Exercises: Plant hormones

What is a plant hormone and how does it differ from an animal hormone in terms of its site of production, transport, and target cells?

Name two plant hormones (for example, auxins, gibberellins, cytokinins, ethylene, or abscisic acid) and describe the main effect of each on the plant.

Define tropism and distinguish between phototropism, geotropism, and thigmotropism, stating the stimulus and an example of a response for each case.

Name the five classical phytohormones (auxins, gibberellins, cytokinins, ethylene and abscisic acid) and associate each one with a characteristic process (stem elongation, fruit growth, cell division, ripening, dormancy and stomatal closure).

Explain what phototropism is and discuss why it is a slow response of differential growth and not a rapid movement like that of Mimosa.

What is gravitropism (geotropism)? Explain why the root grows downwards and the stem upwards even though the seed is sown in any position, indicating what the cells perceive (statoliths containing starch).

Define the dormancy of seeds and of buds, and explain which phytohormones maintain it (abscisic acid) and which break it (gibberellins), as well as its adaptive value (avoiding germinating in autumn).

Explain what abscission is and what role ethylene and the fall of auxins play in the formation of the abscission layer that makes leaves fall in autumn.

A stem illuminated from one side bends towards the light, as shown in the diagram (the purple dots represent auxins). Explain this positive phototropism of the stem at the hormonal level: what happens to the auxins on the shaded side and how this unequal distribution produces the curvature.

Differentiate between nastic movements and tropisms, using the touching-induced closing of Mimosa pudica leaflets and the phototropism of sunflower as examples, and indicating what distinguishes them regarding response speed and permanence.

In fruit growing, plant hormones are used to produce seedless fruit (parthenocarpy) and to root cuttings. Explain which plant hormone is involved in each application and what commercial or agricultural advantage it provides.

Went's experiment demonstrated the existence of auxin. Describe his set-up (coleoptile tips on agar blocks, asymmetric application of the block to decapitated stems) and explain what conclusion it allowed: the growth signal is a diffusible chemical substance.

Distinguish between climacteric fruits (banana, apple, tomato) and non-climacteric ones (orange, grape, strawberry): what happens with ethylene and respiration in each group, what this implies for their harvesting, and how controlled-atmosphere chambers delay ripening.

The "foolish seedling" disease of rice (bakanae) made diseased plants grow out of proportion. Explain what its study discovered and name two agricultural applications of the substances discovered, indicating what they are based on.

Abscisic acid is the "water stress hormone". Explain how it is produced in the root when the soil dries, which order it carries to the leaves (closing of stomata) and what advantage the plant has in anticipating dehydration even though it turns off the $\text{CO}_2$ tap.

The herbicide 2.4-D is a synthetic auxin that kills dicotyledons while sparing cereals. Explain why a growth hormone in high doses kills the plant (uncontrolled and disordered growth) and discuss what advantage and what risk using hormones as herbicides has.

A student claims that plants bend toward an illuminated window because light chemically destroys auxins on the illuminated side, slowing their growth, and not because they are redistributed toward the shaded side. a) Design a controlled experiment that allows discriminating between both hypotheses (photochemical degradation versus lateral transport) using oat coleoptiles, impermeable mica sheets, and agar blocks. b) Reasonably predict the results you would obtain in each treatment if the lateral transport hypothesis were correct. c) Justify which controls are essential to ensure that the mechanical incision of the sheets does not bias the growth results.

A shipping container loaded with green bananas and avocados arrives at port completely overripe and rotten, even though the refrigeration system worked properly at 12 °C throughout the entire voyage. a) Explain the hormonal basis of this problem by identifying the phytohormone responsible, its positive feedback mechanism, and why climacteric fruits carry this risk. b) The shipping company is considering two preventive measures: installing potassium permanganate filters or treating the fruit with 1-methylcyclopropene (1-MCP). Justify the physiological effect that each of these options would have on the preservation of the cargo. c) If the cargo had been oranges instead of bananas, would the same mass ripening collapse have been triggered? Reason your answer by distinguishing between both types of fruit.

In a laboratory bioassay, four batches of bean seedlings subjected to different treatments are analyzed: Batch 1 (intact control plant); Batch 2 (decapitation of the shoot apex); Batch 3 (decapitation of the apex and immediate placement of an agar block rich in indoleacetic acid on the cut surface); Batch 4 (decapitation and continuous topical application of cytokinins directly onto the basal axillary buds). a) Predict the development of lateral buds in each batch after two weeks and biologically justify your predictions based on the auxin/cytokinin balance. b) Explain the origin and transport direction of both phytohormones in an intact plant. c) Based on these mechanisms, physiologically justify why a gardener must carry out periodic pinching/heading-back pruning if they wish to achieve an ornamental hedge that is highly branched and dense from the base.

