The SlCBL10 Calcineurin B-Like Protein Ensures Plant Growth under Salt Stress by Regulating Na⁺ and Ca²⁺ Homeostasis

Abstract

Characterization of a new tomato (<i>Solanum lycopersicum</i>) T-DNA mutant allowed for the isolation of the <i>CALCINEURIN B-LIKE PROTEIN 10</i> (<i>SlCBL10</i>) gene whose lack of function was responsible for the severe alterations observed in the shoot apex and reproductive organs under salinity conditions. Physiological studies proved that <i>SlCBL10</i> gene is required to maintain a proper low Na⁺/Ca²⁺ ratio in growing tissues allowing tomato growth under salt stress. Expression analysis of the main responsible genes for Na⁺ compartmentalization (i.e. <i>Na⁺/H⁺ EXCHANGERs</i>, <i>SALT OVERLY SENSITIVE</i>, <i>HIGH-AFFINITY K+ TRANSPORTER 1;2</i>, <i>H⁺-pyrophosphatase AVP1</i> [<i>SlAVP1</i>] and V-ATPase [<i>SlVHA-A1</i>]) supported a reduced capacity to accumulate Na⁺ in <i>Slcbl10</i> mutant leaves, which resulted in a lower uploading of Na⁺ from xylem, allowing the toxic ion to reach apex and flowers. Likewise, the tomato <i>CATION EXCHANGER 1</i> and <i>TWO-PORE CHANNEL 1</i> (<i>SlTPC1</i>), key genes for Ca²⁺ fluxes to the vacuole, showed abnormal expression in <i>Slcbl10</i> plants indicating an impaired Ca²⁺ release from vacuole. Additionally, complementation assay revealed that <i>SlCBL10</i> is a true ortholog of the Arabidopsis (<i>Arabidopsis thaliana</i>) <i>CBL10</i> gene, supporting that the essential function of CBL10 is conserved in Arabidopsis and tomato. Together, the findings obtained in this study provide new insights into the function of <i>SlCBL10</i> in salt stress tolerance. Thus, it is proposed that SlCBL10 mediates salt tolerance by regulating Na⁺ and Ca²⁺ fluxes in the vacuole, cooperating with the vacuolar cation channel <i>SlTPC1</i> and the two vacuolar H⁺-pumps, <i>SlAVP1</i> and <i>SlVHA-A1</i>, which in turn are revealed as potential targets of <i>SlCBL10</i>.