Boron Deficiency Increases Cytosolic Ca²⁺ Levels Mainly via Ca²⁺ Influx from the Apoplast in <i>Arabidopsis thaliana</i> 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 ([Ca2+]cyt) in Arabidopsis thaliana roots. The aim of this work was to research in Arabidopsis whether [Ca2+]cyt is restored to initial levels when B is resupplied and elucidate whether apoplastic Ca2+ is the major source for B-deficiency-induced rise in [Ca2+]cyt. The use of chemical compounds affecting Ca2+ homeostasis showed that the rise in root [Ca2+]cyt induced by B deficiency was predominantly owed to Ca2+ influx from the apoplast through plasma membrane Ca2+ channels in an IP3-independent manner. Furthermore, B resupply restored the root [Ca2+]cyt. Interestingly, expression levels of genes encoding Ca2+ transporters (ACA10, plasma membrane PIIB-type Ca2+-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 Ca2+ homeostasis after 24 h of B deficiency