37.4 Gravity: the gravitropic response

explain the gravitropic response of plant roots

how to shoots and roots respond to gravity

shoots respond by growing upward and roots respond by growing downward or laterally

gravitropism

the growth or movement of a plant in a particular direction in response to gravity

statolith hypothesis

the hypothesis that amyloplasts (dense, starch-storing plant organelles) serve as statoliths in gravity detection in plant cells

interconnected ideas of the statolith hypothesis

1. amyloplasts are organelles that contain starch granules; which is denser than water. these organelles are pulled to the bottom of the root cap by the force of gravity
2. the position of the amyloplasts activates pressure receptor proteins located in the plasma membrane. these receptors initiate the gravitropic response

statoliths

a tiny stone or dense particle found in specialized gravity-sensing organs in some animals and in gravity-sensing tissues of plants

how do root caps respond to the changes in the direction of gravitational pull

they respond by changing the distribution of auxin in the root cap. they auxin is distributed so that there is a higher concentration at the bottom side of the root cap

steps of root cap response

1. gravity-sensing cells trigger changes in the position of auxin transport proteins that redistribute auxin
2. the cells in the lower portion of the root receive increased concentrations of auxin
3. high \[auxin\] inhibits growth in roots and the difference in concentration triggers differential growth, causing bending down

effect of auxin in root cells

high concentrations of auxin cause a decrease in cell division and elongation. this causes root cells to bend on the upper side and therefore bend downward