Question

To explore how hormones function, researchers have begun to transform plants with particular genes. In one experiment, a gene involved in cytokinin synthesis was introduced into tobacco plants. Which one of the following results would be expected? a. Individuals produced more lateral branches. b. Stems grew extremely tall and slender. c. Roots were incapable of responding to gravity. d. Stomata were closed most of the time.

Short answer

The most expected result after introducing a gene involved in cytokinin synthesis into tobacco plants would be an increase in the number of lateral branches (option a).

Step-by-step solution

Understand the role of cytokinins in plant growth

Cytokinins are plant hormones that play a vital role in various aspects of plant growth and development, including cell division, shoot and root development, and responses to environmental changes. In this context, we need to know how an increased cytokinin synthesis would affect the plant's growth and functions.

Analyze each option

Let's analyze each option based on the role of cytokinins: a. Individuals produced more lateral branches: Cytokinins stimulate lateral shoot growth, so increased cytokinin synthesis can increase the number of lateral branches. b. Stems grew extremely tall and slender: This option describes the effect of increasing another hormone, gibberellin, which is not related to cytokinins. c. Roots were incapable of responding to gravity: This option involves a response to gravity called gravitropism, which is not directly related to cytokinins, although cytokinins can affect root growth and development. d. Stomata were closed most of the time: Stomatal movements are mainly regulated by abscisic acid, not cytokinins.

Choose the most expected result

Among the given options, one that directly relates to the effect of cytokinins on plant growth is: a. Individuals produced more lateral branches. So, the most expected result after introducing a gene involved in cytokinin synthesis into tobacco plants would be an increase in the number of lateral branches.

Key concepts

Plant Hormones and Their Roles

Plant hormones, also known as phytohormones, are crucial to the regulation of plant growth and development. These naturally occurring organic substances influence physiological processes at low concentrations, orchestrating everything from embryogenesis and germination to flowering and fruit ripening. Cytokinins, for example, are a group of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots.

Other notable hormones include auxins, which stimulate stem elongation and root formation, gibberellins, which affect stem elongation and seed germination, and abscisic acid, which primarily moderates stomatal closure and seed dormancy. Each of these hormones has a unique set of roles, and their effects can be understood through experiments that manipulate their levels in plants, such as increasing cytokinin synthesis to study changes in plant morphology and function.

Lateral Shoot Growth

Lateral shoot growth is a phenomenon in plants that enables them to branch out and expand their reach for light and nutrients. Cytokinins play a pivotal role in promoting the development of these lateral branches. When cytokinin levels are increased, as seen in experimental modifications of tobacco plants, there is a corresponding increase in the number of shoots that emerge from the main stem. These shoots can also bear leaves, buds, and flowers, contributing to the plant's reproductive success.

Importance of Lateral Branching

Increased lateral branching can lead to a fuller canopy, which maximizes the plant's ability to photosynthesize by capturing more light. Additionally, it can have significant implications for agricultural practices, where the manipulation of cytokinin levels could be used to enhance crop yields or control plant architecture for better harvesting.

Gravitropism

Gravitropism, also known as geotropism, is the directional growth response of plants to gravity. Roots typically exhibit positive gravitropism, growing downwards, while shoots show negative gravitropism, growing upwards. This response ensures that plant parts grow in the correct orientation, securing the plant firmly in the soil and allowing it to reach towards the light.

How Plants Sense Gravity

Specialized cells known as statocytes in roots and shoots contain amyloplasts, dense organelles that settle under the influence of gravity. These amyloplasts trigger a signal transduction pathway that redistributes growth hormones, leading to differential growth rates on different sides of the root or shoot, causing the plant to bend. While cytokinins may influence overall root and shoot development, they are not the direct regulators of the gravitropic response.

Stomatal Regulation

Stomata are microscopic pores found on the surface of leaves and stems that regulate gas exchange and water evaporation in plants. Guard cells flank each stomatal pore and control its opening and closing. Abscisic acid (ABA) is the primary hormone responsible for signaling the closure of stomata in response to environmental stress, such as drought.

When ABA levels rise, guard cells lose potassium and water, becoming flaccid and causing the stomata to close. This process conserves water but also limits CO₂ uptake, affecting photosynthesis. Cytokinins can have a peripheral role by promoting guard cell division, thus indirectly influencing stomatal density, but they are not primary regulators of stomatal movements.