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Salinity (NaCl) stress impairs plant growth and inflicts severe crop losses. In roots, increasing extracellular NaCl causes Ca2+ influx to elevate cytosolic free Ca2+ ([Ca2+]cyt) as a second messenger for adaptive signaling. Amplification of the signal involves plasma membrane reduced nicotinamide adenine dinucleotide phosphate oxidase activation, with the resultant reactive oxygen species triggering Ca2+ influx. The genetic identities of the Ca2+-permeable channels involved in generating the [Ca2+]cyt signal are unknown. Potential candidates in the model plant Arabidopsis (Arabidopsis thaliana) include annexinl (AtANN1. Here, luminescent detection of [Ca2+]cyt showed that AtANN1 responds to high extracellular NaCl by mediating reactive oxygen species-activated Ca2+ influx across the plasma membrane of root epidermal protoplasts. Electrophysiological analysis revealed that root epidermal plasma membrane Ca2+ influx currents activated by NaCl are absent from the Atann1loss-of-function mutant. Both adaptive signaling and salt-responsive production of secondary roots are impaired in the loss-of-function mutant, thus identifying AtANN1 as a key component of root cell adaptation to salinity. [ABSTRACT FROM AUTHOR]