CAX1 Vacuolar Antiporter Overexpression in Potato Results in Calcium Deficiency in Leaves and Tubers by Sequestering Calcium as Calcium Oxalate

Abstract

Recent studies show that overexpression of a short version of the cation exchanger 1 gene (sCAX1) can cause Ca2+ deficiency symptoms in tomato (Solanum lycopersicum L.). However, the Ca2+ deficiency in relation to the overexpression of this gene has not been investigated in potato (Solanum tuberosum L.). The objective of our study was to investigate the production of known Ca2+ deficiency symptoms in potato in relation to the overexpression of sCAX1. Plantlets of S. tuberosum cultivar ‘Atlantic’ overexpressing the sCAX1 gene were produced using Agrobacterium tumefaciens. Transgenic plants grown with normal amounts of Ca2+ under in vitro or greenhouse conditions showed known Ca2+ deficiency symptoms in potato plants such as shoot tips damage and leaf margin necrosis, as well as tuber internal defects (hollow heart). Growing the transgenic plants with higher amounts of Ca2+ in the media or soil nutrient solution mitigated these symptoms. These results support the notion that both shoot tip necrosis and hollow heart are associated with Ca2+ deficiency. There was abundance of calcium oxalate (CaC2O4) crystals present only in the transgenic plants suggesting that these plants sequester Ca2+ in the form of CaC2O4 in the vacuoles of transgenic plants, reducing Ca2+ in the other pools. Since both shoot tip necrosis and hollow heart are known to be associated with poor cell wall health, our results suggest that CAX1 is a regulator of Ca2+ in the cell wall. In support of this concept, we found reduced cell wall biomass in the transgenic plants compared with the wild type.