explain phytohormones which control growth in plant
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Plant hormones (also known as phytohormones) are signal molecules produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of growth and development, from embryogenesis, the regulation of organ size, pathogen defense, stress tolerance and through to reproductive development. Unlike in animals (in which hormone production is restricted to specialized glands) each plant cell is capable of producing hormones.[1][2] The term 'phytohormone' was coined by Went and Thimann and used in the title of their book in 1937.[3]
Phytohormones are found across the plant kingdom, and even in algae, where they have similar functions to those seen in higher plants.[4] 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.[5
The word hormone is derived from Greek, meaning set in motion. Plant hormones affect gene expression and transcription levels, cellular division, and growth. They are naturally produced within plants, though very similar chemicals are produced by fungi and bacteria that can also affect plant growth.[6] A large number of related chemical compounds are synthesized by humans. They are used to regulate the growth of cultivated plants, weeds, and in vitro-grown plants and plant cells; these manmade compounds are called plant growth regulators or PGRs for short. Early in the study of plant hormones, "phytohormone" was the commonly used term, but its use is less widely applied now.
Plant hormones are not nutrients, but chemicals that in small amounts promote and influence the growth,[7] development, and differentiation of cells and tissues. The biosynthesis of plant hormones within plant tissues is often diffuse and not always localized. Plants lack glands to produce and store hormones, because, unlike animals—which have two circulatory systems (lymphatic and cardiovascular) powered by a heart that moves fluids around the body—plants use more passive means to move chemicals around their bodies. Plants utilize simple chemicals as hormones, which move more easily through their tissues. They are often produced and used on a local basis within the plant body. Plant cells produce hormones that affect even different regions of the cell producing the hormone.
You see plants all around you. But, are they of the same size or height? Of course not! You see big trees, medium-sized shrubs, and even plant saplings. This tells us that plants exhibit some growth changes as well as some movements. This coordination in plants is attributed to the presence of plant hormones. Unlike animals, plants do not have any muscular system or nervous system. But, they are still able to show movement and also coordination. These movements are always controlled and not haphazard. Let us learn more about coordination in plants.
Coordination in Plant
Nervous System
Nerve Impulse and Reflex Action
What is coordination in plants?
Coordination is the ability to use different parts of the plant together, smoothly and efficiently. In plants, coordination is due to the result of a chemical system, wherein plant hormones or phytohormones have a major role.
Movement in plants
Plants exhibit two types of movements.
Growth-dependent movements called the Tropic Movements. ( towards or away from a stimulus)
Non-growth dependent movements called the Nastic Movements. ( independent of stimulus)
Tropic movements
These can be classified again into 5 types. They are:
Phototropism (light)
Geotropism (gravity)
Hydrotropism (water )
Chemotropism (chemicals)
Thigmotropism (touch)
Phototropism – It is the movement of plants in response to light. The shoot system of a plant exhibits this characteristic. The shoot moves towards the light.
Geotropism – It is the movement of a plant part towards the soil. This is a characteristic of the root system. The roots always move in the direction of the earth’s gravity.
Hydrotropism– It is the movement of a plant towards the water. The stimulus here is water.
Chemotropism – It is the movement of plants in response to a chemical stimulus. A classic example of this type of movement is the growth of the pollen tube towards the ovule, during fertilization, in a flower.
Thigmotropism – It is a directional movement in plants in response to touch. For e.g. the plant tendrils climb around any support which they touch.
Nastic Movements
Nastic movements in plants are not directional movements. They are not dependent on stimulus and are growth independent. For example, the leaves of a touch me not plant (Mimosa pudica), fold up immediately when touched. These kinds of changes occur due to the changes in the amount of water in the leaves. Depending on the quantity, they either swell up or shrink.
Plant hormones or phytohormones
They are responsible for the control and coordination of plants. There are different types of hormones, which affect the growth of a plant. Phytohormones are chemical compounds which are released by stimulated cells. These hormones are diffused around the plant cells. They have a role to play in the cell division, cell enlargement, cell differentiation, fruit growth, falling of leaves, ripening of fruits, ageing of plants etc.
The different types of phytohormones are:
Auxins
Gibberellins
Cytokinins
Abscisic acid
Auxins – They help in the cell growth at the shoot tips. By elongating the cells, they help in the growth process.
Gibberellins – These hormones are responsible for the cell growth in the stem, seed germination, and flowering.
Cytokinins – They promote cell division in plants. They also promote the opening of the stomata and delay ageing in leaves.
Abscisic acid – This hormone inhibits the growth of the plant. And therefore, it promotes dormancy in seeds and buds. The detachment of fruits, flowers, and falling of leaves etc. are promoted by this hormone.
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