Identification of a dual-targeted protein belonging to the mitochondrial carrier family that is required for early leaf development in rice.

A dual-targeted protein belonging to the mitochondrial carrier family was characterized in rice (Oryza sativa) and designated 3'-Phosphoadenosine 5'-Phosphosulfate Transporter1 (PAPST1). The papst1 mutant plants showed a defect in thylakoid development, resulting in leaf chlorosis at an early leaf developmental stage, while normal leaf development was restored 4 to 6 d after leaf emergence. OsPAPST1 is highly expressed in young leaves and roots, while the expression is reduced in mature leaves, in line with the recovery of chloroplast development seen in the older leaves of papst1 mutant plants. OsPAPST1 is located on the outer mitochondrial membrane and chloroplast envelope. Whole-genome transcriptomic analysis reveals reduced expression of genes encoding photosynthetic components (light reactions) in papst1 mutant plants. In addition, sulfur metabolism is also perturbed in papst1 plants, and it was seen that PAPST1 can act as a nucleotide transporter when expressed in Escherichia coli that can be inhibited significantly by 3'-phosphoadenosine 5'-phosphosulfate. Given these findings, together with the altered phenotype seen only when leaves are first exposed to light, it is proposed that PAPST1 may act as a 3'-phosphoadenosine 5'-phosphosulfate carrier that has been shown to act as a retrograde signal between chloroplasts and the nucleus.