1. Plant Responses to Internal and External Signals CHAPTER 39

2. 39.3

3. Responses to Light are Critical for Plant Success Light is an especially important environmental factor in the lives of plants (Photosynthesis, Cues key events in growth and development) Photomorphogenesis: the effect of light on plant morphology Plants can detect the presence of light (direction, intensity and wavelength) Action Spectrum: graph which depicts relative effectiveness of different wave lengths of radiation in driving a particular process Blue and red are the most important in regulating a plants photomorphogenesis

4. Blue-Light Photoreceptors Blue light initiates a variety of responses in plants Phototropism (light induces opening of stomata) Slowing of hypocotyl elongation (when seedling hits the ground) Plants use three types of pigment to detect blue light Cryptochromes Phototrophin Zeaxanthin

5. Phytochromes as Photoreceptors Phytochromes regulate many plant responses to light Seed Germination Because of limited nutrient reserves, many types of seeds, especially small ones, germinate only when the light environment and other conditions are near optimal Shade Avoidance Provides information about the quality of light Sunlight includes red and far red radiation, during the day interconversion reaches a dynamic equilibrium, with the ratio of both forms of phytochromes indicating the amounts This enables them to adapt to the light in their environment

6. Photoreversible States of Phytochrome

7. Biological Clocks and Circadian Rhythms A Biological Clock is an internal time keeper that controls an organism’s biological rhythms, it marks time with/without environmental cues but often requires signals to remain tuned A Circadian Rhythm is a cycle with a frequency of 24 hours and is not directly controlled by any factor A plant follow a free-running circadian cycle of about 24 hours, this cycle is entrained to exactly 24 hours. This cycle is goes along with other 24 hour cycles such as Light levels Temperature Relative huidity Some plants also follow a biological clock Such as the opening and closing of a bean plants leaves at certain times

8. The Effect of Light on the Biological Clock The factor that entrains the biological clock to precisely 24 hours a day is light Phytochrome and blue-light photoreceptors can entrain cercadian rhythm in plants, but our understanding of how phytochrome does this is more complete. The mechanism involves turning cellular responses on and off by means of the P r and P fr switching toward equilibrium Phytochrome conversion marks sunrise and sunset, giving the biological clock environmental cues The amount of day and night changing through out the year keeps plants adjusting to the seasons

9. Photoperiodism and Response to Seasons Seasonal events are critically important in life cycle of most plants. The seasons are important in things such as: Seed germination Flowering Onset/breaking of bud dormancy A Photoperiod is the environmental stimulus that plants use most often to detect the time of year (relative length of night and day) Photoperiodism is a physiological response to a photo period, such as flowering Some developmental processes, including flowering in many species, require a certain photoperiod A critical night length sets a minimum (short day plants) or maximum (long day plants) number of hours of darkness required for flowering

10. 39.4 and 39.5

11. Plants respond to a variety of stimuli other than light Gravity Mechanical stimuli Respond to touch Environmental Stresses Drought Flooding Salt Stress Heat Stress Cold Stress

12. Gravity Makes roots grow downwards and sprouts to grow upwards even when it is underground. Gravitropism  A plant’s response to gravity Roots display positive gravitropism Shoots display negative gravitropism Statoliths  specialized plant plastids that contain dense starch grains. In roots, they’re located in the root cap. Because they are denser than the cytoplasm they are influenced by gravity and can cause the root to turn towards the ground.

13. Statoliths 20  m (a) (b)

14. Mechanical Stimuli Thigmomorphogenesis  changes in form that result from mechanical perturbation. Touching a plant can affect the way it grows. “Touch specialists” are plants that respond to touch Vines and other climbing plants that coil around supports. They grow in a straight line and when they contact something begin to wrap around it. This tendency is called thigmotropism. Some plants undergo rapid leaf movements when they’re touched. Action potentials  electrical impulses that makes the surrounding leaves close in once one plant is touched. http://www.youtube.com/watch?v=fc50QniIzVM

16. Environmental Stresses Abiotic  stresses that are caused by nonliving factors like drought, flooding, salt, and extreme temperatures. Biotic  stresses that are caused by attacks from herbivores and pathogens.

17. Drought Plants my be stressed by water deficiency from needing to transport water faster than it can be absorbed from the soil. Many plants save water by reducing the rate of transpiration. The cells lose turgor and close the stomata. It also stimulates the release of absorbic acid in the leaf to keep the stomata closed. Roots grow deeper into the ground to get the water further under the surface.

