Lesson: Gravitropism

Tropism is a growth response between a plant and an external stimulus. The stimulus could be weather, touch, time, gravity or light. A positive response is indicated by growth toward a stimulus and a negative response is indicated by growth away from the stimulus.

Gravitropism is a plant's growth response to gravity. Typically roots grow down into the ground, and stems and leaves grow up above the ground. If one were to place a plant on its side, it would begin to bend in an upright position. This occurs because of the plant hormone also called auxin that plays a part in gravitropism. When the plant is laid on its side, the auxin concentration increases along the lower sides of the roots and stems of the plant. The auxin stimulates cell elongation on the stem which makes the stem bend up toward the sky. The auxin prohibits cell elongation in the roots which causes the roots to bend down into the soil. Gravitropism allows the plant to respond to gravity no matter what position the plant is in.

Sources:
Plants for Kids

Biology of Plants, 6th ed. Raven, Peter H., Everert, Ray F., Eichhorn, Susan E. Worth Publishing 1999.

Botany: An Introduction to Plant Biology. Mauseth, James D. Jones and Bartlett Publishers. Sudbury, Massachusetts 1998.

Herbicide May Have Unintended Consequences

Purdue Univ. researchers have discovered a fine-tuning mechanism involved in plant root growth that has them questioning whether a popular herbicide may have unintended consequences, causing some plants to need more water or nutrients.

Angus Murphy, a professor of horticulture, and Wendy Peer, an assistant professor of horticulture, study the movement of auxin, a plant hormone essential for plant development. They showed that ABCB4, a protein responsible for moving auxin into cells, also removes the hormone when too much has accumulated.

"We knew that the protein took auxin up, but found that it switched to removing auxin when a threshold is reached," says Murphy, whose findings appeared in the early online version of The Plant Journal. "It starts transporting the hormones out."

That fine-tuning mechanism is integral to proper development of plant root hairs, which extend from the main plant root and are where most water and minerals enter.

Read full article in: The LaboratoryEquipment.com

At long last, how plants make eggs

University of California, DavisJune 4, 2009

A gradient (red) in the concentration of the plant hormone auxin determines that only one of the eight nuclei in a plant’s embryo sac will become an egg.

A long-standing mystery surrounding a fundamental process in plant biology has been solved by a team of scientists at UC Davis.

The group's groundbreaking discovery that a plant hormone called auxin is responsible for egg production has several major implications.

First, this is the first definitive report of a plant hormone acting as a morphogen, that is, a substance that directs the pattern of development of cells based on its concentration.

Also, the study's results provide tantalizing new insights into the evolutionary pathway that flowering plants took 135 million years ago when they split off from gymnosperms, the "naked-seeded" plant group that includes conifers, cycads and ginkgo trees.

Finally, the group used their discovery to make additional egg cells within plant reproductive structures, raising the prospects that these techniques may someday be used for enhancing the reproduction and fertility of crop plants.

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