Mitochondria are semi-autonomous organelles that are the powerhouse of the cells. Plant mitochondrial RNA editing guided by pentatricopeptide repeat (PPR) proteins is essential for energy production. We identify a maize defective kernel mutant dek36, which produces small and collapsed kernels, leading to embryos and/or seedlings lethality. Seed filling in dek36 is drastically impaired, in line with the defects observed in the organization of endosperm transfer tissue. Positional cloning reveals that DEK36, encoding a mitochondria-targeted E+ subgroup PPR protein, is required for mitochondrial RNA editing at atp4-59, nad7-383 and ccmFN -302, thus resulting in decreased activities of mitochondrial complex I, complex III and complex IV in dek36. Loss-of-function of its Arabidopsis ortholog At DEK36 causes arrested embryo and endosperm development, leading to embryo lethality. At_dek36 also has RNA editing defects in atp4, nad7, ccmFN1 and ccmFN2 , but at the nonconserved sites. Importantly, efficiency of all editing sites in ccmFN1 , ccmFN2 and rps12 is severely decreased in At_dek36, probably caused by the impairment of their RNA stabilization. These results suggest that the DEK36 orthologue pair are essential for embryo and endosperm development in both maize and Arabidopsis, but through divergent function in regulating RNA metabolism of their mitochondrial targets.
Prolamins, the major cereal seed storage proteins, are sequestered and accumulated in the lumen of the endoplasmic reticulum (ER), and are directly assembled into protein bodies (PBs). The content and composition of prolamins are the key determinants for protein quality and texture-related traits of the grain. Concomitantly, the PB-inducing fusion system provides an efficient target to produce therapeutic and industrial products in plants. However, the proteome of the native PB and the detailed mechanisms underlying its formation still need to be determined. We developed a method to isolate highly purified and intact PBs from developing maize endosperm and conducted proteomic analysis of intact PBs of zein, a class of prolamine protein found in maize. We thus identified 1756 proteins, which fall into five major categories: metabolic pathways, response to stimulus, transport, development, and growth, as well as regulation. By comparing the proteomes of crude and enriched extractions of PBs, we found substantial evidence for the following conclusions: (i) ribosomes, ER membranes, and the cytoskeleton are tightly associated with zein PBs, which form the peripheral border; (ii) zein RNAs are probably transported and localized to the PB–ER subdomain; and (iii) ER chaperones are essential for zein folding, quality control, and assembly into PBs. We futher confirmed that OPAQUE1 (O1) cannot directly interact with FLOURY1 (FL1) in yeast, suggesting that the interaction between myosins XI and DUF593-containing proteins is isoform-specific. This study provides a proteomic roadmap for dissecting zein PB biogenesis and reveals an unexpected diversity and complexity of proteins in PBs.
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