Phototropin switches between cis- and trans-autophosphorylation in light-induced chloroplast relocation in Marchantia polymorpha.

In the accumulation response, chloroplasts move toward weak blue light (BL) to maximize photosynthetic efficiency; in the avoidance response, they move away from strong BL to reduce photodamage. The BL receptor kinase phototropin (phot) mediates these chloroplast relocation responses, and the chloroplast relocation response requires phot kinase activity. Upon receiving BL, phot undergoes autophosphorylation; however, the molecular mechanisms that regulate chloroplast relocation through phot autophosphorylation remain unclear. In this study, we conducted biochemical experiments using phot in the liverwort Marchantia polymorpha and revealed that phot employs cis-autophosphorylation under weak BL and both cis- and trans-autophosphorylation under strong BL. Inhibiting trans-autophosphorylation reduced phot autophosphorylation and suppressed the avoidance response, but not the accumulation response. These findings suggest that phot employs two modes of autophosphorylation to alternate between the accumulation and avoidance responses in plants.