Plant Responses to Internal and External Signals

Flashcards testing knowledge on plant signal transduction, hormones, photoreceptors, biological clocks, physical and abiotic stress responses, and pathogen/herbivore defense mechanisms.

How does the parasitic plant dodder (Cuscuta) locate its host plant?

Dodder seedling growth is steered toward host plants by sensing shade and detecting host-released chemical signals.

What is etiolation, and why is it advantageous for dark-grown seedlings?

Etiolation is a set of morphological adaptations for growing in darkness, characterized by tall spindly stems, unexpanded leaves, and stubby roots. It conserves water and energy while maximizing stem elongation to reach light before seed nutrient reserves run out.

What morphological changes occur during de-etiolation (greening)?

Stem elongation slows, leaves expand, roots elongate, and the shoot produces chlorophyll.

What photoreceptor detects light to trigger de-etiolation, and where is it located in the cell?

Phytochrome, a light-absorbing photoreceptor located in the cytoplasm.

What two second messengers participate in the phytochrome signal transduction pathway during de-etiolation?

Calcium ions ($Ca^{2+}$) and cyclic GMP (cGMP).

What is the resting concentration of cytosolic $Ca^{2+}$ in plant cells, and how does phytochrome activation affect it?

The resting concentration of cytosolic $Ca^{2+}$ is about $10^{-7}\,M$, and phytochrome activation opens $Ca^{2+}$ channels to cause a transient 100-fold increase.

How does polar transport of auxin occur within plant stems?

Auxin moves strictly unidirectionally from shoot tip to base via auxin transport proteins (such as PIN proteins) concentrated at the basal ends of cells.

In what concentration range does auxin stimulate cell elongation in shoot stems?

Auxin stimulates growth within a concentration range of $10^{-8}$ to $10^{-4}\,M$.

According to the acid growth hypothesis, how does auxin promote cell wall loosening?

Auxin stimulates plasma membrane proton ($H^+$) pumps, lowering cell wall pH. The reduced pH activates expansins, which break hydrogen bonds between cellulose microfibrils and cell wall polysaccharides.

What are the commercial applications of the auxins IBA and 2,4-D?

Indolebutyric acid (IBA) is used for vegetative propagation to stimulate root growth on cuttings; 2,4-dichlorophenoxyacetic acid (2,4-D) is a synthetic auxin herbicide used to eliminate eudicot (broadleaf) weeds.

How do cytokinin and auxin ratios control cellular differentiation in cultured plant callus?

When cytokinin and auxin concentrations are balanced, the cells remain an undifferentiated callus. If cytokinin levels increase relative to auxin, shoot buds develop. If auxin levels increase relative to cytokinin, roots form.

What plant hormone was originally isolated from the fungus Gibberella, causing 'foolish seedling disease' in rice?

Gibberellin.

How do gibberellins promote cereal seed germination after water imbibition?

The embryo releases gibberellins, signaling the aleurone layer to synthesize and secrete digestive enzymes like $\alpha$-amylase to hydrolyze endosperm starch into sugars.

How does abscisic acid (ABA) promote drought tolerance in plants?

ABA accumulates in leaves during water deficit and opens $K^+$ channels in guard cell plasma membranes, causing a massive loss of $K^+$ and water, which closes the stomatal pores.

What three growth changes constitute the ethylene-induced triple response in seedlings?

1) Slowing of stem elongation, 2) thickening of the stem, and 3) curvature causing horizontal stem growth.

How do Arabidopsis eto and ctr mutants differ in their ethylene response?

Ethylene-overproducing (eto) mutants produce ethylene at 20 times normal levels and can be restored to wild-type using ethylene synthesis inhibitors. Constitutive triple-response (ctr) mutants have a defect in a negative regulator kinase and display the triple response constantly, even in the absence of ethylene or presence of synthesis inhibitors.

At what concentrations do brassinosteroids induce cell elongation and division?

At concentrations as low as $10^{-12}\,M$.

