Abstract
Phototropins (phot1 and phot2 in Arabidopsis thaliana) relay blue light intensity information to the chloroplasts, which move toward weak light (the accumulation response) and away from strong light (the avoidance response). Chloroplast-actin (cp-actin) filaments are vital for mediating these chloroplast photorelocation movements. In this report, we examine in detail the cp-actin filament dynamics by which the chloroplast avoidance response is regulated. Although stochastic dynamics of cortical actin fragments are observed on the chloroplasts, the basic mechanisms underlying the disappearance (including severing and turnover) of the cp-actin filaments are regulated differently from those of cortical actin filaments. phot2 plays a pivotal role in the strong blue light-induced severing and random motility of cp-actin filaments, processes that are therefore essential for asymmetric cp-actin formation for the avoidance response. In addition, phot2 functions in the bundling of cp-actin filaments that is induced by dark incubation. By contrast, the function of phot1 is dispensable for these responses. Our findings suggest that phot2 is the primary photoreceptor involved in the rapid reorganization of cp-actin filaments that allows chloroplasts to change direction rapidly and control the velocity of the avoidance movement according to the light's intensity and position.
Highlights
Light is essential for photosynthesis; too much light can endanger plant survival (Kasahara et al, 2002; Sztatelman et al, 2010)
The chloroplast avoidance response is crucial for plant survival (Wada et al, 2003; Suetsugu and Wada, 2007; Banas et al, 2012) as evidenced by the fact that leaf necrosis and death have been observed in Arabidopsis thaliana phototropin2 and chloroplast unusual positioning1 mutant plants, which are deficient in this response (Kasahara et al, 2002)
Phototropins are UV-A/blue light receptors controlling a range of responses that serve to optimize the photosynthetic efficiency of plants (Kasahara et al, 2002; Takemiya et al, 2005)
Summary
Light is essential for photosynthesis; too much light can endanger plant survival (Kasahara et al, 2002; Sztatelman et al, 2010). To avoid the danger from excess light, plants have evolved photoprotective mechanisms, including light capture reduction by nastic leaf movements and/or chloroplast avoidance movements, adaptation to the light quality by a state transition, and dissipation of excess absorbed energy by the water-water cycle and/or violaxanthin cycle (Li et al, 2009; Takahashi and Badger, 2011) Among these mechanisms, the chloroplast avoidance response is crucial for plant survival (Wada et al, 2003; Suetsugu and Wada, 2007; Banas et al, 2012) as evidenced by the fact that leaf necrosis and death have been observed in Arabidopsis thaliana phototropin (phot2) and chloroplast unusual positioning (chup1) mutant plants, which are deficient in this response (Kasahara et al, 2002). The activated kinase domain transfers the signals to the downstream targets (Christie et al, 2002; Kong et al, 2006, 2007; Inoue et al, 2008, 2011), the signaling cascade that leads to chloroplast movement has not yet been identified
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