= Gravitropism
<Gravitropism> is directional growth of a <plant> <organ> in response to <gravity>. A downward-growing <root (botany)> shows a positive response; an upward-growing <shoot> shows a negative one. Reorienting a growing <plant> can therefore produce curvature as growth becomes unequal on opposite sides. The response concerns the direction of growth, rather than passive bending under the <organ>'s weight.
In <Arabidopsis thaliana>, <auxin> links perception near the <root (botany)> tip to growth farther back. <Ranjan Swarup> and collaborators used tissue-targeted expression of the influx carrier <AUX1> to map where transport was needed. Their 2005 experiments implicated the <lateral root cap> and elongating <plant epidermis>. Separately, blocking <auxin> responsiveness in expanding <plant epidermis> <cells> abolished the gravity response. The comparison distinguishes delivery of a signal from the capacity of receiving <cells> to respond to it.<Root gravitropism requires lateral root cap and epidermal cells for transport and response to a mobile auxin signal by Ranjan Swarup and others>[Abstract; Figures1,5–7]{ref}
A horizontal <root (botany)> provides a useful observation: record the tip position over time and inspect curvature through the <root elongation zone>, rather than expecting the entire existing <root (botany)> to rotate as a rigid rod. Holding the <light> direction fixed helps distinguish a gravity response from <phototropism>. Transport, cellular growth and final tip direction are related measurements, but they are not interchangeable.
Back to article page