What function do strigolactones perform in host plant interactions with parasitic witchweed (Striga)?

Strigolactones exuded by host roots serve as chemical signals that stimulate the germination of Striga seeds.

What specific functions do cryptochromes and phototropins mediate as blue-light photoreceptors?

Cryptochromes mediate blue-light-induced inhibition of stem elongation, whereas phototropins mediate phototropism, blue-light-driven stomatal opening, and chloroplast movements.

What peak wavelength of radiation causes maximum phototropic curvature in maize coleoptiles?

Blue light with a wavelength near $436\,nm$.

What are the two photoreversible forms of phytochrome, and which wavelengths convert them?

$P_r$ absorbs red light ($660\,nm$) and converts to $P_{fr}$; $P_{fr}$ absorbs far-red light ($730\,nm$) and converts to $P_r$.

Which form of phytochrome ($P_r$ or $P_{fr}$) triggers physiological responses like seed germination and shade avoidance?

$P_{fr}$ is the active form that triggers developmental responses.

What is the duration of a free-running circadian period in plants?

Free-running periods range from about $21$ to $27$ hours (for example, $26$ hours for bean leaf sleep movements in constant darkness).

In photoperiodism, what environmental variable actually controls flowering in short-day and long-day plants?

Continuous night length (dark period duration), not day length.

How does a brief flash of red light during a long dark period affect short-day and long-day plants?

It interrupts the continuous dark period, preventing flowering in short-day plants and inducing flowering in long-day plants.

What protein macromolecule functions as the plant flowering hormone florigen?

The FT protein, encoded by the FLOWERING LOCUS T (FT) gene, which travels via the symplasm from leaf cells to the shoot apical meristem.

How do root cap cells detect gravity according to the statolith hypothesis?

Dense starch-filled plastids called statoliths settle under gravity to the lower portions of root cap cells, triggering a redistribution of calcium and auxin.

What is the difference between thigmomorphogenesis and thigmotropism?

Thigmomorphogenesis refers to systemic morphological changes in plant form resulting from mechanical perturbation (e.g., shorter stems from rubbing), whereas thigmotropism is directional growth in response to touch (e.g., tendrils coiling around a support).

What mechanism causes rapid leaflet folding in the sensitive plant Mimosa pudica?

Stimulation causes motor cells in pulvini to rapidly lose potassium ions ($K^+$), prompting osmotic water loss and turgor loss, accompanied by electrical action potentials traveling at $1\,cm/s$.

How do non-adapted plant roots survive oxygen deprivation during flooding?

Oxygen deprivation stimulates ethylene synthesis, which induces localized apoptosis in root cortex cells to create air tubes that supply oxygen.

At what temperature threshold do temperate plants typically synthesize heat-shock proteins?

Above approximately $40^\circ\text{C}$.

What is the difference between PAMP-triggered immunity and effector-triggered immunity in plants?

PAMP-triggered immunity recognizes general pathogen-associated molecular patterns (like bacterial flagellin) via Toll-like receptors to produce antimicrobial phytoalexins. Effector-triggered immunity relies on specific plant disease resistance ($R$) genes that detect pathogen effectors, triggering localized hypersensitive responses and systemic acquired resistance.

How is systemic acquired resistance (SAR) communicated throughout a plant?

Methylsalicylic acid is produced at the infection site, exported via phloem to distant tissues, and converted to salicylic acid, which activates plant-wide defense pathways.

What cellular defense against herbivores is found in taro (Colocasia esculenta) leaves?

Specialized cells called idioblasts containing needle-shaped calcium oxalate crystals called raphides, which pierce herbivore mouth tissues to allow irritants to enter deeply.

How does the wild tobacco plant Nicotiana attenuata adjust its flowering behavior in response to herbivory?

When infested by hawk-moth larvae, chemical factors in larval saliva trigger the plant to stop blooming at night (which attracts moths) and instead open flowers at dawn to attract hummingbird pollinators.