Chapter 39: Plant Responses to Internal and External SignalsOverview: 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. 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 the stimulus. Upon receiving a stimulus, a receptor initiates a specific series of biochemical steps, a signal transduction 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.- These morphological adaptations for growing in darkness, collectively referred to as etiolation, are seen 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.- When shoot reaches the sunlight, it undergos profound changes, called de-etiolation (greening) 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 thisreception 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).Transcriptional Regulation- Transcription factors bind directly to specific regions of DNA and control the transcription of specific 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 bothPost-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