In: Biology
Explain why auxin does not seem to play the same role in sunflowers and other eudicots as it does in grasses.
Ans: Auxins are synthesized primarily in the shoot apical meristem and young stems and leaves. Their main effect is toae to stimulate plant grwoth by promoting cell elongation in stems and coleoptiles and by goverining grwoth responses to light and gravity.
The great majority of angiosperms are classified either as monocots or eudicots. The plants detect gravity much as animals do that is particles called statoliths in certain cell moce in the direction gravity pulls them. In most of the plants the statoliths are amyloplasts, modified plastids that contain starch grains. In eudicot angiosperm sems, amyloplasts often are present in one or two layer of cells just outside the vascular bundles. In monocots such as cereal grasses, amyloplasts are located in a region of tissue near the base of the leaf sheath. In root its present in the root cap. If ths spatial orientation of a plant cell is shifted experimentally, its amyloplasts sink through the cytoplasm until they come o rest at the bottom of the cell.
If there is alteration in the movement of the amyloplast, then there must be separate mechanism is operating. In stem, the sinking of amyloplast may provides a mechanical stimulus that triggers a gene guided redistribution of auxin (IAA). When the potten sunflower seedling is turned on its side in a dark room. Within 15-20 minutes, cell elongation decreases markedly on the upper side of the growing horizontal stem, but increases on the lower side. With the adjusted grwoth pattern, the stem curve upward, even in the absence of light. Therfore, changing auxin gradient/shifting of IAA from top to the bottom of the stem results in the altered pattern of cell elongation.
In roots, a high concentarion of auxin has the opposite effect-it inhivits cell elongarion. If a roo is placed on its side, amyloplasts in the root cap accumulate neat the side wall that now is the bottom side of the cap. In some way this stimulates cell elongation in the opposite wall and wihtin a few hours the root once again curves downward. In root tips of many plants, especially eudicots, shift in IAA concentraion that corresponds to the changing position of amyloplasts.