n Chapter 39 ~ Plant Responses to Internal and External Signals

Signal Transduction Pathways n Reception n Transduction – w/ the help of second messengers n Response u Generally increase activity of certain enzymes u Enzyme activation F Stimulation of transcription (making enzymes) F Activation of existing enzymes

Ex. Etiolation – plant growth in the dark n Etiolation of potatoes – begin growth underground n Reception – leads to de-etiolation (greening) u Phtyochrome – receptor that picks up light n Transduction u Activation of second messengers – cyclic GMP and calcium ions n Response u Production of enzymes that lead to expansion of leaves and production of chlorophyll

Plant hormones n Hormone: chemical signals that coordinate parts of an organism; produced in one part of the body and then transported to other parts of the body; low concentrations n Tropism: movement toward or away from a stimulus u Positive vs Negative u Caused by cell elongation on the opposite side n Frits Went - experiments w/ phototropism led to discovery of chemical responsible u Hormone: auxin n Others: gravitropism, thigmotropism

Auxin n IAA (indoleacetic acid) n Location: seed embryo; meristems of apical buds and young leaves n Function: stem elongation; root growth, differentiation, branching; fruit development; apical dominance; tropisms n May be used as a herbicide on some plants – death due to hormone overdose

Cytokinins n Zeatin – most common variety n Location: roots (and actively growing tissues) n Function: root growth and differentiation; cell division and growth; germination; delay senescence (aging); apical dominance (terminal bud suppresses the development of axillary buds) n Works w/ auxin – higher levels of auxin = root growth; higher levels of cytokinins = shoot growth

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

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

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

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

Phytochromes n Plant pigment that measures length of darkness in a photoperiod (red light) n P r (red absorbing) 660nm n P fr (far-red absorbing) 730nm

Plant Defenses – Against Herbivores n Physical – thorns n Chemical – u Distasteful or toxic compounds u Symbiosis F Caterpillars eat leaves F Damaged leaf releases chemicals to attract parsitoid wasps F Wasps lay eggs inside caterpillar F Larva eat their way out – killing host u Early warning system to plants close by

Plant Defenses – Against Pathogens n Virulent pathogen – plant has little defense against the pathogen n Avirulent pathogen – pathogens do mild harm, but don’t kill the plant n Epidermis – first line defense n Gene-for-gene recognition – R (resistance) genes in plant recognize certain pathogens that are coded w/ an avirulence (Avr) gene u Both must be present to avoid infection n Elicitors – stimulate the production of phytoalexins (antimicrobial compounds) and PR (pathogenesis-related) proteins