Abstract
Phototropins are UVA/blue-light receptors involved in controlling the light-dependent physiological responses which serve to optimize the photosynthetic activity of plants and promote growth. The phototropin-induced phosphoinositide (PI) metabolism has been shown to be essential for stomatal opening and phototropism. However, the role of PIs in phototropin-induced chloroplast movements remains poorly understood. The aim of this work is to determine which PI species are involved in the control of chloroplast movements in Arabidopsis and the nature of their involvement. We present the effects of the inactivation of phospholipase C (PLC), PI3-kinase (PI3K) and PI4-kinase (PI4K) on chloroplast relocations in Arabidopsis. The inhibition of the phosphatidylinositol 4,5-bisphospahte [PI(4,5)P2]-PLC pathway, using neomycin and U73122, suppressed the phot2-mediated chloroplast accumulation and avoidance responses, without affecting movement responses controlled by phot1. On the other hand, PI3K and PI4K activities are more restricted to phot1- and phot2-induced weak-light responses. The inactivation of PI3K and PI4K by wortmannin and LY294002 severely affected the weak blue-light-activated accumulation response but had little effect on the strong blue-light-activated avoidance response. The inhibitory effect observed with PI metabolism inhibitors is, at least partly, due to a disturbance in Ca2+ (c) signaling. Using the transgenic aequorin system, we show that the application of these inhibitors suppresses the blue-light-induced transient Ca2+ (c) rise. These results demonstrate the importance of PIs in chloroplast movements, with the PI(4,5)P2-PLC pathway involved in phot2 signaling while PI3K and PI4K are required for the phot1- and phot2-induced accumulation response. Our results suggest that these PIs modulate cytosolic Ca2+ signaling during movements.
Highlights
The UV-A/blue-light (BL) photoreceptor phototropin is involved in regulating light dependent processes that are important for the promotion of plant growth and for the optimization of photosynthetic efficiency [1]
We report that phot2, not phot1, activates the PI(4,5)P2-phospholipase C (PLC) pathway upon perceiving BL, and this plays an important role in phot2 signaling during movement responses
PI(4,5)P2-PLC Inhibitors Suppress Chloroplast Movements in phot1 but not in phot2 Mutant Plants In order to determine the involvement of PI(4,5)P2 in the regulation of chloroplast movements, leaves of Arabidopsis thaliana were treated with increasing doses of neomycin or U73122
Summary
The UV-A/blue-light (BL) photoreceptor phototropin is involved in regulating light dependent processes that are important for the promotion of plant growth and for the optimization of photosynthetic efficiency [1]. Identified in various plant species, Arabidopsis thaliana has served as a basic model organism to study phototropin-mediated effects. Arabidopsis contains two phototropins, phot and phot, which regulate various responses, including phototropism, stomatal movements, chloroplast redistribution, hypocotyl elongation inhibition and leaf positioning [1]. Phototropin-induced chloroplast movements are important for optimizing photosynthetic activity [3]. Under weak BL, phot and phot mediate a chloroplast accumulation response in which chloroplasts move to the periclinal cell walls (face position) in order to capture more light for photosynthesis [4]. The avoidance response of chloroplasts is mediated solely by phot2 [7]
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