Plant growth factors

Are chemicals that occur naturally at extremely low concentrations. They regulate various aspects of plant growth and development from seed formation and germination through the aging and death of a plant. They also coordinate many plant responses to enviromental stimuli
Often, plant growth factors, like animal hormones, carry information from on epart of the organism to another. Plant growth factor are transported in the transpiration stream of the Xylem and by mass flow in the phloem. From the xylem and phloem, they usually reach their target cels by diffusion.
Unlike hormones, plant growth factors are not synthesized in special organs, and they sometimes act in the immediate vinictiy of their production. Ethene for example, usually acts on the tissues from which it is released.
Ethene is a gas released from ripening fruits, nodes of stems, aging leaves, and flowers. It is involved in seed dormancy, fruit ripening and leaf abscisson. Other plant growth substances include gibberellins and cytokinins which are growth promoters while abscisic acid (ABA) is a powerful growth inhibitor that often acts antagoniscally to growth promoters. ABA appears to promote dormancy in some seeds, and it stimulates the closing of stomata.
The main plant growth factor involved in phototropism is indoleacetic acid (IAA) which is synthesized mainly in the shoot tips. Its major effect is to promote growth by increasing the rate of cell elongation.
Phtotropisms. Most of our knowledge about phototropism has been gained from experiments on coleoptiles, the specialised, protective sheaths around the germinating shoots of grasses. Coleoptiles have been used because their response to light is easy to observe, they are small, and they are easy to grow in large numbers
If a grass seeding is exposed to light from one side, it grows towards that light. microscopic examination reveals that the cells on the shaded side of the coleoptiles are significantly longer than those on the lit side. It seems that coloeptiles bend towards the light by differential growth; Cells on the side (shaded) elongate faster and become longer that those on the lit side. Early exeperiments on phototropism involved coleoptiles, decapitated coleoptiles, and coleoptiles shaded in various ways. Later experiments used agar blocks to collect a chemical diffusing downwards from shoot tips.
In the 1930's, the chemical messenger responsible for phototropic repsonses was extracted and given the name auxin. Auxin was later found to consist mainly of indoleactic acid (IAA)
Coleoptile tips appear to produce equal amounts of IAA in the dark or the light, but its distribution can vary. It seems that when light strikes one side of the coleoptile, a receptor triggers the redisdribution if IAA so that more travels down the shaded side of the coleoptile. The increased IAA concentration causes cells on the shaded side to grow longer than those of the lit side
IAA is thought to stimulate cell elongation by causing target cells to secrete proteins into their cell walls. The resulting increase in acidity is thought to weaken the bond between cellulose microfibrils, allowing the cell wall to expand when the cell takes in water

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	Criado por harry_bygraves mais de 11 anos atrás