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Article|19 Jan 2022|OPEN
Overexpression of apple Ma12, a mitochondrial pyrophosphatase pump gene, leads to malic acid accumulation and the upregulation of malate dehydrogenase in tomato and apple calli 
Meng Gao1 ,† , Haiyan Zhao1 ,† , Litong Zheng1 , Lihua Zhang1 , Yunjing Peng1 , Wenfang Ma1 , Rui Tian1 , Yangyang Yuan1 , Fengwang Ma1 , Mingjun Li1 , and Baiquan Ma,1 ,
1State Key Laboratory of Crop Stress Biology for Arid Areas/Shaanxi Key Laboratory of Apple, College of Horticulture, Northwest A&F University, Yangling 712100, Shaanxi, China
*Corresponding author. E-mail:,
Both authors contributed equally to the study.

Horticulture Research 9,
Article number: uhab053 (2022)
Views: 450

Received: 06 May 2021
Revised: 13 Jul 2021
Accepted: 09 Aug 2021
Published online: 19 Jan 2022


Acidity is an important factor influencing the organoleptic quality of apple fruits. In this study, an apple pyrophosphate-energized proton pump (PEPP) gene was isolated and designated MdMa12. On the basis of a phylogenetic analysis in Rosaceae species, PEPP genes were divided into three groups, with apple PEPP genes most closely related to pear PEPP genes. Gene expression analysis revealed that high malic acid content was generally accompanied by high MdMa12 expression levels. Moreover, MdMa12 was mainly expressed in the fruit. A subcellular localization analysis suggested that MdMa12 is a mitochondrial protein. The ectopic expression and overexpression of MdMa12 in “Micro-Tom” tomato and apple calli, respectively, increased the malic acid content. One (MDH12) of four malate dehydrogenase genes highly expressed in transgenic apple calli was confirmed to encode a protein localized in mitochondria. The overexpression of MDH12 increased the malate content in apple calli. Furthermore, MdMa12 overexpression increased MdDTC1MdMa1, and MdMa10 expression levels, which were identified to transport malate. These findings imply that MdMa12 has important functions related to apple fruit acidity. Our study explored the regulatory effects of mitochondria on the complex mechanism underlying apple fruit acidity.