A role for root carbonic anhydrase βCA4 in the 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 deme and the sensitive deme, T6. Exposure to 10 mM NaCl caused a transient depolarization of the root cell membranes, and in contrast, the supply of 10 mM NaHCO caused hyperpolarization. This hyperpolarization was much stronger in A1 than in T6. Acetazolamide (AZ), a specific inhibitor of CAs, abolished the hyperpolarizing effect in A1, indicating the implication of CAs in this fast membrane response. The time-dependent (3 to 72 h) expression profiles of 14 CAs in roots of A1 and T6 exposed to either control (0 mM NaHCO, pH 5.9), or bicarbonate (10 mM NaHCO,pH 8.3) conditions revealed a bicarbonate specific upregulation of BCA4.1 (from 3 to 12 h) in A1. Contrastingly, in T6 BCA4.1 was downregulated by NaHCO. Exclusively in A1, the enhanced expression of BCA4.1 under bicarbonate was parallelled by an increase of PIP1,3, SLAH1, SLAH3, AHA2, and FRO2 gene expression levels. Under HCO exposure, a bca4 knockout mutant had a lower number of lateral roots, lower root diameters, and higher root lipid peroxidation than the WT. These results indicate that bicarbonate-induced root membrane hyperpolarization is the fast (minutes) initial signalling event in the tolerance response. This is followed by the specific upregulation of BCA4.1 and genes involved in HO and CO transport, apoplast acidification, and iron acquisition.