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.

Figure 10. *Insect fresh weight gain (%) after 3 days of feeding on select Arabidopsis plant treatments.* Insects feeding on wild-type *Arabidopsis* had a fresh weight gain of 215.24%. When feeding on jar1, weight gain was 238.77%. Standard error bars indicate this is statistically significant.

Quiz

Question

Which of the statements below is supported by the results displayed in Figure 10 of this experiment?

A. The cabbage loopers did not like to eat the Arabidopsis plants, so they lost weight during the three day study.

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B. The cabbage loopers will eat both the wild-type and jar1 plants, but they gain less weight eating the wild-type, either because they eat less of it or because they are less able to assimilate its nutritive value.

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.

C. The gene altered in the jar1 mutant does not have an influence on the cabbage looper insects as they feed on these plants.

Your answer is incorrect. Try again.

Looks Good! 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.

Figure 11. *Insect fresh weight gain (%) after 3 days of feeding on select Arabidopsis plant treatments.* Insects gained 215.24% in fresh weight on wild-type treatment, 238.77% on jar1 treatment, and 313.36% on coi1 treatment. All means are statistically different from each other.

Quiz

Question

Based on the results of the insect response experiment using the mutant coi1 displayed in Figure 11, select the best conclusion.

A. The COI1 gene does not influence cabbage looper weight gain.

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B. The COI1 gene must be responsible for the JA response, not the JAR1 gene.

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C. The COI1 gene has a stronger influence on cabbage looper weight gain than the JAR1 gene, but both impact the cabbage looper weight gain.

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.

Looks Good! 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.

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