Figure 9.

Model for Phototropic Adaptation Mechanisms.

Phosphorylated NPH3 proteins interact with phot1 proteins under darkness. When the seedlings are irradiated with slightly strong blue light, in which the refractory state is induced, NPH3 proteins are dephosphorylated. The proteins are then detached from phot1 complexes, resulting in the formation of aggregates. Accordingly, the phot signal cannot be transduced efficiently downstream. Both red light and blue light upregulate the transcription of RPT2, accumulating RPT2 proteins gradually, and the RPT2 proteins bind to dephosphorylated NPH3 proteins. The RPT2-NPH3 complexes interact with phot1 proteins, and the phot signal is transduced through the RPT2-NPH3 complexes. Therefore, the RPT2-mediated regulation of blue light-induced dephosphorylation of NPH3 proteins is necessary for establishing the effector adaptation mechanism that separates the first positive and second positive phototropism. Small red circles represent phosphorylated phot1 and NPH3 proteins.