The Mitochondrial AtPHT3;3 Transporter Is Required for Resource Allocation During Late Embryo Development and Seed Maturation

Abstract Mitochondrial transport processes are critical for oxidative phosphorylation; however, the physiological roles of many members of the plant mitochondrial carrier family remain poorly characterized. In Arabidopsis thaliana, three PHOSPHATE TRANSPORTER 3 (PHT3) proteins are identified as potential mitochondrial phosphate carriers, but only PHT3;1 and PHT3;2 have been investigated. Here, we examined the least studied member of this family, AtPHT3;3, which exhibits elevated expression in reproductive tissues. After confirming the subcellular localization of PHT3;3 to mitochondria, we performed investigations on two T-DNA insertion alleles, pht3;3-1 and pht3;3-2 by integrating physiological, metabolomics, and phylogenetics analysis. Both mutants exhibited higher rates of aborted and missing seeds, embryonic arrest at the early torpedo stage, and impaired seedling growth. These phenotypes were partially restored through complementation with a constitutively expressed AtPHT3;3 transgene. Metabolite profiling of pre-anthesis and post-anthesis flowers revealed significant accumulation of glycolytic intermediates and altered pools of amino acids and organic acids in the pht3;3 mutants, indicating that loss of AtPHT3;3 is associated with changes in primary metabolism. Phylogenetic and protein sequence analyses showed that AtPHT3;3 diverged from AtPHT3;1 and AtPHT3;2, sharing ancestry with yeast Pic2 and human SLC25A3. However, the transport substrate and biochemical activity of AtPHT3;3 remain to be established. Overall, these findings identified AtPHT3;3 as a mitochondrial carrier protein required for late embryo development, seed formation and seedling performance, and reproductive-tissue metabolism. These results highlight the importance of mitochondrial transport processes in plant reproductive development.