**Plant Hormone Characteristics**:
– Plant hormones affect gene expression, cellular division, and growth.
– Manmade compounds used to regulate plant growth are called plant growth regulators (PGRs).
– Plants lack glands to produce and store hormones.
– Hormones are transported within plants using cytoplasmic streaming, diffusion, and vascular tissues.
– Plant hormones regulate cell growth, development, and differentiation at specific stages.
**Major Plant Hormone Classes**:
– Major classes of plant hormones include abscisic acid, auxins, brassinosteroids, cytokinins, and ethylene.
– Additional major plant hormones include jasmonates, salicylic acid, and strigolactones.
– Some compounds serve functions similar to major hormones but are still debated as hormones.
**Specific Plant Hormones**:
– **Abscisic Acid**:
– Inhibits bud growth, seed dormancy, and influences metabolic reactions.
– **Auxins**:
– Mainly represented by indole-3-acetic acid, influencing cell enlargement, bud formation, and root initiation.
– **Brassinosteroids**:
– Steroid-based hormones controlling cell elongation, division, and stress resistance.
– **Cytokinins**:
– Influence cell division, shoot formation, and counter apical dominance.
– **Ethylene**:
– Affects cell growth, shape, and fruit ripening.
**Additional Plant Hormones**:
– **Gibberellins**:
– Promote stem elongation, seed germination, and break seed dormancy.
– **Jasmonates**:
– Important for plant defense and interact with other hormones.
– **Salicylic Acid**:
– Critical for plant defense against pathogens and stress responses.
– **Strigolactones**:
– Stimulate parasitic weed germination and play roles in plant growth and development.
**Applications and Significance**:
– Plant hormones are utilized in horticulture for propagation techniques like cuttings and tissue culture.
– They play a significant role in seed dormancy and germination.
– Some plant hormones have found applications in human medicine, such as salicylic acid for pain relief and potential cancer treatments.
Plant hormones (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, including embryogenesis, the regulation of organ size, pathogen defense, stress tolerance and reproductive development. Unlike in animals (in which hormone production is restricted to specialized glands) each plant cell is capable of producing hormones. Went and Thimann coined the term "phytohormone" and used it in the title of their 1937 book.
Phytohormones occur across the plant kingdom, and even in algae, where they have similar functions to those seen in vascular plants ("higher plants"). Some phytohormones also occur in microorganisms, such as unicellular fungi and bacteria, however in these cases they do not play a hormonal role and can better be regarded as secondary metabolites.