Experiment 2: Insect Response

The magnitude of plant response to wounding by leaf crushing was different from insect feeding. JA-Ile synthesis is much higher in crushed plants, for both the WT and the jar1 mutant.

Table 2. JA-Ile levels (pmole g - 1) after leaf crushing and cabbage looper feeding
Time sample collected WT crushing jar1crushing WT with insect feeding jar1 with insect feeding WT unwounded (No feeding) jar1 unwounded (No feeding)
0 min. 0 0 17 1 1 1
60 min. 275 25 37 3 1 1
120 min. 200 25 7 2 1 1

Is it possible that this lower response level in the plants to insect wounding still allows the plants to mount some sort of defense against the cabbage loopers to slow down their feeding?  Would there be a difference between the WT and the jar1 mutant in their insect defense?  These questions were answered when Scott conducted the insect response experiment.

The second experiment focused on the insect response to the two types of Arabidopsis plants. If JAR1 signals help the Arabidopsis plants mount a defense, the cabbage loopers might feed less on the normal (wild-type) compared with the mutant plants. This experiment required a 3-day feeding time. Scott weighed all the cabbage loopers, let some feed exclusively on wild-type plants, let others feed exclusively on jar1 mutants, and reweighed them after 3 days. Because some insects ate more than others, he averaged all the initial and final weights for both plant treatment types and then calculated their changes in weight as a percentage.

Because Scott placed the cabbage loopers on islands of plants surrounded by water, he created an environment that gave the cabbage loopers only one feeding choice. As Scott monitored the experiment he could predict that the functioning of the JAR1 gene in the wild-type plants would not dramatically repel these insects. There was not a mass escape attempt of cabbage loopers from the wild-type or the jar1 plants.  The impact would be measured in the weight changes of these insects after the three day feeding.

Inects feeding on wild-type arabidopsis had a weight gain of 215.24%. When feeding on jar1, weight gain was 238.77%. Standard error bars indicate this is statistically significant.



Which of the statements below is supported by the results of this experiment?

Looks Good! Correct: The graph tells us that the cabbage loopers gained weight on both kinds of plants, but there is a difference in weight gain between the insect feeding on the wild type vs. the jar1 plants. Our conclusion would be that the mutated jar-1 cannot induce some plant response that impacts the insects.

Because Scott was a part of a research team using Arabidopsis as the model plant subject in their experiments, they could take advantage of the previous discoveries of other mutants known to impact JA response.  One mutant, coi1, was available to Scott to repeat the insect response experiment. This provided a comparison of cabbage looper weight gain on wild-type plants that had no mutant genes, jar1 plants, and coi1 plants.

Insects gained 215.24% on wild-type treatment, 238.77% on jar1 treatment, and 313.36 on coi1 treatment. All means are statistically different from each other.



 Based on the results of the insect response experiment using the mutant coi1, select the best conclusion.

Looks Good! Correct: Choice C fits with the data. Plants with the coi1 mutation support more weight gain than plants with the jar1 mutation. The experiment supports the hypothesis that both genes are involved in the insect feeding response but the JAR1 gene has an intermediate effect compared with COI1 gene.