A group of plant biologists led by Qian Li investigated the role of the protein GRV1 in the gravitropic response, or growth directed by gravity, of sunflower (Helianthus annuus) roots. Using CRISPR, they produced GRV1-knockout seedlings and compared them with unmodified seedlings. When the pots were rotated 90 degrees, unmodified roots began bending downward within 3 hours, whereas knockout roots did not show noticeable bending until 12 hours later. After 24 hours, however, the two groups had reached the same average bend angle and total root length. The researchers then restored GRV1 in only one of two regions in separate knockout seedlings: the root cap, the tissue at the tip that senses the direction of gravity, or the elongation zone, the tissue behind the cap where unequal growth causes bending. Restoring GRV1 in the root cap produced the timing seen in unmodified roots, whereas restoring it in the elongation zone did not shorten the delay. This pattern suggests that ______
Which choice most logically completes the text?
For a hard experimental-inference question, separate each measured outcome, such as onset time, final extent, and overall growth, before evaluating the choices. Then use any follow-up manipulation to determine what it establishes and reject choices that combine a supported conclusion with an unsupported or contradicted claim.
Hints
Distinguish onset from endpoint
Compare when bending begins with the bend angle and root length measured after 24 hours. Do these observations support the same kind of conclusion?
Use the restoration evidence
Ask what can be concluded when producing the protein in one tissue restores normal timing but producing it in another tissue does not.
Avoid adding an unsupported effect
Check whether each choice claims that GRV1 changes final bend extent, general elongation, or a tissue requirement that the experiment contradicts.
Step-by-step Explanation
Separate timing from the final outcome
After rotation, the unmodified and knockout roots differ in when they begin bending: the unmodified roots respond much sooner. By 24 hours, however, the groups have the same average bend angle and total root length. Thus, the evidence concerns the timing of the response, not its eventual extent or general root growth.
Compare the tissue-specific restorations
Restoring GRV1 only in the root cap recovers the timing seen in unmodified roots. Restoring it only in the elongation zone does not shorten the knockout seedlings' delay. Therefore, GRV1 activity in the root cap is sufficient to restore the rapid response, whereas activity in the elongation zone alone is not.
Limit the conclusion to what the results establish
The experiment identifies the tissue in which GRV1 must be active to contribute to rapid bending, but it does not establish every molecular step by which the protein works. The safest inference is about the dependence of the rapid response on root-cap activity.
Select the best-supported completion
The best completion states that GRV1's contribution to rapid gravitropic bending depends on its activity in the gravity-sensing root cap rather than on direct activity in the elongation zone.