Studying ER-membrane contact sites in plants using the optogenetic approach: Taking the LiMETER as an example.

The endoplasmic reticulum (ER) links to multiple organelles through membrane contact sites (MCS), which play critical roles in signal transduction, cell homeostasis and stress response. However, studying the behaviour and functions of MCS in plants is still challenging, partially due to the lack of site-specific markers. Here, we used an optogenetic reporter, LiMETER (Light-inducible Membrane-Tethered cortical ER), to study the structure and dynamics of ER-PM contact sites (EPCS) in plants. Upon blue light activation, LiMETER is recruited to the EPCS rapidly, while this process is reversible when blue light is turned off. Compared with other EPCS reporters, LiMETER specifically and reversibly labels the contact sites, causing little side-effects on the ER structure and plant development. With its help, we re-examined the formation of ER-PM connections induced by cell-intrinsic factors or extracellular stimuli. We found that EPCSs are preferably localised at ER tubules and the edge of ER cisternae, and their number increased significantly under abiotic stress conditions. The abundance of ER and PM interaction is also developmental dependent, suggesting a direct link between ER-PM interaction, ER function and cell homeostasis. Taken together, we showed that LiMETER is an improved marker for functional and microscopical studies of ER-PM interaction, demonstrating the effectiveness of optogenetic tools in future research.