Blue light-induced kinetics of H ⁺ and Ca ²⁺ fluxes in etiolated wild-type and phototropin-mutant Arabidopsis seedlings

Abstract

Ion flux kinetics associated with blue light (BL) treatment of two wild types (WTs) and the phot1, phot2 and phot1͞phot2 mutants of Arabidopsis were studied by using the MIFE noninvasive ionselective microelectrode technique. BL induced significant changes in activity of H ؉ and Ca 2؉ transporters within the first 10 min of BL onset, peaking between 3 and 5 min. In all WT plants and in phot2 mutants, BL induced immediate Ca 2؉ influx. In phot1 and phot1͞ phot2 mutants, net Ca 2؉ flux remained steady. It is suggested that PHOT1 regulates Ca 2؉ uptake into the cytoplasm from the apoplast. Changes in ion fluxes were measured from cotyledons of intact seedlings and from the cut top of the hypocotyl of decapitated seedlings. Thus the photoreceptors mediating BL-induced Ca 2؉ and H ؉ fluxes are present in the rest of the decapitated seedling and probably in the cotyledons as well. The H ؉ and Ca 2؉ flux responses to BL appear not to be linked because, (i) when changes were observed for both ions, Ca 2؉ flux changed almost immediately, whereas H ؉ flux lagged by about 1.5 min; (ii) in the Wassilewskija ecotype, changes in H ؉ fluxes were small. Finally, wave-like changes in Ca 2؉ and H ؉ concentrations were observed along the cotyledon-hook axis regardless of its orientation to the light. Blue light (BL) is a key factor controlling plant growth and morphogenesis. Among numerous physiological processes controlled by BL are phototropism (bending toward or away from the light source), cotyledon expansion, and inhibition of hypocotyl elongation (1-3). These reactions are preceded or accompanied by significant changes in electrochemical properties of cells and tissues, including changes in membrane potential and ion transport across membranes (4-6).For more than 60 years, the interpretation of the multiphasic transient surface electrical responses of plants to light (7-11) was problematic and speculative (4). More recently, with microelectrode impalements, those responses have been located at the plasma membrane (12, 13), and it is now possible to interpret them in terms of specific ionic movements (5). BL induces transient extracellular acidification of epidermal cells of young pea leaves (14, 15). This acidification was found to be linked to Ca 2ϩ entry and Ca 2ϩ -calmodulin binding (14). BL also induces opening of anion (16) Unfortunately, there are pitfalls in inferring causal links between electrophysiological changes and BL-induced morphogenetic responses, based only on the similarity of the time scales of the measured responses. From their time scale, the photomorphogenetic reactions could be divided into several groups. The quickest observed (within minutes) are reactions associated with inhibition of hypocotyl elongation (11, 13). Next are bending responses, operating within the time scale of hours. The slowest are reactions of cotyledon expansion, which take days (2). However, it is quite possible that all these reactions may be induced simultaneously (moreover, they could be linked), and the observed difference ...

Blue light activates calcium-permeable channels inArabidopsismesophyll cells via the phototropin signaling pathway

“…Measurements of Ca 2ϩ changes in hypocotyls showed that blue light induced a Ca 2ϩ signal, which peaked around 3-12 min (23). Further support for the involvement of Ca 2ϩ signals in the phototropin-controlled growth of hypocotyls was provided by the observation that most of the Ca 2ϩ signal is eliminated in phot1 (23). Similar to the present results obtained on mesophyll cells, hypocotyls in phot1 phot2 were characterized by a complete loss of blue-light activation.…”

“…To date, little is known about the signal transduction elements following the blue-light perception by phot1 and phot2 (2,16,(20)(21)(22)(23)(24). In guard cells, blue-light activation of the plasma membrane H ϩ -ATPase depends on phototropins, driving turgor formation and stomatal opening (16).…”

An Inositol Polyphosphate 5-Phosphatase Functions in PHOTOTROPIN1 Signaling in Arabidopis by Altering Cytosolic Ca2+

“…Several studies have shown that blue light induces elevation of [Ca 2þ ] cyt , and mutations in genes encoding blue light receptors, such as PHOT1 or PHOT2, have compromised calcium elevation, leading to changes in photomorphogenesis (Babourina et al, 2002;Harada et al, 2003;Stoelzle et al, 2003). It is interesting to note that defects in PHOT1 and [Ca 2þ ] cyt appear to affect only the short term initial phase of the hypocotyl growth response, and further inhibition of hypocotyl elongation requires the CRY1-COP1 pathway (Parks et al, 1998;Folta and Spalding, 2001).…”

“…How might [Ca 2þ ] cyt mediate cell elongation? A simple interpretation is that blue light induces Ca 2þ and H þ fluxes (Babourina et al, 2002), both of which are critical for cell growth. 5PTase13 expression was similarly suppressed by both blue and red light compared with darkness.…”

“…These five proteins participate in the regulation of phototropic responses Briggs, 1995, 1996;Motchoulski and Liscum, 1999;Sakai et al, 2000;Inada et al, 2004), chloroplast movement (Inada et al, 2004), and stomatal aperture control (Lascè ve et al, 1999;Inada et al, 2004). Other studies have shown that calcium might be part of the blue light signaling pathway (Baum et al, 1999;Babourina et al, 2002;Folta et al, 2003;Stoelzle et al, 2003;Christie, 2007;Harada and Shimazaki, 2007).…”