Abstract
Phototropins, namely, phototropin 1 (phot1) and phototropin 2 (phot2), mediate chloroplast movement to maximize photosynthetic efficiency and prevent photodamage in plants. Phot1 primarily functions in chloroplast accumulation process, whereas phot2 mediates both chloroplast avoidance and accumulation responses. The avoidance response of phot2-mediated chloroplasts under high-intensity blue light (HBL) limited the understanding of the function of phot1 in the chloroplast accumulation process at the HBL condition. In this study, we showed that the phot2 mutant exhibits a chloroplast accumulation response under HBL, which is defective when the root phototropism 2 (RPT2) gene is mutated in the phot2 background, mimicking the phenotype of the phot1 phot2 double mutant. A further analysis revealed that the expression of RPT2 was induced by HBL and the overexpression of RPT2 could partially enhance the chloroplast accumulation response under HBL. These results confirmed that RPT2 also participates in regulating the phot1-mediated chloroplast accumulation response under HBL. In contrast, RPT2 functions redundantly with neural retina leucine zipper (NRL) protein for chloroplast movement 1 (NCH1) under low-light irradiation. In addition, no chloroplast accumulation response was detected in the phot2 jac1 double mutant under HBL, which has been previously observed in phot2 rpt2 and phot1 phot2 double mutants. Taken together, our results indicated that phot1 mediates the HBL-induced chloroplast accumulation response in an RPT2-dependent manner and is also regulated by j-domain protein required for chloroplast accumulation response 1 (JAC1).
Highlights
Chloroplast photorelocation is a critical physiological process that optimizes photosynthetic efficiency and ensures plant survival
Both phot1 and phot2 have been shown to be functionally redundant in the regulation of chloroplast accumulation under low-intensity blue light (LBL) (0.01–1 μmol m−2 s−1), whereas only phot1 is involved in chloroplast accumulation under high-intensity blue light (HBL) (100 μmol m−2 s−1) (Sakai et al, 2001)
The phenotypes of the rpt2, jac1, phot1 rpt2, phot1 jac1, and rpt2 jac1 mutants were similar to those of phot1 or the WT plants under HBL irradiation (Figure 1). These results indicated that the phot1-mediated chloroplast accumulation response was mainly dependent on root phototropism 2 (RPT2) under HBL irradiation
Summary
Chloroplast photorelocation is a critical physiological process that optimizes photosynthetic efficiency and ensures plant survival. Upon the absorption of light, the binding of LOV2 to the kinase domain is inhibited, resulting in the activation of kinase activity (Christie et al, 1999; Salomon et al, 2000; Inoue et al, 2008; Sullivan et al, 2008) Both phot and phot mediated chloroplast movement response, the abundance and phosphorylation levels of phototropins and the interactions between phototropin molecules and phototropin homo and heterocomplexes vary under different light conditions in regulating chloroplast movement (Sztatelman et al, 2016). Consistent with this result, we have previously shown that phot plays a role in chloroplast avoidance in Gossypium hirsutum (Shang et al, 2019)
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