An aquatic angiosperm Vallisneria (Alismatales: Hydrocharitaceae) has been used as an excellent experimental material over a century to study the light regulation of dynamic intracellular movements including chloroplast redistribution and cytoplasmic streaming (Senn, 1908; Seitz, 1987; Takagi, 1997). However, understanding for the molecular mechanisms lagged behind because of difficulty in applying modern techniques such as gene transformation to this plant. Especially, which kind of photoreceptors function in these intriguing responses has long been an unsolved topic. Recently, genes encoding plant-specific blue-light receptor phototropins were isolated in Vallisneria, for the first time from aquatic plants (Sakai et al., 2015). Phototropins were identified first as the photoreceptor for hypocotyl phototropism in Arabidopsis thaliana, and now known to regulate many responses including chloroplast photorelocation movements in various plant species (Christie, 2007). Phototropins are localized mainly on the plasma membrane and their auto-phosphorylation induced by blue light is the critical step of signal transduction pathway (Sakamoto and Briggs, 2002; Kong et al., 2006; Kong et al., 2013; Inoue et al., 2010). Here we describe a protocol for in vitro protein phosphorylation assay using crude-microsomal and plasma-membrane-enriched fractions of Vallisneria, which enabled us to verify the presence of phototropins and characterize their auto-phosphorylation responses. After these analyses, Sakai et al. (2015) proposed that Vallisneria phototropins mediate the high-intensity-blue-light-induced chloroplast avoidance response.
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