Phosphorylation blues: Cracking the phototropin phosphocode

Phototropins are blue-light receptors that regulate diverse light-driven processes, including phototropism, chloroplast movement, stomatal opening, leaf positioning, and leaf flattening, all of which optimise photosynthesis and promote plant growth. These autophosphorylating serine/threonine kinases initiate signalling from the plasma membrane by altering protein phosphorylation patterns. Nearly 3 decades of research have uncovered numerous phosphorylation events pivotal for distinct phototropin-mediated responses. Here, we summarise major advances in understanding phototropin autophosphorylation, the identification of substrate proteins, and how phosphorylation at specific sites, collectively termed the phototropin phosphocode, controls protein function. We also discuss the emerging role of protein phosphatases in modulating this phosphocode and the potential to engineer these regulatory mechanisms to enhance photosynthetic performance and plant biomass.