Abstract
Blue-light-induced chloroplast 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 in addition to chloroplast movements. Seed plants such as Arabidopsis (Arabidopsis thaliana) have 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. Phots 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.
Highlights
Plants recognize UV-B, blue (BL), red (RL), and farred light with photoreceptor molecules (Kami et al, 2010; Paik and Huq, 2019), i.e. a red/far-red photoreceptor phytochrome (Franklin and Quail, 2010), a photolyase-like BL receptor cryptochrome (Liu et al, 2016), a UV-B photoreceptor, UVR8 (Jenkins, 2017), and so on
Most phototropins are constitutively associated with the plasma membrane (PM), we previously showed that some Arabidopsis phototropins are localized on the outer envelope of the chloroplast (Kong et al, 2013c)
To reveal the functions of phot2 localized on the chloroplast outer membrane, we produced transgenic Arabidopsis plants expressing phot2-GFP fusion protein targeted to the chloroplast outer membrane with the N-terminal, 47-amino acid sequence of OUTER ENVELOPE MEMBRANE PROTEIN7 (OEP7; Fig. 1A; Lee et al, 2001), hereafter called the CP-P2G lines
Summary
Plants recognize UV-B, blue (BL), red (RL), and farred light with photoreceptor molecules (Kami et al, 2010; Paik and Huq, 2019), i.e. a red/far-red photoreceptor phytochrome (Franklin and Quail, 2010), a photolyase-like BL receptor cryptochrome (Liu et al, 2016), a UV-B photoreceptor, UVR8 (Jenkins, 2017), and so on These plant photoreceptors mediate light responses including photomorphogenesis and photoperiodic flowering by regulating gene expression. A single species of phot of the liverwort Marchantia polymorpha (Mpphot) regulates chloroplast accumulation and avoidance (Komatsu et al, 2014) It is localized on the chloroplast outer envelope and the PM (Kodama, 2016). We produced transgenic plants in which phot was tethered to the chloroplast outer envelope or the PM in the Arabidopsis phot1phot mutant background, and we observed phot-associated phenotypes in the transgenic plants
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