Gravitropism
Gravitropism (or geotropism) is a turning or growth movement by a plant in response to gravity. Charles Darwin was one of the first to 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 behaviour can be easily demonstrated with a potted plant. When laid onto its side, the growing parts of the stem begin to display negative gravitropism, bending (biologists say, turning; see tropism) upwards. Herbaceous (non-woody) stems are capable of a small degree of actual bending, but most of the redirected movement occurs as a consequence of root or stem growth in a new direction.
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Gravitropism in the root
If the root cap is removed, root growth ceases to respond to gravity. The root cap is vital for gravitropism since it contains cells with sensors called statoliths, which are amyloplasts packed with starch. Amyloplasts are a type of plastid similar to chloroplasts. Statoliths are dense organelles that settle to the lowest part of the root cap cells in response to a change in the gravity vector. This initiates differential cell expansion in the root elongation zone causing a reorientation of the root growth (see below). The location of the elongation zone is many cells above the root cap, so intercellular signal transduction must occur from the site of gravity perception, in the root cap, to the growth response in the elongation zone. As of 2005, the nature of this signal is an active area of research in Plant Biology.
Roots bend in response to gravity due to a regulated movement of the plant hormone auxin known as polar auxin transport. In roots, an increase in the concentration of auxin will inhibit cell expansion, therefore, the redistribution of auxin in the root can initiate differential growth in the elongation zone resulting in root curvature.
Gravitropism in the stem
A similar mechanism is known to occur in plant stems except that the shoot cells have a different dose response curve with respect to auxin. In shoots, increasing the local concentration of auxin promotes cell expansion; this is the opposite of root cells.
The differential sensitivity to auxin helps explain Darwin's original observation that stems and roots respond in the opposite way to the gravity vector. In both roots and stems auxin accumulates towards the gravity vector on the lower side. In roots, this results in the inhibition of cell expansion on the lower side and the concomitant curvature of the roots towards gravity (positive gravitropism). In stems, the auxin also accumulates on the lower side, however, in this tissue it increases cell expansion and results in the shoot curving up (negative gravitropism).
Gravitropic mutants
There are cultivars known that are mutants for mechanisms thought to be required for gravitropism. Usually these are trees that have a weeping or pendulate growth habit. The branches still respond to gravity, but with a positive response, rather than the normal negative response. Some agravitropic mutants have also been isolated in Arabidopsis thaliana (one of the genetic model systems for plant research) and their roots have a weak response to gravity and grow in random directions. One agravitropic mutant cannot produce starch causing the statoliths to be less dense and thus reducing their ability to function as sensors.
See Also
Other tropisms:
- Thigmotropism - movement in response to touch/contact.
- Phototropism - movement in response to light.
- Skototropism - inverse movement in response to light (negative phototropism)