E11 Tropisms

Topic

Tropisms are the directional curvature that plant organs develop in response to a physical stimulus; regardless of the specific stimulus that triggers it, this directional curvature always results from the same common mechanism: an asymmetric distribution of the hormone auxin within the organ, between one side and the other.

When the physical stimulus triggering the directional curvature is light, the resulting tropism is called phototropism: the organ detects the direction of the light and develops its directional curvature by orienting itself toward the light source or—in the case of certain organs—away from it.

When the physical stimulus is gravity, the resulting tropism is called gravitropism: the organ detects the direction of gravity and develops its directional curvature by orienting the stem upward (against gravity) or the root downward (with gravity).

When the physical stimulus is contact with a surface or another object, the resulting tropism is called thigmotropism: the organ detects the point of contact and develops its directional curvature by coiling around the object it touched, as seen in the tendrils of a climbing plant.

When the physical stimulus is a moisture gradient in the soil, the resulting tropism is called hydrotropism: the root detects which side of the soil has higher moisture levels and develops its directional curvature by growing toward that wetter zone, even when that direction does not align with the pull of gravity.

Regardless of the physical stimulus triggering any of these four tropisms, the common mechanism producing the directional curvature is always the same: PIN proteins—which act as polar auxin transporters within the organ's cells—redistribute toward one of the organ's two sides in response to the detected physical stimulus, thereby creating an asymmetric distribution of auxin between the two sides. This asymmetric distribution of auxin produces differential elongation between the two flanks of the organ: the flank with more auxin elongates more than the flank with less auxin, and this difference in elongation between the two flanks is ultimately what bends the organ and generates the directional curvature characteristic of tropisms—whether phototropism, gravitropism, thigmotropism, or hydrotropism.

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