Plant Responses to Signals Chapter 39

Plants have to respond to gravity and other stimuli in environment. Growth pattern in plants - reaction to light.

Shoot reaches sunlight - starts process called greening. Shoots start to grow - entire plant begins to make chlorophyll. Begins signal transduction pathway like one seen in animal cells.

Signal transduction pathway promotes cell activity in plant.

Hormones Plants produce hormones that regulate growth and development. Hormones - chemical signals produced in one part of body, transported to other parts. Growth towards or away from stimuli (regulated by hormones) - tropism.

Growth of shoot towards light - phototropism (positive). Hormone responsible for growth - auxin.

Auxin produced in large quantities in apical meristem - growth occurs. Auxin used on cut stems to promote root growth. Auxins used as growth inhibitor for some plants - used as pesticides.

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Cytokinins stimulate cytokinesis (cell division) Cytokinins produced in actively growing tissues, particularly roots, embryos, and fruits. Both cytokinins and auxins present, cells divide.

Shoots forming with addition of cytokinins

Cytokinin levels raised, shoot buds form. Auxin levels raised, roots form. Cytokinins also slow down aging process of some plant organs - florists use sprays to keep flowers fresh.

Gibberellins stimulate growth in leaves and stems - little effect on root growth. Stems, gibberellins stimulate cell elongation and cell division. Gibberellins applied to dwarf plants - grow to normal height. Applied to normal plants - nothing happens.

Many plants - both auxin and gibberellins must be present for fruit to set. Seeds have large amount of gibberellins - signals seed to break dormancy.

Abscisic acid promotes plant to become dormant; thought to help leaves drop in fall. Sometimes seed will need to have all abscisic acid removed (through washing) to break dormancy. Also helps to withstand drought - sends plant into dormancy until the conditions are favorable again.

Ethylene promotes leaf dropping as well as fruit ripening. If fruit producing ethylene placed with fruits that are not, those fruits will also ripen in response to hormone. By losing leaves during fall, plants prevent drying out during winter.

Responses to light Plants require light to grow; can absorb various aspects of spectrum of light. Respond differently to different wavelengths of light. 2 different types of plants, short day and long day.

Short-day plants - long-night plants -require minimum length of uninterrupted darkness. Long-day plans - short-night plants - require period of continuous darkness interrupted by few minutes of light. Response to light - photoperiodism.

Typically, red light used to interrupt nighttime cycle.

Tropisms Roots - positive gravitropism (grow in direction of gravity); shoots - negative gravitropism (grow against direction of gravity). Thigmotropism - response to touch; in some plants, causes plant to coil around an object (like tendril).

Some plants cannot grow in extreme temperatures or salinities; others thrive in them. Freezing of cytoplasm can kill plant because excess ions can accumulate.

Marsh grasses are often tolerate of extreme salinities

Plant defenses Plants susceptible to many different bacteria and viral infections because of place in food chain. Eaten by herbivores - need protection against excess herbivory – use physical defenses, such as thorns, and chemical defenses, such as production of toxic compounds.

Some plants able to secrete compounds that kill insect eating it. Most plants resistant to pathogens automatically because they are able to detect infection and kill it off right away.