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Unformatted text preview: surface of cells in roots and to the upper surface of cells in shoots. Calmodulin, a calcium binding protein, and secondary messenger in signal transduction pathways, mediates the calcium migration. Elongation occurs in the walls with the lowest calcium levels. Cytokinins are also important to gravitropism. Roots grown normally have uniform distribution of cytokinins in their root tips. Within minutes of placing a root tip horizontally, cytokinins are distributed asymmetrically in the root. Concentration of cytokinins on the lower side of the root inhibits elongation on that side, while lowered concentration of cytokinins on the upper side of the root favors elongation that will orient the growing tip back towards gravity....
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- Fall '09