Calcium signalling in Arabidopsis thaliana responding to drought and salinity

Abstract

SummaryChanges in cytosolic free calcium concentration ([Ca2+]cyt) in response to mannitol (drought) and salt treatments were detected in vivo in intact whole Arabidopsis seedlings.Transient elevations of [Ca2+]cyl to around 1.5 p,M were observed, and these were substantially inhibited by pretreatment with the calcium-channel blocker lanthanum and to a lesser extent, the calcium-chelator EGTA. The expression of three genes, p5cs, which encodes Al-pyrroline-5-carboxylate synthetase (P5CS), the first enzyme of the proline biosynthesis pathway, rab18 and Iti78 which both encode proteins of unknown function, was induced by mannitol and salt treatments. The induction of all three genes by mannitol was inhibited by pretreatment with lanthanum. Salt-induced p5cs, but not rab18 and Iti78, expression was also inhibited by lanthanum. Induction of pScs by mannitol was also inhibited by the calcium channel-blockers gadolinium and verapamil and the calcium chelator EGTA, further suggesting the involvement of calcium signalling in this response. Mannitol induced greater levels of pScs gene expression than an isoosmolar concentration of salt, at both relatively high and low concentrations. However, calcium transients were of a similar magnitude and duration in response to both mannitol and isoosmolar concentrations of salt, suggesting that a factor other than calcium is involved in the discrimination between drought and salinity signals in Arabidopsis. In order to gauge the involvement of the vacuole as an intracellular calcium store in the response of Arabidopsis to mannitol, [CaZ+]cyt was measured at the microdomain adjacent to the vacuolar membrane. The results obtained were consistent with a significant calcium release from the vacuole contributing to the overall mannitol-induced [Ca2+]cyt response. Data obtained by using inhibitors of inositol signalling suggested that this release was occurring through IP3-dependent calcium channels.

Inositol(1,4,5)trisphosphate production in plant cells: an early response to salinity and hyperosmotic stress

“…We now describe the e¡ect of salt and osmotic stress on Ins(1,4,5)P 3 production in suspension-cultured carrot (Daucus carota L.) cells. Our data show that stresses induced by both salt and hyperosmotic shock lead to a rapid and sustained elevation of cellular levels of Ins(1,4,5)P 3 and that the increases in Ins(1,4,5)P 3 occur in a dose-dependent manner. A direct e¡ect of salt upon phospholipase C activity in vitro was not observed which supports the idea that a more complex set of mechanisms is involved in the sensing of salt and osmotic stresses.…”

“…[9]) we did not carry out microscopic examinations of cells exposed to these osmotic agents. Knight et al [3] have shown that mannitol concentrations of up to 660 mM do not adversely a¡ect normal cell function.…”

“…Extraction and assay of Ins(1,4,5)P 3 All samples were immediately quenched with 100 Wl ice-cold HClO 4 (10% w/v). After 15 min on ice, protein and cellular debris was sedimented by centrifugation (15 000Ug) for 7 min and 100 Wl of the supernatant removed and neutralised by 1.5 M KOH/60 mM HEPES.…”

“…It is still not clear whether stress-induced cytoplasmic Ca 2 increases are due to in£ux of extracellular Ca 2 or mobilisation of Ca 2 from intracellular stores. However, the results of Knight et al [3], using a number of approaches and pharmacological tools, suggest that both processes are likely to be involved.…”

“…Development of tolerance to such stress situations by higher plants involves a complex set of cellular interactions which ultimately result in altered expression patterns of numerous genes involved in the adaptive responses. Relatively little is known about the early events that trigger the induction of adaptive processes but rapid changes in cytosolic free Ca 2 concentrations have been shown to be one of the early responses to both salinity and drought stress in Arabidopsis thaliana [3]. In addition it has recently been demonstrated that salt and hyperosmotic stress result in increased turnover of polyphosphoinositides in Chlamydomonas and a number of higher plants [4,5].…”