Phototropism allows plants to orient their photosynthetic organs towards the light. In Arabidopsis, phototropins 1 and 2 sense directional blue light with phot1 triggering phototropism in response to low fluence rates, while both phot1 and phot2 mediate this response in higher light. Phototropism results from asymmetric growth in the hypocotyl elongation zone that depends on an auxin gradient across the embryonic stem. How phototropin activation leads to this growth response is still poorly understood. Members of the Phytochrome Kinase Substrate (PKS) family may act early in this pathway because PKS1, PKS2 and PKS4 are needed for a normal phototropic response and they associate with phot1 in vivo. Here we show that PKS proteins are needed both for phot1 and phot2-mediated phototropism. The phototropic response is conditioned by the developmental asymmetry of dicotyledonous seedlings with a faster growth reorientation when the cotyledons face away from the light compared to seedlings with the cotyledons facing the light. The molecular basis for this developmental effect on phototropism is unknown, here we show that PKS proteins play a role at the interface between development and phototropism. Moreover we present evidence for a role of PKS genes in hypocotyl gravi-reorientation that is independent of photoreceptors. pks mutants have normal levels of auxin and normal polar auxin transport however they show altered expression patterns of auxin marker genes. This suggests that PKS proteins are involved in auxin signaling and/or lateral auxin redistribution. This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

PMID:

 

24286493

 

[PubMed – as supplied by publisher]