Objective

The purpose of this experiment was to determine the effect of indole-3-acetic acid (IAA), a plant growth hormone, on the mitosis of onion root cells. It was hypothesized that the presence of IAA would increase the number of onion root cells undergoing division.

Procedure

Two plastic cups were filled with sand and labeled. Two green onion plants were then placed in either one of the two cups. The control cup received treatment of water while the experimental cup received treatment of a 1 mg/L solution of IAA. Both cups were soaked with their respective solutions and then fitted with plastic wrap in order to avoid losing solution from evaporation. Both the experimental and control cups were placed in front of a window in order to provide plants with access to sunlight and allowed to sit for 48 hours.
After 48 hours, the onion plants were removed from the cups and three (for redundancy) root tips were cut from both stems using a razor. The root tips were cut to a length of 0.50 cm. Both sets of root tips were then allowed to sit in a 1M solution of HCl for 5 minutes in order to soften the tissue for easier access to the stain. The root tips were then removed from the HCl solution and placed in deionized water for 1 minute to rinse. Then the tips were stained in a carbol fuchsin stain solution for 3 minutes. After 3 minutes, the tips again rinsed in deionized water for 1 minute.
One tip from both sets were placed on a slide and firmly crushed between the slide and the cover slip. This allowed for better observation of cells underneath the microscope. While under the microscope, the number of cells in both mitosis and interphase were counted for both groups. This data was collected and then a chi-square analysis was performed to test the statistical difference of the groups.

Data

Table 1: Final Values for Number of Cells in Mitosis and Interphase for Control and Experimental Samples

Calculations

Table 2: Calculation Variable Key

𝑊𝑖𝑡h 1 𝑑𝑒𝑔𝑟𝑒𝑒 𝑜𝑓 𝑓𝑟𝑒𝑒𝑑𝑜𝑚 ⇒ (𝑃)𝑣𝑎𝑙𝑢𝑒 ≪ 0. 01 (see discussion section)

Discussion

Two green onion plants were placed in cups with sand and either one of two solutions: water (control) or a 1 mg/L IAA solution (experimental). After being allowed to grow over 48 hours, root tips were trimmed from the plants and then stained using carbol fuchsin. A tip from each set was then crushed between a slide and cover slip and placed under a microscope. The number of cells in mitosis and interphase were then counted for both the control and the experimental group. Using that data, statistical tests were performed to determine whether a statistical difference existed.
After performing a chi-square test (see calculations section), the results demonstrated that with the (calculated) one degree of freedom, there is a statistical difference between the IAA-treated and water-treated groups. This demonstrates that the plant growth hormone IAA has a significant positive influence on the number of cells entering mitosis.
IAA belongs to the auxin plant hormone family [1]. They play key roles in plant cell growth, division, and differentiation. Auxin perception is mediated by receptors that physically bind auxin, allowing it to travel from the extracellular to the intracellular environment, where it then initiates signal transduction cascades that trigger specific responses [2]. Responses to IAA reception include the up regulation of genes which promote cell division [1,2].
Due to its important role in regulation of plant cell growth and division, IAA has been widely used in commercial agriculture products for enhancing crop development [3]. In horticulture, auxins have been used to promote adventitious root growth, to create root stem cuttings and to promote uniform fruit and flowering growth [3].
References

1. Zhao Y. Auxin biosynthesis and its role in plant development. Annu Rev Plant Biol. 2010;61:49-64. doi: 10.1146/annurev-arplant-042809-112308. PMID: 20192736; PMCID: PMC3070418.
2. Lavy M, Estelle M. Mechanisms of auxin signaling. Development. 2016 Sep 15;143(18):3226-9. doi: 10.1242/dev.131870. PMID: 27624827; PMCID: PMC5047657.
3. Foliar application of IAA at different growth stages and their influence on growth and productivity of bread Wheat (triticum aestivum l.) H Hanaa and A Safaa 2019 J. Phys.: Conf. Ser. 1294 092029