Phototropism in Arabidopsis

Introduction

Being sessile organisms, plants posses various mechanisms to react to different and changing environmental stimuli. One of these mechanisms allows plants to adjust their growth direction to the direction of incoming blue light. This phototropic response involves sensing of light by photoreceptors, here mainly the membrane-associated proteins phot1 and phot2 kk2006a, redirection of the flux of the hormone auxin bbpm2004a,etls2006a,nbps2003a,pbbm2004a, as well as other downstream signaling events ddri2010a,flds2003a,ikmn2008a,hs2007a,lshp2006a,week2008a. Although these key players in phototropism in Arabidopsis thaliana are known, detailed means of interaction remain hidden.

The current view on phototropism can be summarized as follows: phototropism is a blue light- initiated process with its response being fluence rate dependent. For simplicity, here only low fluence rates of maximally 0.1 μmol m^-2s ^-1 are considered—a scenario in which the phototropic response depends mostly on the activity of the photo receptor phot1. Under these fluence conditions, the second receptor of the same family, phot2, can be neglected. In addition, the two cryptochromes cry1 and cry2 have a mild effect on phototropism kk2006a but are not further considered here.

Open Questions

Considering the fact that during phototropism a lateral auxin gradient with its maximum on the shaded side is formed, the question arises how it is possible that such a gradient is established. Here, it is of special interest why the maximum of the gradient is located on the shaded side since the original blue light stimulus is applied to the opposite side and photo-activation seems to be positively fluence correlated. Still, one can argue that the light absorption of a tissue like a dark grown hypocotyl (with a diameter of about 250μm) hardly absorbs any light but then one would need to question why a gradient is formed at all.

In the course of this project, it is planned to investigate this gradient formation relying on both, experimental techniques as well as computational modeling, collaborating with the groups of Richard Smith and Christian Fankhauser as part of the Plant Growth project from SystemsX.ch.

References

de Carbonnel M, Davis P, Roelfsema MR, Inoue S, Schepens I, Lariguet P, Geisler M, Shimazaki K, Hangarter R, Fankhauser C
The Arabidopsis PHYTOCHROME KINASE SUBSTRATE2 protein is a phototropin signaling element that regulates leaf flattening and leaf positioning.
Plant Physiol: 2010 Mar, 152(3);1391-405
[PubMed:20071603] [WorldCat.org: ISSN ESSN ] [DOI] ( o)

Blakeslee JJ, Bandyopadhyay A, Peer WA, Makam SN, Murphy AS
Relocalization of the PIN1 auxin efflux facilitator plays a role in phototropic responses.
Plant Physiol: 2004 Jan, 134(1);28-31
[PubMed:14730061] [WorldCat.org: ISSN ESSN ] [DOI] ( p)

<biblio>

1. bbpm2004a pmid=14730061
2. ddri2010a pmid=20071603
3. etls2006a pmid=16371470
4. flds2003a pmid=14645723
5. hs2007a pmid=16906793
6. ikmn2008a pmid=18378899
7. kk2006a pmid=16870491
8. lshp2006a pmid=16777956
9. nbps2003a pmid=12827205
10. pbbm2004a pmid=15208397
11. week2008a pmid=20031918

</biblio>