Question

In plant tissue culture experiments, high auxin to cytokinin ratio favours _____ development and high cytokinin to auxin ratio favours _____ development. (a) root, shoot (b) shoot, root (c) root, root (d) shoot, shoot

Short answer

A high auxin to cytokinin ratio favours root development, and high cytokinin to auxin ratio favours shoot development. Hence, the answer is (a) root, shoot.

Step-by-step solution

- Understanding Plant Hormones

First, understand the role of auxin and cytokinin in plant development. Auxins are a class of plant hormones that are primarily involved in the elongation of cells in plants. They stimulate root development at higher concentrations relative to other hormones. Cytokinins, on the other hand, promote cell division and are more involved in shoot development, especially when in higher concentrations than auxins.

- High Auxin to Cytokinin Ratio

Determine what development is favored by a high auxin to cytokinin ratio. With a higher concentration of auxin compared to cytokinin, root development is enhanced.

- High Cytokinin to Auxin Ratio

Determine what development is favored by a high cytokinin to auxin ratio. If the concentration of cytokinins is higher than that of auxins, then shoot development is promoted.

Key concepts

Auxin and Cytokinin Ratio

Understanding the delicate balance between two pivotal plant hormones, auxin and cytokinin, is essential in plant tissue culture. This balance acts as a molecular switch determining the fate of plant cells, whether they will grow into shoots or roots.

Auxins, which include compounds like indole-3-acetic acid (IAA), play a critical role in cell elongation and root initiation. An increased ratio of auxin to cytokinin tips the scale toward root development. In a culture medium rich in auxins, cells are more likely to differentiate into root tissues due to the stimulation of certain genes that favor root growth.

Conversely, higher levels of cytokinins compared to auxins shift the balance toward shoot formation. Cytokinins, such as kinetin and benzylaminopurine (BAP), boost cell division and shoot initiation. When cytokinin levels outpace those of auxins in the medium, plant cells receive a signal that it's time to form leaves and stems instead of roots.

This interplay is easily observed in tissue culture labs, where specific concentrations of these hormones are manipulated to achieve the desired outcome, making it a powerful tool in plant propagation and research.

Root Development

Root development is a fundamental aspect of plant growth and stability. In tissue culture, the intentional manipulation of hormone levels allows researchers to induce root formation from plant cells.

Auxins serve as root-inducing hormones, with higher concentrations leading to the differentiation of cultured plant cells into root tissues. The process is finely tuned; intermediate levels of auxins would stimulate cell elongation and division essential for root proliferation and elongation, ensuring the developing roots have an optimal environment.

The roots not only anchor the plant but are also responsible for water and nutrient uptake. Thus, developing a healthy root system in vitro provides a robust foundation for plants once they are transferred to soil or another growing medium. It's a prime example of how hormonal control in the lab mirrors and supports the natural growth processes of plants.

Shoot Development

In contrast to root development, shoot development is propelled by a higher cytokinin to auxin ratio. Shoots, which make up the aerial portions of the plant including stems, leaves, and eventually flowers and fruits, are crucial for photosynthesis and reproduction.

Cytokinins promote the formation of new shoots by encouraging cell division at the shoot's apical meristems. These hormones effectively signal that the plant's energy should be directed toward growing upwards and outwards, capturing light and expanding the plant's ability to photosynthesize.

In a plant tissue culture setting, increasing the level of cytokinins while maintaining lower auxin levels stimulates the cultured cells to form shoots. This manipulation enables the culturing of multiple shoots from a single explant, a process known as micropropagation. This is particularly useful for the rapid cloning of plants to produce large numbers of identical offspring with desirable traits.