Gravity Signaling

Following gravity perception, the sedimentation of amyloplasts must be converted into cellular information. Foremost among the candidates thought to specify this information is the formation of an auxin gradient.

Several different experimental approaches have shown that an auxin gradient does form during gravitropism. Using a radiolabelled-IAA approach, many researchers have confirmed that an auxin gradient forms throughout the elongation zone of gravity-stimulated roots. One of the limitations of this approach is the need to isolate a significant quantity of tissue, making it difficult to correlate the formation of the auxin gradient within the cells thought to be sensing gravity at the cap. More recently, an auxin gradient has been detected across the root cap columella cells by using an auxin-responsive promoter fused to GFP. This approach allows for the visualization of the effects of auxin within the cell, namely the resulting expression of a gene having a synthetic auxin response element. Because the GFP reporter protein must be produced and folded, there is a lag of approximately 1.5 h before a gradient of GFP appears following gravistimulation.

Another line of evidence that links gravity sensing with auxin redistribution is the very rapid relocalization that occurs to PIN3 proteins following gravistimulation. In vertical roots, PIN3 is localized uniformly throughout the plasma membrane. Within minutes after reorientation, PIN3 becomes localized to the new lower face of the cell, where it presumably directs auxin flux toward the lower flank of the root and inhibits their elongation. This relocalization is not due to the production of new PIN3 proteins, but rather results from the re-uptake of PIN3 from the plasma membrane via endocytosis. This is followed by the targeted exocytosis of vesicles containing PIN3 to a specified face of the plasma membrane.

While both of these lines of evidence support a link between gravity perception and auxin transport, the connection between the sedimentation of amyloplasts and redirection of PIN proteins remains loose. By crossing the auxin-responsive GFP into starchless mutants, we have shown that starchless roots fail to produce an auxin gradient across the root cap. This further supports the link between statolith sedimentation and the control of auxin transport.

In addition to auxin, several other potential growth regulating signals have been implicated in gravity signaling. For example, one of the fastest changes to be observed in gravity sensing cells is a change in ionic currents, measured as both membrane potential changes and surface potential changes. These changes have been reported within seconds after gravistimulation. Similarly, changes in pH have also been measured within minutes of gravistimulation, with root cap columella cells becoming alkaline and the apoplasts of these cells becoming acidic as a result of proton pumping. As of now, it is unclear whether or how these signals carry information about polarity that influences growth.