Boron deficiency increases the levels of cytosolic Ca2+ and expression of Ca2+-related genes in Arabidopsis thaliana roots

Boron (B) deficiency affects the expressions of genes involved in major physiological processes. However, signal transduction pathway through which plants are able to sense and transmit B-deprivation signal to the nucleus is unknown. The aim of this work was to research in Arabidopsis thaliana roots whether the short-term B deficiency affects cytosolic Ca2+ levels ([Ca2+]cyt) as well as expression of genes involved in Ca2+ signaling. To visualize in vivo changes in root [Ca2+]cyt, Arabidopsis seedlings expressing Yellow Cameleon (YC) 3.6 were grown in a nutrient solution supplemented with 2 μM B and then transferred to a B-free medium for 24 h. Root [Ca2+]cyt was clearly higher in B-deficient seedlings upon 6 and 24 h of B treatments when compared to controls. Transcriptome analyses showed that transcript levels of Ca2+ signaling-related genes were affected by B deprivation. Interestingly, Ca2+ channel (CNGC19, cyclic nucleotide-gated ion channel) gene was strongly upregulated as early as 6 h after B deficiency. Expression levels of Ca2+ transporter (ACA, autoinhibited Ca2+-ATPase; CAX, cation exchanger) genes increased when seedlings were subjected to B deficiency. Gene expressions of calmodulin-like proteins (CMLs) and Ca2+-dependent protein kinases (CPKs) were also overexpressed upon exposure to B starvation. Our results suggest that B deficiency causes early responses in the expression of CNGC19 Ca2+-influx channel, ACA- and CAX-efflux, and Ca2+ sensor genes to regulate Ca2+ homeostasis. It is the first time that changes in the levels of in vivo cytosolic Ca2+ and expression of Ca2+ channel/transporter genes are related with short-term B deficiency in Arabidopsis roots.