Boron Deficiency Increases Cytosolic Ca(2+) Levels Mainly via Ca(2+) Influx from the Apoplast in Arabidopsis thaliana Roots

Boron (B) is a micronutrient for plant development, and its deficiency alters many physiological processes. However, the current knowledge on how plants are able to sense the B-starvation signal is still very limited. Recently, it has been reported that B deprivation induces an increase in cytosolic calcium concentration ([Ca(2+)](cyt)) in Arabidopsis thaliana roots. The aim of this work was to research in Arabidopsis whether [Ca(2+)](cyt) is restored to initial levels when B is resupplied and elucidate whether apoplastic Ca(2+) is the major source for B-deficiency-induced rise in [Ca(2+)](cyt). The use of chemical compounds affecting Ca(2+) homeostasis showed that the rise in root [Ca(2+)](cyt) induced by B deficiency was predominantly owed to Ca(2+) influx from the apoplast through plasma membrane Ca(2+) channels in an IP(3)-independent manner. Furthermore, B resupply restored the root [Ca(2+)](cyt). Interestingly, expression levels of genes encoding Ca(2+) transporters (ACA10, plasma membrane P(IIB)-type Ca(2+)-ATPase; and CAX3, vacuolar cation/proton exchanger) were upregulated by ethylene glycol tetraacetic acid (EGTA) and abscisic acid (ABA). The results pointed out that ACA10, and especially CAX3, would play a major role in the restoration of Ca(2+) homeostasis after 24 h of B deficiency.