Chapter 39 Plant Responses to Internal and External Signals Overview Stimuli and a Stationary Life At every stage in the life of a plant sensitivity to the environment and coordination of responses are evident 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 Being stationary a plant generally responds to environmental cues by adjusting its pattern of growth and development As a result plants of the same species vary in body form much more than animals of the same species Concept 39 1 Signal transduction pathways link signal reception to response All organisms receive specific environmental signals and respond to them in ways that enhance survival and reproductive success the stimulus Animals and plants have cellular receptors that they use to detect important changes in their environment For a stimulus to elicit a response certain cells must have an appropriate receptor a molecule affected by Upon receiving a stimulus a receptor initiates a specific series of biochemical steps a signal transduction These morphological adaptations for growing in darkness collectively referred to as etiolation are seen pathway that couples reception to the response Plants are sensitive to a wide range of internal and external stimuli and each of these initiates a specific signal transduction pathway also in seedlings germinated in the dark and make sense for plants sprouting underground The shoot is supported by the surrounding soil and does not need a thick stem Expanded leaves would hinder soil penetration and be damaged as the shoot pushes upward Because little water is lost in transpiration an extensive root system is not required The production of chlorophyll is unnecessary in the absence of light A plant growing in the dark allocates as much energy as possible to the elongation of stems to break ground before the nutrient reserves in the tuber are exhausted The elongation rate of the stems slows leaves expand roots elongate the shoot produces chlorophyll The de etiolation response is an example of how a plant receives a signal in this case light and how this When shoot reaches the sunlight it undergos profound changes called de etiolation greening reception is transduced into a response greening Studies of mutants have provided valuable insights into the roles played by various molecules in the three stages of cell signal processing reception transduction and response Reception Signals are first detected by receptors proteins that change shape in response to a specific stimulus The receptor for de etiolation 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 The importance of this phytochrome was confirmed through investigations of a tomato mutant called aurea which greens less when exposed to light Injecting additional phytochrome into aurea leaf cells and exposing them to light produced a normal de etiolation response Transduction Receptors such as phytochrome are sensitive to very weak environmental and chemical signals For example just a few seconds of moonlight slow stem elongation in dark grown oak seedlings These weak signals are amplified by second messengers small internally produced chemicals that transfer and amplify the signal from the receptor to proteins that cause the specific response In the de etiolation response each activated phytochrome may give rise to hundreds of molecules of a second messenger each of which may lead to the activation of hundreds of molecules of a specific enzyme Light causes phytochrome to undergo a conformational change that leads to increases in levels of the second messengers cyclic GMP cGMP and Ca2 Changes in cGMP levels can lead to ionic changes within the cell by influencing properties of ion channels 1 Cyclic GMP also activates specific protein kinases enzymes that phosphorylate and activate other proteins The microinjection of cyclic GMP into aurea tomato cells induces a partial de etiolation response even without the addition of phytochrome Changes in cytosolic Ca2 levels also play an important role in phytochrome signal transduction The concentration of Ca2 is generally very low in the cytoplasm Phytochrome activation can open Ca2 channels and lead to transient 100 fold increases in cytosolic Ca2 Response 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 by stimulating transcription of mRNA for the enzyme transcriptional regulation or by activating existing enzyme molecules post translational modification Transcription factors bind directly to specific regions of DNA and control the transcription of specific Transcriptional Regulation genes In the case of phytochrome induced de etiolation several transcription factors are activated by phosphorylation in response to the appropriate light conditions The activation of some occurs through the cyclic GMP pathway while activation of others requires Ca2 The mechanism by which a signal promotes a new developmental course may depend on the activation of positive transcription factors proteins that increase transcription of specific genes or negative transcription factors proteins that decrease transcription or both Post Transcriptional Modification of Proteins Most often the activities of existing proteins are modified by phosphorylation Many second messengers such as cyclic GMP and some receptors including some forms of phytochrome activate protein kinases directly Often one protein kinase will phosphorylate other protein kinases creating a kinase cascade that eventually link initial stimuli to responses at the level of gene expression By such mechanisms many signal pathways ultimately regulate the synthesis of new proteins usually by turning specific genes on and off De Etiolation Greening Proteins What sort of proteins are either newly transcribed or activated by phosphorylation during the de etiolation process Many are enzymes that function in photosynthesis directly others are enzymes involved in supplying the chemical precursors necessary for chlorophyll production still others affect the levels of