Plant Responses to Internal and External Signals

Root cells of a grass detect a molecule released by a soil fungus and, about an hour later, begin secreting a defensive enzyme. Which sequence best describes what happens inside each root cell?

Two related wildflower species have seeds holding enough stored food for about 6 days of growth in darkness. In the dark, species X's stem grows 1.5 cm per day and species Y's grows 0.4 cm per day. Assuming each stem grows straight up, from about how deep in the soil could each species' seedling still reach the light?

In a model plant signaling pathway, one activated receptor switches on 12 molecules of an enzyme, and each of those enzyme molecules then makes 400 molecules of a second messenger. How many second-messenger molecules result from one activated receptor?

Leaf cells are treated with a drug that blocks protein phosphatases. The cells then get a short pulse of a signal that works through a protein kinase pathway. Compared with untreated cells, what would most likely happen?

Young stem segments were floated for 3 hours in different solutions, and their growth was measured. Results: weak pH 7 buffer, 0.2 mm; weak pH 7 buffer + auxin, 1.4 mm; pH 4.5 buffer (no auxin), 1.3 mm; strong pH 7 buffer that stops the cell walls from turning acidic + auxin, 0.3 mm. Which conclusion do the data best support?

A thale cress seedling is lit from its left side only and begins bending to the left. Researchers then measure auxin in the left and right halves of the stem's growing zone. Which result would they most likely find?

A gall-forming soil bacterium (Agrobacterium) inserts two genes into plant cells: one for making auxin and one for making cytokinin. The infected cells keep dividing into a disorganized lump called a tumor. Researchers infect kalanchoe stems with a mutant strain whose auxin gene is broken but whose cytokinin gene still works. What would the tumors most likely do?

Gibberellin (GA) promotes stem growth by causing DELLA proteins, which repress growth genes, to be destroyed. Many high-yield 'semi-dwarf' wheat varieties carry a DELLA protein that GA can no longer mark for destruction. If these semi-dwarf plants were sprayed with extra GA, what would most likely happen?

In dim light, the chloroplasts in spinach leaf cells gather along the cell walls that face the light. Researchers measured how well each color of light caused this movement (1.0 = most effective): 400 nm, 0.70; 450 nm, 1.00; 500 nm, 0.40; 550 nm, 0.05; 660 nm, 0.05; 730 nm, 0.00. Which kind of photoreceptor most likely controls the movement?

Small seeds of a meadow herb were kept moist under four conditions for 5 days. Germination: open sunlight, 88%; sunlight filtered through a layer of green leaves held above the dish, 12%; sunlight filtered through a clear plastic sheet held the same way, 85%; darkness, 9%. Which explanation fits best?

Bean plants in a growth chamber get bright white light. In one group, LED lamps add extra far-red light while the amount of red light stays the same. Compared with controls, how would the extra-far-red group most likely grow?

Plants grown on a normal day-night cycle were moved into constant dim light at hour 0. The mRNA for one gene peaked at hours 6, 31, 56 and 81. Which conclusion is best supported?

A seedling is laid on its side in the dark. In both its root and its shoot, auxin builds up on the lower side. Yet the root curves down and the shoot curves up. Which explanation fits best?

A student thinks roots sense gravity when dense starch-filled plastids (statoliths) in root cap cells settle toward the lower cell wall. Which observation would most strongly support this idea?

To toughen tomato transplants, a grower runs gentle fans over one bench of seedlings for three weeks so the plants sway all day, and keeps another bench in still air. Results: swayed plants, 31 cm tall with 6.1 mm stems; still-air plants, 42 cm tall with 5.2 mm stems. Which description is correct?

Each evening, the leaves of a prayer plant fold upward. The fold happens at a swollen joint at the base of the leaf (a pulvinus), where a group of 'motor' cells suddenly loses turgor. What would most likely happen if those motor cells were treated with a drug that blocks their potassium (K⁺) channels?

In gene-for-gene recognition, a plant resists a pathogen strain when one of its resistance (R) genes matches one of the strain's avirulence (Avr) genes. Plant line A has R1, line B has R2 and line C has R1 and R2. Strain X has Avr1, strain Y has Avr2 and strain Z has neither. Which pairing is most likely to result in disease?

In the hypersensitive response, leaf cells around an infection site make antimicrobial compounds, toughen their walls and then kill themselves. How does destroying its own cells help the plant?

One lower leaf of each cucumber plant is infected with a mild bacterial strain. Five days later, an upper leaf is challenged with a fungus. Average fungal lesions per upper leaf: normal plants, no first infection, 24; normal plants, first infection, 6; plants with a gene that breaks down salicylic acid, first infection, 22. Which conclusion is best supported?

In a two-choice test, 40 predatory bugs could each walk toward air from caterpillar-damaged plants or air from undamaged plants. 30 chose the damaged plants and 10 chose the undamaged plants. With a null hypothesis of no preference, the chi-square value is 10.0 (critical value 3.84 at p = 0.05, 1 degree of freedom). Which conclusion is correct?