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Abstract
Although sessile, plants are able to grow toward or away from an environmental stimulus. Important examples are stem or leaf orientation of higher plants in response to the direction of the incident light. The responsible photoreceptors belong to the phototropin photoreceptor family. Although the mode of phototropin action is quite well understood, much less is known of how the light signal is transformed into a bending response. Several lines of evidence indicate that a lateral auxin gradient is responsible for asymmetric cell elongation along the light gradient within the stem. However, some of the molecular key players leading to this asymmetric auxin distribution are, as yet, unidentified. Previously, it was shown that phototropin gets autophosphorylated upon illumination and binds to a scaffold protein termed NPH3 (for nonphototropic hypocotyl 3). Using a yeast three-hybrid approach with phototropin and NPH3 as a bait complex, we isolated a protein, termed EHB1 (for enhanced bending 1), with a so far unknown function, which binds to this binary complex. This novel interacting factor negatively affects hypocotyl bending under blue light conditions in Arabidopsis (Arabidopsis thaliana) and thus seems to be an important component regulating phototropism. Interestingly, it could be shown that the gravitropic response was also affected. Thus, it cannot be ruled out that this protein might also have a more general role in auxin-mediated bending toward an environmental stimulus.
Highlights
Phototrophic responses of higher plants are mainly mediated by blue light perceived by photoreceptors of the phototropin family (Briggs and Christie, 2002; Christie, 2007; Holland et al, 2009)
Several lines of evidence indicate that PIN1 and PIN3 redistribution plays a key role in this tropic response (Friml et al, 2002; Friml et al, 2002; Blakeslee et al, 2004)
Two conclusions can be deduced from these results: firstly, phosphorylation of NPH3 (Pedmale and Liscum, 2007), might not be essential for EHB1 binding to NPH3, and secondly, EHB1 interaction with the BTB domain might compete with the binding of a Cullin-3 like protein to NPH3 recently mentioned by Pedmale and Liscum (Pedmale and Liscum, 2007)
Summary
Phototrophic responses of higher plants are mainly mediated by blue light perceived by photoreceptors of the phototropin family (Briggs and Christie, 2002; Christie, 2007; Holland et al, 2009). Several proteins have been identified to interact with phototropin (Sullivan et al, 2009), up to now only two proteins directly interacting with phototropin, namely NPH3 (Motchoulski and Liscum, 1999) and RPT2 (root phototropism 2) have been intensively characterised They were reported to be key players involved in further downstream signalling (Sakai et al, 2000). RPT2 seems to be required for both, phototropin-mediated hypocotyl bending as well as stomatal opening (Inada et al, 2004) It is still unclear, which component is responsible for the temporary attachment of the phot1/NPH3 complex to the plasma membrane (Sakamoto and Briggs, 2002; Kaiserli et al, 2009). We have identified a negative acting element in phototropin signal transduction directly interacting with NPH3, which seemed to be involved gravitropic response of both root and hypocotyl and named the mutant enhanced bending 1 (ehb1) and the corresponding gene EHB1
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