In a recent study on the role of the ubiquitin-proteasome system in seedling and chloroplast development, a ubiquitin variant ubK48R was inducibly expressed in Arabidopsis thaliana, (Talloji et al. 2022). In addition to inhibiting proteolysis, its expression during germination caused both defects in chloroplast development and seedling death. This seedling lethality could be suppressed by mutating PHOT2, the gene that encodes the blue light receptor, phototropin2. The aim of this study was to establish how the mutations of phototropins introduced into the Arabidopsis genome to counteract the lethal effect of UbK48R expression affect phototropin-mediated chloroplast responses to blue light. We compared blue-light-controlled chloroplast redistribution in the leaves of several phototropin mutants, in particular in two frameshift mutants and a deletion mutant of PHOT2 with three amino acids deleted in the photosensory LOV1 domain. The deletion mutant showed decreased functionality of the photoreceptor protein, which was manifested in a marked decrease in the chloroplast avoidance response. A comparison of AlphaFold3 predicted structural models for the wild type and mutated PHOT2 proteins revealed that the lack of the three amino acids disrupts interdomain interactions near the deletion site and affects a potential dimerization interface. Our results point to an important function of the PHOT2 LOV1 domain in light signal transduction, which results in chloroplasts avoiding excess light.
