The effect of salt stress on the herbivory of Trichoplusia ni on Arabidopsis thaliana columbia glabra Dominic J. Acri, Alec E. Biscopink, Matthew E. Krach —
University of Notre Dame, Biological Science II Laboratory – Protista North, MPM Section
Introduction
Results
Conclusion
• Environmental stresses on plants can activate plant defenses, such as toxicity and morphological structures • In environments across the globe where soil salinity is high, such as Pakistan and other eastern countries, plants with adaptations for defenses under salt stress will be more likely to survive. • Under a salt stress, or other environmental stress, Brassica plants will produce more glucosinolates, which prevent herbivores from eating the weakened plant (Jahangir 2009; López-Berenguer et al. 2008). • Salinity negatively affects plants at the molecular level by causing ion disruption which results in osmotic changes (Bojović et al. 2010). • In this experiment, a minor salt stress was applied to Arabidopsis thaliana to determine salinity’s effect on the herbivory of Trichoplusia ni. on Arabidopsis thaliana columbia glabra. • It was hypothesized that Trichoplusia ni larvae would feed less readily on the Arabidopsis thaliana that was induced by a salt stress. • It was also hypothesized that salt stressed A. thaliana would not grow as wide or as many leaves as A. thaliana under natural conditions.
There was no significant difference between the average widths or average leaf numbers for the salt stressed and control plants (Figure 4A, Figure 4B). The weight and length of the larvae feeding on the salt stressed plants were greater on average than those feeding on control plants (Figure 5A, Figure 5B). • 38 of 40 plants survived initial transplantation (20 control, 18 experimental). To correct for trial size, two additional plants were transplanted for the experimental group prior to the addition of salt stress. • 18 of 20 cabbage loopers survived through the entire experiment (10 control, 8 experimental), data for animals that died prematurely were excluded. • There was a significant difference in body weight between the experimental and control feeding Trichoplusia ni (t = -2.60, df = 16, p = 0.019). • There was a significant difference in body length between the experimental and control feeding Trichoplusia ni (t = -2.32, df = 16, p = 0.036).
Arabidopsis thaliana increases glucosilonate defense chemicals under salt stress, deterring T. ni herbivory (López-Berenguer et al. 2008, Jahinger et al. 2009) However, the plants produce less of certain other defense chemicals and increase production of nitrogenous nutrients under a salt stress (Mattson and Haack 1987). The greater herbivory on the salt stressed plants indicates that these two factors serve to aid T. ni in its herbivory more than the increased production of glucosinolates hinder it. The lack of significant difference in size or leaf number between the control and salt stressed plants, shows that the salt stress has more of an effect on biochemical pathways, than growth rate or size of the plant. This lack of significant difference, while not in line with the hypothesis, shows that the size of the plant did not affect herbivory; regardless of the lack of difference, the T. ni still fed more on the salt stressed plants. Not every plant used in the study may have received the exact same amount of sunlight or water, and the plants had the potential to become infected. In two of the control pots (C8 and C9), an infection or other hindrance, caused the plants to stop growing before the larvae were added. These plants were wholly consumed prior to completion of the study, which may have resulted in decreased herbivory and subsequently, decreased weight in the caterpillars. In future studies, one could more closely monitor each plant, and if certain ones have serious trouble growing (prior to addition of larvae), they could be removed from the study, in order to obtain more consistent data.
Materials and methods Arabidopsis thaliana columbia glabra plants were grown in standard greenhouse conditions and the experimental group received a tri-weekly salt stress. Trichoplusia ni (cabbage loopers) were allowed to feed ad libitum upon either the control or the experimental plants for a time period of one week. • A. thaliana columbia glabra seeds were planted in a soil tray, two sprouting plants were transplanted into one pot (Figure 1). • Experimental plants were stressed with 1.5 L of 100 mM NaCl solution triweekly; control plants were exposed to 1.5 mL of 0 mM NaCl solution. • One cabbage looper (Figure 2) was placed into an isolation unit (Figure 3) with two plants. • Plants growth was recorded after each salt stress and cabbage looper growth was recorded at the end of the experiment.
Literature cited Fig. 4. Plant growth of control and salt stressed plants. There was no significant difference between plant growth, measured in leaf number and diameter (p > 0.05).
Fig. 5. Trichoplusia ni herbivory on control and salt stressed plants. (a) Animals that fed on salt stress plants grew larger in body weight. (b) Animals that fed on salt stress plants grew larger in body length.
Data points were checked for normality using a Shapiro-Wilk Normality test. After all the data passed, Student’s t-tests were run for each day of plant data and the final animal data. As the SEM bars show (Figure 4), there was no significance in plant growth. There was however a significant difference in animal growth (Figure 5). [Another figure, perhaps of Important equipment, or showing experimental design] isolation unit
Discussion Fig. 1. The Arabidopsis Thaliana columbia glabra is a mutant of the columbia ecotype, modified to not have trichomes. Two of these plants were transplanted into one pot; plant diameter and leaf number were recorded before each stress.
Fig. 2. The Trichoplusia ni (cabbage looper) is a generalist herbaviore. One animal was placed into each pot after one week of salt stress; length and mass were recorded for each T. ni larva at the end of the experiment.
Fig. 3. An isolation unit ensures that the cabbage looper will not escape without hindering gas exchange, light exposure, or experimental temperature for the plants.
The hypothesis was that T. ni larvae would feed less readily on A. thaliana, that was induced by a salt stress. It was also hypothesized that the salt stressed plants would not grow as wide or as many leaves as the control Arabidopsis thaliana. The first hypothesis was not verified by the results; T. ni fed more readily on the salt stressed plants, as indicated by their greater average mass and length. The second hypothesis was also not verified by the results; there was not a statistically significant difference between the control and experimental plants for width and leaf number.
Bojović, B., Đelić,G., Topuzović, M., Stanković, M. 2010. Effects of NaCl on seed germination in some species from families Brassicaceae and Solanaceae. Science. 32: 83-87. Web. Jahangir, M., Abdel-Farid, I. B., Kim, H. K, & Verpoorte, R. 2009. Healthy and unhealthy plants: The effect of stress on metabolism of Brassicaceae. Environmental and Experimental Botany. López-Berenguer, C., Martínez-Ballesta, M. C., García-Viguera, C., & Carvajal, M. 2008. Leaf water balance mediated by aquaporins under salt stress and associated glucosinolate synthesis in broccoli. Plant Science. 174: 321–328. Mattson, W. J., Haack, R.A. 1987. The Role of Drought in Outbreaks of Plant-Eating Insects. BioScience. 37.2: 110.
Acknowledgments We thank T.M. Olsen and S.F. Ryan for their advice, guidance, and review of this project, N. Keller and M. Shockley for their technical assistance in growing the plants and animals, and A.D. Sheppard for his suggestions for data analysis. This work was supported by the Department of Biology at the University of Notre Dame.