Beschreibung:
<jats:title>Summary</jats:title><jats:p>Phototropism allows plants to orient their photosynthetic organs towards the light. In <jats:styled-content style="fixed-case">A</jats:styled-content>rabidopsis, phototropins 1 and 2 sense directional blue light such that phot1 triggers phototropism in response to low fluence rates, while both phot1 and phot2 mediate this response under higher light conditions. 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 (<jats:styled-content style="fixed-case">PKS</jats:styled-content>) family may act early in this pathway, because <jats:styled-content style="fixed-case">PKS</jats:styled-content>1, <jats:styled-content style="fixed-case">PKS</jats:styled-content>2 and <jats:styled-content style="fixed-case">PKS</jats:styled-content>4 are needed for a normal phototropic response and they associate with phot1 <jats:italic>in vivo</jats:italic>. Here we show that <jats:styled-content style="fixed-case">PKS</jats:styled-content> proteins are needed both for phot1‐ and phot2‐mediated phototropism. The phototropic response is conditioned by the developmental asymmetry of dicotyledonous seedlings, such that there is a faster growth reorientation when cotyledons face away from the light compared with seedlings whose cotyledons face the light. The molecular basis for this developmental effect on phototropism is unknown; here we show that <jats:styled-content style="fixed-case">PKS</jats:styled-content> proteins play a role at the interface between development and phototropism. Moreover, we present evidence for a role of <jats:italic><jats:styled-content style="fixed-case">PKS</jats:styled-content></jats:italic> genes in hypocotyl gravi‐reorientation that is independent of photoreceptors. <jats:italic>pks</jats:italic> mutants have normal levels of auxin and normal polar auxin transport, however they show altered expression patterns of auxin marker genes. This situation suggests that <jats:styled-content style="fixed-case">PKS</jats:styled-content> proteins are involved in auxin signaling and/or lateral auxin redistribution.</jats:p>