Question 63·Hard·Inferences
A group of plant biologists led by Qian Li investigated the role of the protein GRV1 in the gravitropic response (growth directed by gravity) of sunflower (Helianthus annuus) roots. Using CRISPR, they produced GRV1‐knockout seedlings and compared them with unmodified (wild-type) seedlings. When grown in a vertical orientation, roots of both groups elongated directly downward at similar rates. When the pots were suddenly rotated 90°, wild-type roots began bending downward within 3 hours, whereas the knockout roots did not show noticeable bending until 12 hours after rotation. This outcome suggests that ______
Which choice most logically completes the text?
For SAT science-based inference questions, restate the setup and results, then ask what changed and what stayed the same between groups/conditions. Avoid answers that overclaim (e.g., “required for any response”) or shift to an unmeasured variable (e.g., degree of bending instead of timing). Choose the option that matches exactly the observed pattern without adding assumptions.
Hints
Separate the two conditions in the experiment
Compare the results when the plants are kept vertical versus when the pots are rotated 90°. What is the same in both groups, and what changes?
Focus on the key difference between wild-type and knockout
Is the difference about whether roots grow at all, how fast they elongate, or how quickly they start bending after rotation?
Watch for “speed vs. ability vs. degree” traps
Some choices may claim GRV1 is required for any bending, or that GRV1 changes how far roots bend. Stick to what the data directly show: the main measured difference is when bending becomes noticeable.
Step-by-step Explanation
Restate the experiment in simple terms
First, summarize what the scientists did and observed:
- They created two groups of sunflower seedlings: normal (wild-type) and GRV1-knockout (missing the GRV1 protein).
- When grown vertically, roots in both groups grew straight down at similar rates.
- After the pots were rotated 90°, wild-type roots began bending downward within 3 hours, while knockout roots didn’t show noticeable bending until about 12 hours.
Keep the two situations separate: normal vertical growth vs. response to rotation.
Identify what stays the same (what GRV1 likely doesn’t control)
In the vertical orientation, both groups:
- elongated directly downward, and
- did so at similar rates.
So the data do not support the idea that GRV1 is needed for roots to grow downward under normal orientation or that it makes roots grow faster in general.
Identify what changes (what GRV1 likely does influence)
After rotation, the key difference is how soon bending starts:
- Wild-type: bending within 3 hours
- Knockout: noticeable bending around 12 hours
That pattern points to GRV1 affecting the speed/timing of the gravitropic response to a change in orientation.
Choose the option that matches the limited conclusion
The most logical conclusion is that GRV1 helps roots reorient more quickly after the direction of gravity changes for the plant.
Therefore, the correct choice is: the presence of GRV1 enhances the speed at which sunflower roots adjust their growth in response to gravitational changes.