Phytohormones
Introduction
Phytohormones, also known as plant hormones, are chemical substances produced naturally within plants that regulate growth and development. They are a fundamental part of plant biology, influencing processes such as cell division, flowering, fruit development, and responses to environmental stimuli.
Classification
Phytohormones are broadly classified into five main groups: auxins, gibberellins, cytokinins, abscisic acid, and ethylene. Each group has a unique set of roles and functions within the plant.
Auxins
Auxins are a group of phytohormones that play a crucial role in the coordination of many growth and behavioral processes in a plant's life cycle. They are primarily involved in cell elongation, apical dominance, and tissue differentiation.
Gibberellins
Gibberellins are a large family of diterpenoid compounds that regulate a wide range of growth and developmental processes, including stem elongation, germination, dormancy, flowering, sex expression, enzyme induction, and leaf and fruit senescence.
Cytokinins
Cytokinins are a class of plant growth substances that promote cell division, or cytokinesis, in plant roots and shoots. They are involved in various processes of plant growth and development, including cell division, shoot initiation and growth, leaf senescence, nutrient mobilization, and seed germination.
Abscisic Acid
Abscisic acid (ABA) is a plant hormone that regulates plant growth and development. It plays an essential role in stress tolerance, mediating responses to abiotic stresses such as drought, salinity, and cold.
Ethylene
Ethylene is a gaseous plant hormone that regulates a wide range of biological processes in plants. It is involved in many processes such as fruit ripening, flower wilting, leaf fall, and response to mechanical stress.
Biosynthesis
Phytohormones are synthesized in the plant through a series of biochemical reactions. The biosynthesis of each type of phytohormone involves unique pathways and enzymes.
Auxin Biosynthesis
The biosynthesis of auxins primarily occurs in the shoot apical meristem and young leaves. The key precursor in auxin biosynthesis is the amino acid tryptophan.
Gibberellin Biosynthesis
Gibberellins are synthesized from the acetyl CoA via the mevalonate pathway. The process involves a series of enzymatic reactions and leads to the production of various forms of gibberellins.
Cytokinin Biosynthesis
Cytokinins are synthesized in roots and transported to other parts of the plant. The biosynthesis of cytokinins involves the conversion of adenine to isopentenyl adenine.
Abscisic Acid Biosynthesis
Abscisic acid is synthesized from carotenoids in the chloroplasts. The biosynthesis of ABA involves several enzymatic reactions, leading to the production of ABA from a C40 carotenoid precursor.
Ethylene Biosynthesis
Ethylene is synthesized from the amino acid methionine. The process involves a series of enzymatic reactions, leading to the production of S-adenosylmethionine, which is then converted to ethylene.
Function
Phytohormones play a crucial role in nearly every aspect of plant growth and development. They regulate various physiological processes, from seed germination to fruit ripening.
Auxin Function
Auxins regulate cell elongation, apical dominance, and tissue differentiation. They also play a role in phototropism and gravitropism, processes that allow plants to respond to light and gravity, respectively.
Gibberellin Function
Gibberellins promote stem elongation, seed germination, and leaf and fruit senescence. They also regulate sex expression and enzyme induction.
Cytokinin Function
Cytokinins promote cell division and shoot initiation and growth. They also delay leaf senescence and are involved in nutrient mobilization and seed germination.
Abscisic Acid Function
Abscisic acid regulates stress tolerance, mediating responses to abiotic stresses such as drought, salinity, and cold. It also plays a role in seed dormancy and germination.
Ethylene Function
Ethylene regulates fruit ripening, flower wilting, leaf fall, and response to mechanical stress. It also plays a role in pathogen defense and stress responses.