The formation of pineapple translucency was regulated by Ca2+/H+ antiporter gene AcoCAX2

Flesh translucency is a significant physiological disorder in pineapple cultivation. Calcium deficiency in the fruit is a primary cause of flesh translucency. However, simply adding calcium to the soil or applying calcium fertilizer to the leaves and fruits does not eliminate flesh translucency. The mechanism behind this phenomenon has not been previously documented. Our research discovered that the calcium content in the stalk was significantly higher than in the fruit. Calcium ions are primarily transported through the phloem. The calcium concentration in the phloem sap of pineapple stalk at different stages ranged from 14 to 17 µmol/L. The transporter responsible for unloading Ca2+ across the membrane of the end sieve vessel in the stalk phloem is the Ca2+/H+ antiporter exchanger (CAX). Among the four CAX genes in pineapple, only AcoCAX2 is expressed in the stalk. The AcoCAX2 protein is located in the plasma membrane and cytoplasm. Calcium-sensitive yeast K667 transformed with AcoCAX2 absorbed more calcium ions from the medium compared to the control. The promoter proNtPRB1b-proAcoCAX2 is specifically expressed in the junction between the stalk and fruit. Plants overexpressing AcoCAX2 in this junction absorbed more calcium in the fruit, resulting in a lower incidence of translucency compared to the control. Fruit with a mutant AcoCAX2 gene contained less calcium and exhibited a higher incidence of translucency than control. The mutant flesh with AcoCAX2 contained more liquid in the intercellular space compared to the wild type and control. AcoCAX2 is the main gene responsible for transferring calcium ions from the stalk phloem into the fruit.