C) A hawthorn tree ( Crataegus sp.) subjected to prevailing onshore winds responds slowly by growing away from the wind. • years Fig 12.1 • (D) Tree trunks and branches can respond slowly to mechanical stress by producing reaction wood. In this case the tree is an angiosperm, which produces tension wood on the upper surface. Gymnosperms produce compression wood on the lower surface. Fig 12.1 • (E) Cross section through a gymnosperm tree branch with compression wood (arrow), creating an asymmetric ring structure. Temporal and Spatial Aspects of Signaling cont’d • Plant responses to environmental signals also differ spatially. • In a cell autonomous response to an environmental signal, both signal reception and response occur in the same cell (e.g., guard cell response to blue light) • A non - cell autonomous response is one in which signal reception occurs in one cell and the response occurs in distal cells, tissues or organs. (e.g., additional stomata in response to high light intensity) Fig 12.2 • General scheme for signal transduction. Environmental or developmental signals are perceived by specialized receptors. A signaling cascade is then activated that involves second messengers and leads to a response by the plant cell. When an optimal response has been achieved, feedback mechanisms attenuate the signal. Perception and Amplification • Receptors can be located at the plasma membrane, cytosol, endomembrane system, or nucleus. • Some receptors are found in more than one cellular location, such as mechanosensitive stretch receptors that help cells and chloroplasts adjust to osmotically induced swelling. Figure 12.3 Primary locations of plant hormone receptors and mechanosensitive receptors in the cell Perception and Amplification cont’d • Signals must be amplified intracellularly to regulate their target molecules. • Signal transduction pathways can consist of a few signaling steps or an elaborate cascade of signaling events. • The most common modification is the transfer of a phosphate from ATP to a protein. • This phosphorylation is catalyzed by a class of enzymes called kinases Perception and Amplification cont’d • Some kinases are components of receptor complexes. • Other kinases function in amplification cascades, either to elevate weak initial signaling events above the threshold of detection or to propagate them throughout the cytoplasm. • Some signal amplification cascades, such as the MAP kinase cascade, can involve multiple levels of amplification. Perception and Amplification cont’d • Second messengers represent another strategy to enhance or propagate signals. • Ca 2+ is the most ubiquitous second messenger in all eukaryotes. • In plants, Ca 2+ is involved in symbiotic interactions, and plant defense responses to various hormones and abiotic stresses. Perception and Amplification cont’d • Cytosolic Ca 2+ levels can increase rapidly when Ca 2+ is taken up into the cytosol, either from external stores, such as the cell wall, or from internal stores, such as vacuoles. • In both cases, Ca 2+ influx is mediated primarily by Ca 2+ - permeable ion channels. • Several families of Ca 2+ – permeable channels have been identified in plants. Perception and Amplification cont’d • Once receptor mediated signaling activates Ca 2+ – permeable channels, Ca 2+ sensor proteins play a pivotal role as signaling intermediaries, linking Ca 2+ signals to changes in cellular activities. • Plant genomes contain four major multigene families of Ca 2+ sensors: • The calmodulin ( CaM ) and calmodulin - like proteins • The Ca 2+ - dependent protein kinases • Ca 2+ /calmodulin - dependent protein kinases, and • Calineurin - B like (CBL) proteins