Plant Responses

Plant Hormones

Hormones are chemical messengers that coordinate the different parts of a multicellular organism. They are produced by one part of the body and transported to another.

Signal transduction is the series of steps between an organism's perception of an environmental change and the response to that change

Hormones help coordinate growth, development, and response to stimuli.

There are 5 main plant hormone classes: auxins (IAA), cytokinins, gibberellins, abscisic acid and ethylene.

See p.759

Hormone Where Found in Plant Major FunctionsAuxin Seed embryo, young leaves, Stimulates cell elongation; involved in

meristems of apical buds phototropism, gravitropism, apical dominance, and vascular differentiation; stimulates ethylene synthesis; regulates fruit development

Cytokinin Synthesized in roots and Stimulates cell division, stimulates germination,transported to other organs growth, and flowering, delays leaf senescence

Gibberellin Meristems of apical buds and Stimulates shoot elongation, stimulates roots, young leaves, embryo flowering and development of fruit, promotes

seed and bud germination

Ethylene Tissues of ripening fruits, Stimulates fruit ripening, leaf and flower nodes of stems, senescent senescence, opposes some auxin effectsleaves and flowers

Abscisic Acid Leaves, stems, green fruit Stimulates stomate closure during water stress,inhibits growth, promotes seed dormancy

The effect of gibberellin on Thompson seedless grapes

Light Responses

Tropism: A plant growth response that results in the plant growing either toward or away from a stimulus.

Phototropism: The growth of a shoot in a certain direction in response to light.

Positive phototropism is growth toward light; negative phototropism is growth away from light.

The direction of growth is due to the fact that cells on one side of the plant elongate more quickly than the cells on the other side (fig.35.2)

Responses to light are critical for plant successPhotomorphogenesis: The effects of light on plant morphologyBlue light has the greatest effect on plant growth and movement. Phytochromes are pigments that are involved in many of a

plant’s responses to light.

Photoperiods

Photoperiodism: A physiological response to a photoperiod (the relative lengths of night and day), such as flowering

Note: It is night length - NOT day length - that controls flowering and other responses

Short-day plants require a period of light shorter than a certain critical length in order to flower.

Long-day plants flower in the late spring or early summer; they require the most daylight to flower.

Day-neutral plants can flower in days of any length.

Sleep movements of a bean plant (Phaseolus vulgaris): http://plantsinmotion.bio.indiana.edu/plantmotion/movements/leafmovements/bean/beansleep.html

Noon Midnight

Other Stimuli

Gravitropism: A response to gravity. Roots show positive gravitropism and shoots show negative

gravitropism Thigmomorphogenesis: The change in form of a plant that results

from mechanical disturbance (repeatedly touching a plant with a ruler to measure its height can affect its growth pattern)

Thigmotropism: Directional growth in a plant as a response to a touch.

Example: climbing tendrils of plants such as the sweet pea. The tendrils actually "feel" the solid object, which results in the coiling response.

Human skin can detect a thread weighing 0.002mg being drawn across it, but a feeding tentacle of the insectivorous sundew plant responds to a thread of 0.0008mg.

Thigmonasty: Touch-induced movement rather than growth (mimosa leaf)

Mimosa Video: http://plantsinmotion.bio.indiana.edu/plantmotion/movements/nastic/mimosa/mimosa.html

Rapid turgor movements by the sensitive plant (Mimosa pudica)

(a) Unstimulated (b) Stimulated

Side of pulvinus withflaccid cells

Side of pulvinus withturgid cells

Vein

0.5 m(c) Motor organs

Leafletsafterstimulation

Pulvinus(motororgan)

Other Responses

In times of drought, the guard cells lose turgor. This causes the stomata to close. Young leaves stop growing, and they will roll into a shape that slows transpiration rates. Deep roots continue to grow, while those near the surface grow slowly if at all.

In times of flooding, certain cells in the root cortex die, which creates air tubes that bring in oxygen and enable the plant to continue cellular respiration.

Plants respond to salt stress by producing compounds that keep the water potential of cells more negative than that of the soil solution, which encourages water to flow into the roots. Most plants cannot survive salt stress for long periods of time.

In heat stress environments, plants produce heat-shock proteins, which act to ensure that newly formed proteins are folded properly and mis-folded proteins are removed from the cell.

In cold stress environments, plants alter the lipid and protein composition of their cell membranes.