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Concept
Gravitropism
Summary
Gravitropism (also known as geotropism) is a coordinated process of differential growth by a plant in response to gravity pulling on it. It also occurs in fungi. Gravity can be either "artificial gravity" or natural gravity. It is a general feature of all higher and many lower plants as well as other organisms. Charles Darwin was one of the first to scientifically document that roots show positive gravitropism and stems show negative gravitropism. That is, roots grow in the direction of gravitational pull (i.e., downward) and stems grow in the opposite direction (i.e., upwards). This behavior can be easily demonstrated with any potted plant. When laid onto its side, the growing parts of the stem begin to display negative gravitropism, growing (biologists say, turning; see tropism) upwards. Herbaceous (non-woody) stems are capable of a degree of actual bending, but most of the redirected movement occurs as a consequence of root or stem growth outside. The mechanism is based on the Cholodny–Went model which was proposed in 1927, and has since been modified. Although the model has been criticized and continues to be refined, it has largely stood the test of time.
Root growth occurs by division of stem cells in the root meristem located in the tip of the root, and the subsequent asymmetric expansion of cells in a shoot-ward region to the tip known as the elongation zone. Differential growth during tropisms mainly involves changes in cell expansion versus changes in cell division, although a role for cell division in tropic growth has not been formally ruled out. Gravity is sensed in the root tip and this information must then be relayed to the elongation zone so as to maintain growth direction and mount effective growth responses to changes in orientation to and continue to grow its roots in the same direction as gravity.
Abundant evidence demonstrates that roots bend in response to gravity due to a regulated movement of the plant hormone auxin known as polar auxin transport. This was described in the 1920s in the Cholodny-Went model.
Official source
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