Question

Which of the plant hormones is known as the master plant hormone and why? a. cytokinin, because many plant functions require cell division b. gibberellins, because growth is essential to many plant responses c. abscisic acid, because it is necessary for leaf and fruit drop d. brassinosteroids, because water uptake is so fundamental to plant growth e. auxin, because there are many different auxin-responsive genes

Short answer

The master plant hormone is Auxin because there are many different Auxin-responsive genes, indicating its broad influence in plant development and functions.

Step-by-step solution

Understand the role of each plant hormone

Auxins: They control the growth of plants in response to light and gravity. Cytokinins: They stimulate cell division and differentiation. Gibberellins: They are important for stem elongation, germination and flowering. Abscisic acid: It plays a role in seed dormancy and environmental stress response. Brassinosteroids: They are essential for plant's resistance to diseases and environmental stresses.

Identify the master plant hormone

Comparing the roles of the hormones, Auxins affect a broad range of processes in plant growth and development. Especially, the wide range of auxin-responsive genes indicate that Auxin could have a massive impact on plant life.

Confirm the Answer

Auxins influence a number of plant functions and are responsive to numerous genes, suggesting their Master role in plant growth and development. The 'why' part of the question gets answered by stating the multiplicity of auxin-responsive genes.

Key concepts

Auxins

Plant hormones, often described as the chemical messengers of the botanical world, play critical roles in regulating various growth processes. One of the most prominent of these hormones is auxin. It's known as the 'master plant hormone' for its overwhelming influence on plant development.

Auxins are primarily associated with the elongation of cells in plant stems and are actively involved in the process of phototropism—where plants grow towards light. They also play a pivotal role in gravitropism, which describes a plant's growth response to gravity. Additionally, auxins are crucial for the formation of fruit without fertilization, a process known as parthenocarpy, and are involved in the differentiation of xylem and phloem, the water and nutrient-conducting tissues.

The hormone's interaction with auxin-responsive genes suggests that it integrates various developmental signals, which qualifies it as a master plant hormone. Its widespread effects on plants affirm its position at the top of the hormonal hierarchy.

Cytokinins

Cytokinins, although not as expansive in function as auxins, hold a valuable place in plant hormone regulation. These hormones are fundamental in promoting cell division, also known as cytokinesis, after which they are named.

Besides supporting cellular replication, cytokinins also influence the path of development cells take, guiding them towards forming the various types of plant tissues. They work in tandem with auxins to control the balance between root and shoot growth, ensuring that a plant's architecture remains balanced and sturdy.

Moreover, cytokinins play an important role in delaying the aging of plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from older tissues to newer ones. They are also involved in the expansion of leaves and the enhancement of photosynthesis by promoting chloroplast development.

Gibberellins

Gibberellins are synonymous with growth and development, contributing notably to stem elongation. These hormones are vital in breaking seed dormancy and initiating germination, a critical step in a plant's life cycle.

Aside from germination, gibberellins are involved in other growth aspects such as the elongation of internodes, which are the segments between two nodes or joints along the plant stem. They also regulate the sex expression of certain plants and influence leaf and fruit senescence. In flowering, gibberellins can help in inducing bolting, where the stem rapidly elongates before flowering occurs in response to long days or specific environmental cues.

Furthermore, gibberellins can interplay with auxins to promote fruit set and growth, particularly in seedless varieties, and have been used in agriculture to increase fruit size and improve yields.

Abscisic Acid

Abscisic acid (ABA) stands as a key influencer during times of environmental stress and plays an essential role in maintaining the plant's internal water balance.

This hormone is primarily responsible for the inhibition of growth, which is a protective response to environmental stresses like drought, salinity, and cold temperatures. It brings about the closure of stomata in the leaves, reducing water loss through transpiration. Moreover, ABA is involved in seed and bud dormancy, ensuring that seeds only germinate and buds only start to grow when conditions are favorable for survival.

Despite these inhibitory effects, ABA also has a role in developmental processes such as root growth and root branching, allowing the plant to adjust its water absorption capabilities as per the environmental conditions.

Brassinosteroids

Brassinosteroids, while less well-known than their hormonal counterparts, are integral for a plant's development and its ability to respond to external stresses. They share some functional similarities with animal steroid hormones, despite differing significantly in structure.

These hormones contribute to cell division and cell elongation, impacting the overall plant growth pattern. They aid in increasing the resistance of plants to various environmental stresses like extreme temperatures, pathogens, and pollutants. Brassinosteroids also modulate light-dependent plant growth, and therefore, they are indispensable in ensuring that seedlings establish themselves after germination, by promoting expansion during etiolation, the process where plants grow in partial or complete absence of light.

Moreover, brassinosteroids play a role in enhancing resistance against fungal infections and some pests, fortifying plant cell walls and thus, bolstering the plant's defensive capabilities.