MdCAX5 plays a critical role in calcium homeostasis and bitter pit development in apple

Abstract

Bitter Pit (BP) is a prevalent physiological disorder in apple that significantly reduces fruit quality and market value. While numerous studies have investigated the mechanisms underlying BP occurrence, the molecular processes, particularly the role of the Ca2+/H+ exchanger (CAX), remain unclear. This study aims to elucidate the function of the MdCAX5 gene in relation to BP development. To achieve this, we utilized transient transformation in apple, as well as stable transformation in Arabidopsis and tomato, to measure the mineral content in transgenic plants, thereby validating the function of MdCAX5. The overexpression of the MdCAX5 gene significantly reduced calcium (Ca) content in plants and disrupted the mineral element balance within the plant. Analysis of the MdCAX5 gene promoter revealed that Ca2+ can enhance promoter activity, indicating that the MdCAX5 gene can effectively respond to Ca signaling. Transcriptomic analysis of tomato plants stably overexpressing the MdCAX5 gene revealed significant alterations in the expression of genes involved in Ca signal transduction and transport, which in turn impacted the biosynthesis of secondary metabolites and metabolic pathways within the plants. These changes resulted in a reduction in Ca content, imbalanced Ca distribution, increased hydrolase activity, and disrupted cellular structures, including compromised organelles, cellular membranes, and membrane components. These disruptions culminated in the manifestation of Ca deficiency symptoms in the plants. This study provides theoretical insights into the mechanisms underlying the occurrence of apple BP disease.