No abstract

Plant cells perceive cold temperatures and initiate cellular responses to protect themselves against cold stress, but which cellular compartment mediates cold sensing has been unknown. Chloroplasts change their position in response to cold to optimize photosynthesis in plants in a process triggered by the blue-light photoreceptor phototropin (phot), which thus acts as a cold-sensing molecule. However, phot in plant cells is present in multiple cellular compartments, including the plasma membrane (PM), cytosol, Golgi apparatus, and chloroplast periphery, making it unclear where phot perceives cold and activates this cold-avoidance response. Here, we produced genetically encoded and modified variants of phot that localize only to the cytosol or the PM and determined that only PM-associated phot induced cold avoidance in the liverwort Marchantia polymorpha. These results indicate that the phot localized to the PM constitutes a cellular compartment for cold sensing in plants.

Background Organelle motility is essential for the correct cellular function of various eukaryotic cells. In plant cells, chloroplasts move towards the intracellular area irradiated by a weak light to maximise photosynthesis. To initiate this process, an unknown signal is transferred from the irradiated area to distant chloroplasts. Quantification of this chloroplast movement has been performed using visual estimations that are analyst-dependent and labour-intensive. Therefore, an objective and faster method is required. Results In this study, we developed the cellssm package of R (https://github.com/hnishio/cellssm.git), which is a user-friendly tool for state-space modelling to statistically analyse the directional movement of cells or organelles. Our method showed a high accuracy in estimating the start time of chloroplast movement in the liverwort Marchantia polymorpha over a short period. The tool indicated that chloroplast movement accelerates during transport to the irradiated area and that signal transfer speed is uneven within a cell. We also developed a method to estimate the common dynamics among multiple chloroplasts in each cell, which clarified different characteristics among cells. Conclusions We demonstrated that state-space modelling is a powerful method to understand organelle movement in eukaryotic cells. The cellssm package can be applied to various directional movements (both accumulation and avoidance) at cellular and subcellular levels to estimate the true transition of states behind the time-series data.

SUMMARY Plant cells alter the intracellular positions of chloroplasts to ensure efficient photosynthesis, a process controlled by the blue light receptor phototropin. Chloroplasts migrate toward weak light (accumulation response) and move away from excess light (avoidance response). Chloroplasts are encircled by the endoplasmic reticulum (ER), which forms a complex network throughout the cytoplasm. To ensure rapid chloroplast relocation, the ER must alter its structure in conjunction with chloroplast relocation movement, but little is known about the underlying mechanism. Here, we searched for interactors of phototropin in the liverwort Marchantia polymorpha and identified a RETICULON (RTN) family protein; RTN proteins play central roles in ER tubule formation and ER network maintenance by stabilizing the curvature of ER membranes in eukaryotic cells. Marchantia polymorpha RTN1 (MpRTN1) is localized to ER tubules and the rims of ER sheets, which is consistent with the localization of RTNs in other plants and heterotrophs. The Mprtn1 mutant showed an increased ER tubule diameter, pointing to a role for MpRTN1 in ER membrane constriction. Furthermore, Mprtn1 showed a delayed chloroplast avoidance response but a normal chloroplast accumulation response. The live cell imaging of ER dynamics revealed that ER restructuring was impaired in Mprtn1 during the chloroplast avoidance response. These results suggest that during the chloroplast avoidance response, MpRTN1 restructures the ER network and facilitates chloroplast movement via an interaction with phototropin. Our findings provide evidence that plant cells respond to fluctuating environmental conditions by controlling the movements of multiple organelles in a synchronized manner.