18. Vascular cylinder Air tubes Epidermis 100  m (a) Control root (aerated) (b) Experimental root (nonaerated)

19. Flooding Soil needs airspace to provide oxygen for cellular respiration in the roots. Some plants have adaptations like aerial roots. In other kinds of plants, oxygen deprivation makes ethylene, causing cells to do apoptosis, making air tubes to provide oxygen to submerged roots.

20. Salt Stress Salt lowers the water potential of soil solution, causing water deficit in plants. Sodium in high concentrations can be toxic to plants. Many plants can produce solutes to make sodium tolerated at high concentrations. This keeps the water potential more negative than soil solution without admitting toxic quantities of salt.

21. Heat Stress Heat can denature enzymes and damage its metabolism. Transpiration is used for effective cooling. Plants close stomata to conserve water and keep it from evaporating, but then water cannot evaporate and it gets hotter. Heat shock proteins help protect other proteins from heat stress. This also can occur in heat stressed animals and microorganisms.

22. Cold Stress If the temperature changes the fluidity of cell membranes and alters solute transportation. Plants alter the lipid composition of their membranes to help gradually fix this, so plants are more likely to survive if the weather changes gradually. If freezing temperatures occur, ice can form in cell walls. The solutions in the cells prevent the cytoplasm from freezing, but it makes the water leave the cell, causing an imbalance of ions that can potentially kill the cell.

23. Plants are at the bottom of the food chain Most plants’ interactions with other organisms do not benefit the plant, because they are producers so they get eaten. They are also subject to infection by disease viruses, bacteria, and fungi that can damage tissues or even kill the plant.

24. Defenses against herbivores Physical and chemical defenses, like thorns or distasteful/toxic compounds. Some plants try to “recruit” predatory animals to help plants defend themselves against herbivores. Recruitment of parasitoid wasps that lay their eggs within caterpillars 4 3 Synthesis and release of volatile attractants 1 Chemical in saliva 1 Wounding 2 Signal transduction pathway

25. Some plants respond to herbivore attacks with an early warning system. Ex: the lima bean plant releases a mix of chemicals, and neighboring lima bean plants respond to the chemicals by expressing a defensive gene to help it survive.

26. Defenses against pathogens A plant’s first line of defense against pathogens is its outer layer, like our skin. Pathogens can still enter the plant from wounds or natural openings like stomata. Once a pathogen enters a plant the plant lets out a chemical defense system to kill the pathogen and prevent it from spreading. Plants can recognize some pathogens to defend against them.

27. Virulent pathogens are pathogens which the plant has little specific defense for. Plants cannot kill these because they would both perish, so a kind of compromise has happened between plants and their pathogens. The pathogen gains access to its host to enable it to perpetuate itself without killing the plant. Avirluent pathogens are ones which only mildly harm the plant

28. Gene for gene recognition is a widespread form of plant disease resistance that involves recognition of pathogen-derived molecules by protein products of specific disease resistance genes. (R genes) Plants have many of these for the different kinds of pathogens Hypersensitive response  the genetically programmed death of infected cells Systematic acquired resistance is a long lasting systemic response that primes the plant for resisting a broad spectrum of pathogens.

29. 3In a hypersensitive response (HR), plant cells produce anti- microbial molecules, seal off infected areas by modifying their walls, and then destroy themselves. This localized response produces lesions and protects other parts of an infected leaf. 4 Before they die, infected cells release a chemical signal, probably salicylic acid. 6 In cells remote from the infection site, the chemical initiates a signal transduction pathway. 5 The signal is distributed to the rest of the plant. 2 This identification step triggers a signal transduction pathway. 1 Specific resistance is based on the binding of ligands from the pathogen to receptors in plant cells. 7Systemic acquired resistance is activated: the production of molecules that help protect the cell against a diversity of pathogens for several days. Signal 7 6 5 4 3 2 1 Avirulent pathogen Signal transduction pathway Hypersensitive response Signal transduction pathway Acquired resistance R-Avr recognition and hypersensitive response Systemic acquired resistance Plant Responses to Pathogen Invasions A hypersensitive response against an avirulent pathogen Seals off the infection and kills both pathogen and host cells in the region of the infection