The photosensory function of Zmphot1 differs from that of Atphot1 due to the C-terminus of Zmphot1 during phototropic response

The role of phot1 in triggering hypocotyl phototropism and optimizing growth orientation has been well-characterized in Arabidopsis, whereas the role of Zmphot1 in maize remains largely unclear. Here, we show that Zmphot1 is involved in blue light-induced phototropism. Compared with Atphot1, Zmphot1 exhibited a weaker phototropic response to very low-fluence rates of blue light (< 0.01 μmol m⁻² s⁻¹), but stronger phototropic response to high-fluence rates of blue light (> 10 μmol m⁻² s⁻¹) than Atphot1. Notably, blue light exposure induced Zmphot1-green fluorescent protein (GFP), but not Atphot1-GFP, to form the aggregates in the cytoplasm of Nicotiana benthamiana cells. Furthermore, by generating the chimeric phot1 proteins, we found that the serine-threonine kinase (STK) domain at the C-terminus is responsible for a more volatile membrane association of Zmphot1. Consistently, the chimeric phot1 protein fusing the STK domain of Zmphot1 with other domains of Atphot1 responded similarly as Zmphot1 to both low and high fluence rates of blue light. Interestingly, although both Zmphot1 and Atphot1 interact with AtNPH3, Zmphot1 induced weaker dephosphorylation of NON-PHOTOTROPIC HYPOCOTYL 3 (NPH3) than Atphot1. Together, our findings indicate that Zmphot1 and Atphot1 exhibit different photosensory function during phototropic response and that the STK domain may play a key role in determining their properties.