Pentatricopeptide 336 and Mitochondrial Sorting in Cucumber

Cucumber is a unique model plant for organellar genetics because its three genomes show differential transmission: maternal for chloroplast, paternal for mitochondrial and bi-parental for nuclear. A cucumber line has been selected showing a paternally transmitted, strongly mosaic (MSC) phenotype associated with an under-representation of ribosomal protein S7 (rps7), a key component of the small ribosomal subunit in the mitochondrion. We identified a nuclear locus (Psm for Paternal sorting of mitochondria) that preferentially sorts for rare wild-type sublimons in crosses with MSC16 as the male parent. Fine mapping of Psm revealed pentatricopeptide repeat (PPR) 336 as the likely candidate gene. PPR336 stabilizes mitochondrial polysomes in Arabidopsis (J Mol Biol 375:626) and because MSC16 shows reduced transcription of rps7, the cucumber homolog of PPR336 as the candidate for Psm is consistent with a nuclear effect on assembly or stability of mitochondrial ribosomes.