A role for root carbonic anhydrase βCA4 in bicarbonate tolerance of Arabidopsis thaliana

Carbonic anhydrases (CAs) are the main enzymes handling bicarbonate in the different cell compartments. This study analyses the expression of CAs in roots of Arabidopsis thaliana demes differing in tolerance to bicarbonate: the tolerant A1(c+) deme and the sensitive deme, T6(c-). While 10 mM NaCl caused a transient depolarization of the root cell membranes, 10 mM NaHCO3 caused hyperpolarization. This hyperpolarization was much stronger in A1(c+) than in T6(c-). Acetazolamide (AZ), a specific inhibitor of CAs, abolished the hyperpolarizing effect in A1(c+), indicating the implication of CAs in this fast membrane response. The time dependent (3 to 72 h) expression profiles of 14 CAs (αCA1-8 and βCA1-6) in roots of A1(c+) and T6(c-) exposed to either control or NaHCO3 (pH 8.3) revealed a bicarbonate specific upregulation of BCA4.1 (from 3 to 12 h) and, although to a lesser extent, of βCA3 in A1(c+). Contrastingly, in T6(c-) βCA4 was downregulated by NaHCO3. Exclusively in A1(c+), the enhanced expression of βCA4 under bicarbonate was parallelled by an increase of PIP1,3, SLAH1, SLAH3, AHA2, and FRO2 gene expression levels. Under HCO3- exposure, a bca4 knockout mutant had lower number of lateral roots, lower root diameter and higher MDA root concentrations than the WT. The obtained results indicate that bicarbonate induced root membrane hyperpolarization is the fast (minutes) initial signalling event in the tolerance response, followed by the specific upregulation of BCA4.1 and the genes involved in H20 and CO2 transport, apoplast acidification, ion homeostasis and iron acquisition.