Plant hormones / Phytohormones

Phytohormones, also known as plant hormones, are crucial chemical substances that influence various aspects of plant growth, development, and behavior. These hormones can function at very low concentrations and are essential in coordinating and managing the physiological responses of plants to their environment. Here are the major types of phytohormones:

1. Auxins: These are primarily involved in cell elongation, root initiation, and growth regulation. They play a pivotal role in processes like phototropism (growth towards light) and gravitropism (growth in response to gravity). Indole-3-acetic acid (IAA) is the most common naturally occurring auxin.

2. Cytokinins: These promote cell division and are involved in shoot and root morphogenesis, leaf senescence, and the stimulation of seed germination. Cytokinins are synthesized in roots and are transported to other plant parts.

3. Gibberellins: Over 100 different gibberellins have been identified. They are crucial in promoting stem elongation, triggering the germination of dormant seeds, and other growth processes like flowering and fruit maturation.

4. Abscisic acid (ABA): Often termed the "stress hormone," ABA facilitates the plant's ability to withstand drought and salinity stress. It plays a significant role in seed dormancy and helps regulate stomatal closure in response to water stress.

5. Ethylene: This gaseous hormone is involved in various aspects of growth, development, and senescence, such as fruit ripening, flower wilting, and leaf fall. It is unique among phytohormones due to its gaseous state under natural conditions.

6. Brassinosteroids: These are steroid hormones that promote cell expansion and elongation, pollen tube growth, and vascular differentiation. Brassinosteroids are also critical in the plant's response to environmental and internal cues.

7. Jasmonates: These include jasmonic acid and its derivatives, playing key roles in plant defense mechanisms against herbivores and pathogens. They also regulate processes such as fruit ripening, floral development, and senescence.

8. Salicylic acid: It is primarily involved in the plant defense response against pathogens. Salicylic acid plays a key role in the resistance to biotrophic pathogens and in systemic acquired resistance, which is a whole-plant immune response triggered by localized exposure to a pathogen.

Each of these hormone types has distinct but sometimes overlapping roles within the plant. The balance and interaction of these hormones regulate complex plant behaviors and physiological responses, making them fundamental to plant adaptation and survival. For more detailed insight into the mechanisms and effects of these hormones, consulting specific botany and plant physiology texts or academic papers would be beneficial.