Chapter 39: Plant Responses to Internal and External Signals

Fig. 39-1

Signal transduction pathways Plant cell receptors detect environment changes Stimulus must find specific receptors to cause a response Receptor initiates a specific signal transduction pathway Etiolation: morphological adaptations to growing in darkness (potatoes); pale, unhealthy, poor roots – After exposure to light de-etiolation occurs; shoots and roots grow normally

(a) Before exposure to light (b) After a week’s exposure to natural daylight Image included due to my love for potatoes

Fig CELL WALL CYTOPLASM Reception TransductionResponse Relay proteins and second messengers Activation of cellular responses Hormone or environmental stimulus Receptor Plasma membrane 1 23

Plant hormones Hormone: chemical signals, coordinate parts of an organism produced in one part of the body and then transported to other parts of the body used in low concentrations Tropism: movement toward or away from a stimulus Went experiments (phototropism) Hormone: auxin Others: gravitropism, thigmotropism

Auxin IAA (indoleacetic acid) Location: seed embryo; meristems of apical buds and young leaves Function: stem elongation; root growth, differentiation, branching; fruit development; apical dominance; tropisms Video: Phototropism Video: Phototropism

Cytokinins Zeatin Location: roots (and actively growing tissues) Function: root growth and differentiation; cell division and growth; germination; delay senescence (aging); apical dominance (w/ auxin)

Gibberellins GA 3 Location: meristems of apical buds and roots, young leaves, embryo Function:germination of seed and bud; stem elongation; leaf growth; flowering (bolting); fruit development; root growth and differentiation

Abscisic acid ABA Location: leaves, stems, roots, green fruit Function:inhibits growth; closes stomata during stress; counteracts breaking of dormancy

Ethylene Gaseous hormone Location:ripening fruit tissue; stem nodes; aging leaves and flowers Function:fruit ripening; oppositional to auxin (leaf abscission); promotes/inhibits: growth/development of roots, leaves, and flowers; senescence

Table 39-1

Daily and Seasonal Responses Circadian rhythm (24 hour periodicity) Photoperiodism (phytochromes) Short-day plant: light period shorter than a critical length to flower (flower in late summer, fall, or winter; poinsettias, chrysanthemums) Long-day plant: light period longer than a critical length to flower (flower in late spring or early summer; spinach, radish, lettuce, iris) Day-neutral plant: unaffected by photoperiod (tomatoes, rice, dandelions) Critical night length controls flowering

Phytochromes Plant pigment that measures length of darkness in a photoperiod (red light) P r (red absorbing) 660nm P fr (far-red absorbing) 730nm Blue-Light Photoreceptors Various blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism Tested in action spectrum studies: testing effects of different colors of light on plant light responses

Fig b Light Time = 0 min (b) Coleoptile response to light colors Time = 90 min

Plant Defenses Plants use defense systems to deter herbivory, prevent infection, and combat pathogens – Counter excessive herbivory with physical defenses (thorns and chemical defenses) – Some “recruit” predatory animals to defend them – Plants damaged by insects can release volatile chemicals to warn other plants of the same species

Fig Recruitment of parasitoid wasps that lay their eggs within caterpillars Synthesis and release of volatile attractants Chemical in saliva Wounding Signal transduction pathway

Defenses Against Pathogens A plant’s first line of defense against infection is the epidermis and periderm The second line of defense is a chemical attack (phytoalexins and PR proteins) that kills the pathogen and prevents its spread Gene-for-gene recognition involves recognition of pathogen-derived molecules by protein products of specific plant disease resistance (R) genes These defenses include the hypersensitive response (kill/wall off affected area) and systemic acquired resistance (salicylic acid triggers defense genes)

Fig Signal Hypersensitive response Signal transduction pathway Avirulent pathogen Signal transduction pathway Acquired resistance R-Avr recognition and hypersensitive response Systemic acquired resistance

You should now be able to: 1.Compare the growth of a plant in darkness (etiolation) to the characteristics of greening (de- etiolation) 2.List six classes of plant hormones and describe their major functions 3.Describe the phenomenon of phytochrome photoreversibility and explain its role in light- induced germination of lettuce seeds 4.Explain how light entrains biological clocks

5.Distinguish between short-day, long-day, and day- neutral plants; explain why the names are misleading 6.Describe how plants tell up from down 7.Distinguish between thigmotropism and thigmomorphogenesis 8.Describe the challenges posed by, and the responses of plants to, drought, flooding, salt stress, heat stress, and cold stress

9.Describe how the hypersensitive response helps a plant limit damage from a pathogen attack