Chapter 40: Sensory Systems in Plants

Response to Light

• Photomorphogenesis is non-directional, light triggered development • Light can change the conformation of light-receptor molecules that initiate biological change Red Light Receptors • Phytochorme, a pigment containing protein, exists in two forms P r and P fr • P r absorbs red light and P fr absorbs far-red light • When each form absorbs a photon of this light they change into the other form.

• P fr is biologically active while P r is not • When Pfr is present, a reaction affected by phytochorme will occur and when it has been replaced with Pr the reaction will not occur • Such reactions include seed germanation and signals plant spacing

• Phototropisms are directional • Contribute to the branching patterns seen within a species as shoots to grow towards the light • Stems and other plant parts, except roots, bend and grow toward light with blue wavelengths

Blue Light Receptors • When blue light strikes the plant phototropin I is activated and changes conformation(shape) resulting in autophosphorylation, signaling a transduction pathway that leads to phototropic growth

Response to Gravity

• Gravitropism can occur when a potted plant is pushed over or a storm pushes plants over • A shoot grows away form gravity(negative response) while the root grows towards the gravity(positive response)

Response to Touch

• Thigmotripism is a directional growth response of a plant to contact with an object, animal, plant, or wind • Thigmonasty is the non-directional growth response pf a plant to contact with an object, animal, plant, or wind • The snapping of the Venus flytrap is one of the most dramatic responses to touch

• Touch responses can also result in reversible trugor movements • Turgor is the pressure within a living cell resulting from diffusion of water into it • When water leaves the cell it will collapse, while water entering a cell will create movement, becoming more turgid • Changes in leaf orientation are mostly due to turgor pressure changes in pulvini, multicelluar swelling located at the base of a plant • Turgor movement can also be stimulated by light, wind, heat, electricity

• Circadian clocks are endogenous timekeepers that keep plant movements and other responses synchronized with the environment

Dormancy

• A mature plant can turn dormant for survival against extreme temperatures. • Environmental triggers for growth and dormancy are temperature, water, and light.

• Most dormant plants lose their leaves and form winter buds.

• Unfavorable weather conditions can be bypassed by producing dormant seeds.

Surviving Temperature Extremes

• The lipid composition of a plants membrane will help determine how resistant it is to freezing temperatures.

• The more unsaturated the membrane lipids are the higher the resistance to cold temperatures.

• Supercooling When ice crystals are stored in extracellular spaces preventing them from damaging any cells.

• It is also important for a plant's cells to be tolerant of gradual dehydration.

• If temperatures are too hot HSPs (heat shock proteins) are produced to help stabilize other proteins, preventing them from misfolding or unfolding.

• Thermotolerance Plants can tolerate extreme temperatures if exposed to them gradually overtime.

Plant Hormones

• Plants use hormones, chemical substances produced in one part of a plant then transported to another in response to environmental and internally regulated development.

• This would include leaf abscission and the development of mature fruit.

• There are 7 important kinds of hormones in plants: Auxin, Cytokinins, Gibberellins, Brassinosteroids, Oligosacchorins, Ethylene, and Abscisic acid.

Auxin

• Promotes elongation of the stem (This helps a plant grow torwards a light source).

• Plant cells that are in shade have more auxin because the auxin migrates from the light side to the dark side therefore growing faster than cells in the light causing the plant to bend. (As shown in Frit Went's experiment) • Other functions include the formation of adventitious roots, prevents leaf abscission, promotes cell division and ethylene production, and lateral bud dormancy. Because of these functions it has been found useful in agriculture and horticulture.

Auxin continued...

• Acid growth hypothesis provides a model linking auxin to cell wall expansion. The hormone causes cells to transport hydrogen ions from the cytoplasm to the cell wall.

• This mechanism produces a very quick response.

• Like the snapping of a Venus flytrap involving an acid growth response allowing cells to grow in 0.3 seconds closing the trap.

Cytokinins

• stimulates cell division, but only with Auxin • promotes the development of chloroplasts • delays leaf aging • and promotes bud formation • a sideaffect of cytokinin is when it is used against the plant by pathogens where increased levels of the hormone cause too much cell division which creates tumors.

Gibberellins

• promotes stem elongation • found in the apical regions of shoots and roots • stimulates enzyme production in germinationg seeds

Brassinosteroids and Oligosaccharins

Brassinosteroids • structurally similar to animal hormones • Effects plant growth and development parallel to auxins and gibberellines Oligosaccharins • Carbohydrates released from cell walls • Regulates pathogen responses and growth and development in some plants.

Ethylene and Abscisic Acid

Ethylene • gaseous hydrocarbon • stimulates ripening in fruit • ethylene production can increase due to plant stress.

Abscisic Acid • produced mostly in mature green leaves and in fruits • inhibits the growth of buds and promotes leaf senescence • Helps control opening and closing stomata • Ensures plant survival under water stress