Pathways for signal transduction connect signal receipt to the response.

Dodder plants receive particular environmental cues and respond to them in ways that improve survival and reproductive success, although dodder is not exceptional in this sense.

Consider a less dramatic example: a forgotten potato in the back corner of a kitchen cabinet. Shoots have grown from the "eyes" of this modified subterranean stem, or tuber (axillary buds). These shoots, on the other hand, have little resemblance to those of a normal plant. This plant features ghostly pale stalks and unexpanded leaves, as well as tiny, stubby roots, as opposed to robust stems and wide green leaves.

These morphological adaptations for growing in the darkness, known as etiolation, make sense when we consider that a young potato plant in nature often faces continuous darkness when sprouting underground.

Expanded leaves would obstruct soil penetration and be destroyed when the shoots are pushed through the soil in these conditions. Because the leaves are unexpanded and subterranean, there is less evaporative water loss and no need for a large root system to replace the water lost by transpiration. Furthermore, because there is no light for photosynthesis, the energy invested in generating green chlorophyll would be squandered.

Instead, a potato plant developing in the dark devotes all of its energy to elongating its stems. This adaptation allows the shoots to break ground before the tuber's nutritional supplies are depleted. The etiolation response is one example of how intricate interactions between external and internal signals tailor a plant's morphology and physiology to its surroundings.

A generic model for signal transduction pathways is reviewed and shown in the attatched image above.

A hormone or other type of stimulus engaging with a particular receptor protein can cause the successive activation of relay proteins as well as the generation of second messengers involved in the route.

The signal is sent, resulting in cellular reactions.

The receptor is shown on the surface of the target cell in this picture; in other situations, the stimulus interacts with receptors inside the cell.

Signals are recognized initially by receptors, which are proteins that change form in response to a specific stimulus. The receptor involved in de-etiolation is a phytochrome, a type of photoreceptor that we'll go over in more detail later in the chapter.

The kind of phytochrome that works in de-etiolation is found in the cytoplasm, as opposed to most receptors, which are built into the plasma membrane. Through experiments on the tomato, a close relative of the potato, researchers established the importance of phytochrome in de-etiolation.

When exposed to light, the area mutant of tomato, which has lower amounts of phytochrome, greens less than wild-type tomatoes. (Aurea is a Latin.)

The process by which a signal induces developmental changes may be dependent on transcription factors which are catalysts (which increase transcription of specific genes)

or repressors (proteins that inhibit transcription), or both. For some Arabidopsis mutants, for example, save for their light color, when developed in the light, they have a light-grown morphology.

They are black, with enlarged leaves and short, strong stalks, but are not green because the final stage in the synthesis of chlorophyll necessitates direct exposure to light. These mutants contain flaws in a particular gene, for which the repressor typically prevents other genes from expressing themselves that are triggered by light. When the repressor is removed, they are usually blocked by a route that is allowed to progress due to mutation.

Plant hormones aid in the coordination of growth, development, and response to stimuli.

Hormones regulate plant growth and development by influencing cell division, elongation, and differentiation. Some also mediate plant responses to external cues. Light responses are important for plant survival.

Photoreceptors that respond to blue light regulate hypocotyl elongation, stomatal opening, and phototropism.

Phytochromes function as molecular "on-off" switches that govern shade avoidance and seed germination in a variety of seed types. Red light activates phytochrome, but far-red light deactivates it.

The conversion of phytochromes also gives information about the length of the day (photoperiod) and hence the time of year.

Many species' blooming times are regulated by photoperiodism. To blossom, short-day plants require a night that is longer than a crucial duration. Long-day plants require a night length that is less than a crucial time in order to bloom.

An intrinsic circadian clock regulates several daily cycles in plant activity. The length of free-running circadian rhythms is roughly 24 hours, but it is entrained to exactly 24 hours by dawn, and dusk influences phytochrome form.

Other than light, plants respond to a broad range of stimuli.

Gravitropism is the act of bending as a result of gravity. Positive gravitropism is observed in the roots, whereas negative gravitropism is observed in the stems. Statoliths, which are starch-filled plastids, allow roots to sense gravity.

Thigmotropism is a touch-induced growth response. Electrical impulses are transmitted during rapid leaf motions.

Plants are vulnerable to environmental stressors such as drought, floods, excessive salt, and temperature extremes.

Plants respond to diseases and herbivore assaults.

The hypersensitive reaction isolates an infection and kills both pathogen and host cells in the area. Systemic acquired resistance is a broad-based defensive response in organs far from the source of infection.

Plants generate unpleasant or poisonous compounds, as well as attractants that draw herbivore-killing animals, in addition to physical defenses such as thorns and trichomes.