Phototropin2 Contributes to the Chloroplast Avoidance Response at the Chloroplast-Plasma Membrane Interface

Abstract

Blue-light-induced chloroplasts movements play an important role in maximizing light utilization for photosynthesis in plants. Under a weak light condition, chloroplasts accumulate to the cell surface to capture light efficiently (chloroplast accumulation response). Conversely, chloroplasts escape from strong light and move to the side wall to reduce photodamage (chloroplast avoidance response). The blue light receptor phototropin (phot) regulates these chloroplast movements and optimizes leaf photosynthesis by controlling other responses as well as chloroplast movements. Seed plants such as Arabidopsis thaliana have the phototropins phot1 and phot2. They redundantly mediate phototropism, stomatal opening, leaf flattening, and the chloroplast accumulation response. However, the chloroplast avoidance response is induced by strong blue light and regulated primarily by phot2. Phototropins are localized mainly on the plasma membrane. However, a substantial amount of phot2 resides on the chloroplast outer envelope. Therefore, differentially localized phot2 might have different functions. To determine the functions of plasma membrane-and chloroplast envelope-localized phot2, we tethered it to these structures with their respective targeting signals. Plasma membrane-localized phot2 regulated phototropism, leaf flattening, stomatal opening, and chloroplast movements. Chloroplast envelope-localized phot2 failed to mediate phototropism, leaf flattening, and the chloroplast accumulation response but partially regulated the chloroplast avoidance response and stomatal opening. Based on the present and previous findings, we propose that phot2 localized at the interface between the plasma membrane and the chloroplasts is required for the chloroplast avoidance response and possibly for stomatal opening as well.

Green light attenuates blue‐light‐induced chloroplast avoidance movement in Arabidopsis and Landoltia punctata

“…Phototropins are associated with the inner side of the plasma membrane in darkness, and a portion of light‐activated phot2, and to a lesser extent phot1, also move to the outer chloroplast membrane (Kong et al, 2012). In a recent study, Ishishita et al (2020) engineered a phot2‐green fluorescent protein fusion protein with either a chloroplast outer membrane protein tag or a plasma membrane protein tag into a phot1phot2 mutant. They were able to visualize where phot2 bound and measure known phototropin‐controlled responses.…”

Light quality, oxygenic photosynthesis and more

“…Further, CRY1 and CRY2, together with PHOT1 and PHOT2, serve to entrain the plant circadian clock to the phase of daily light-dark cycles (Somers et al 1998). We note that PHOT1 and PHOT2 are bound on plasma membranes, while the other photoreceptors are localized in the nucleus or the cytoplasm (Ronald and Davis 2019, Hayes 2020, Ishishita et al 2020. This difference probably makes the phototropins particularly useful in deciphering directional light cues.…”

Toward Multiplexed Optogenetic Circuits

“…This light cycle will be specific to each engineered pathway and requires optimization from the microplate strain screening to scale-up in bioreactors. An original strategy could be at the cell level to mimic organelle motion observed in nature: the blue light receptor phototropin (phot) regulates intracellular chloroplast movements ( Ishishita et al, 2020 ). At low illumination, chloroplasts accumulate to the cell surface to capture light efficiently (chloroplast accumulation response).…”