To determine how the phototropism signal travels, four treatments are designed using grass coleoptiles unilaterally illuminated from the right: 1) intact coleoptile; 2) apex cut off and placed back on a water-permeable gelatin sheet; 3) apex cut off and placed back with an impermeable mica sheet inserted horizontally between apex and stump only in the illuminated half; 4) apex cut off and placed back with the impermeable mica sheet inserted horizontally only in the shaded half. a) Reasonably predict in which of the four treatments bending toward the light will be observed and in which it will not. b) Interpret what the gelatin treatment demonstrates regarding the physical or chemical nature of the signal compared to the mica treatment. c) Design a variant in total darkness, using asymmetrical agar blocks on decapitated coleoptiles, that demonstrates that bending does not require direct light, but rather a differential concentration of the active substance.

A farmer detects that his young apple trees are growing in a columnar shape with a single dominant vertical axis, which reduces flower and fruit production on the lower branches. On the other hand, he needs to quickly densify a newly planted perimeter windbreak hedge. a) Justify the physiological cause of the columnar architecture of the apple trees and reason which type of pruning (heading back the main axis or thinning out lower branches) he should apply in each case to solve both problems. b) If, instead of pruning the apple trees, he decided to use hormonal treatments, which phytohormone should he apply and to what specific anatomical area to induce the outgrowth of the lateral buds without cutting the apex? c) Predict what long-term consequence drastic and continuous pruning of the entire aerial canopy would have on the root system, taking into account auxin synthesis and polar transport.

In intensive horticulture, synthetic phytohormones are used to direct plant development. Analyze the following three practical cases: 1) When unpollinated tomato flowers are sprayed with low-dose auxins, normal-sized but seedless fruits develop; 2) When applying a high dose of the auxin analogue 2.4-D onto a mixed field, broadleaf weeds (dicots) die from metabolic disruption while the wheat crop (monocot) suffers no damage; 3) A batch of cuttings submerged in a concentrated solution of indolebutyric acid (IBA) exhibits basal necrosis instead of developing roots. a) Justify the biological mechanism that generates parthenocarpy in case 1 and reason its commercial interest. b) Explain the paradox of case 2: how can a cell growth-promoting hormone act as a lethal selective herbicide? c) Design a bioassay to identify the optimal concentration range of IBA that maximizes adventitious rooting without reaching the phytotoxic levels of case 3.

In an experimental greenhouse, two chambers are connected by a unidirectional ventilation circuit: Chamber A contains tomato plants subjected to herbivory by caterpillars (Spodoptera littoralis); Chamber B contains intact, healthy tomato plants that only receive air coming from Chamber A. After 48 hours, the plants in Chamber B, despite having had no contact with any insect, significantly increase their concentration of protease inhibitors in their leaves. a) Explain the systemic signaling cascade within the attacked plants in Chamber A, detailing the role of jasmonic acid from the initial wound to the defensive response. b) Formulate a plausible hypothesis about the chemical mechanism by which the plants in Chamber B anticipate the attack and detect danger. c) Predict what would happen if the plants in Chamber A were mutants deficient in jasmonic acid synthesis and what would happen if the plants in Chamber B were insensitive to volatile organic compounds (VOCs).

On an urban avenue, the autumn phenology of two London plane trees (Platanus × hispanica) of the same age is analyzed: Tree A is located under an LED streetlight that brightly illuminates its crown throughout the night, while Tree B grows in a dark stretch. By late November, Tree B has lost all its leaves after turning yellow, while the branches of Tree A close to the streetlight retain green leaves that freeze and die after the first frost in December. a) Interpret why the streetlight delays leaf senescence, identifying the altered environmental stimulus, the photoreceptors involved, and the hormonal balance (auxins versus ethylene) in the abscission zone. b) Explain what metabolic advantages Tree B gained by orderly degrading chlorophyll and remobilizing nutrients toward the trunk before leaf drop, compared to the losses suffered by Tree A. c) If at the beginning of autumn the blade of a leaf is excised leaving only the petiole attached to the stem, predict the timing of its abscission in two situations: untreated petiole versus petiole with auxin-rich lanolin paste applied to its cut end. Justify your prediction.