The plastid ribosome proteins (PRPs) play important roles in plastid protein biosynthesis, chloroplast differentiation and early chloroplast development. However, the specialised functions of individual protein components of the chloroplast ribosome in rice (Oryza sativa) remain unresolved. In this paper, we identified a novel rice PRP mutant named asl2 (Albino seedling lethality 2) exhibiting an albino, seedling death phenotype. In asl2 mutants, the alteration of leaf colour was associated with chlorophyll (Chl) content and abnormal chloroplast development. Through map-based cloning and complementation, the mutated ASL2 gene was isolated and found to encode the chloroplast 50S ribosome protein L21 (RPL21c), a component of the chloroplast ribosome large subunit, which was localised in chloroplasts. ASL2 was expressed at a higher level in the plumule and leaves, implying its tissue-specific expression. Additionally, the expression of ASL2 was regulated by light. The transcript levels of the majority of genes for Chl biosynthesis, photosynthesis and chloroplast development were strongly affected in asl2 mutants. Collectively, the absence of functional ASL2 caused chloroplast developmental defects and seedling death. This report establishes the important role of RPL21c in chloroplast development in rice.

BackgroundPentatricopeptide repeat (PPR) proteins play essential roles in modulating the expression of organelle genes and have expanded greatly in higher plants. However, molecular mechanisms of most rice PPR genes remain unclear.ResultsIn this study, a new rice PPR mutant, asl3 (albinoseedlinglethality3) exhibits an albino lethal phenotype at the seedling stage. This albino phenotype was associated with altered photosynthetic-pigment and chloroplast development. Map-based cloning showed that ASL3 encodes a novel rice PPR protein with 10 tandem PPR motifs, which localizes to the chloroplast. ASL3 showed tissue-specific expression, as it was highly expressed in the chlorenchyma, but expressed at much lower levels in roots and panicles. RNAi of ASL3 confirmed that ASL3 plays an essential role in the early development and chloroplast development in rice. Moreover, expression analysis revealed that the asl3 mutation severely affected the transcriptional levels of important genes associated with plastid translation machinery and photosynthesis, which may impair photosynthesis and finally led to the seedling death in asl3 mutant. These results evidenced the important role of ASL3 in the early development of rice, especially chloroplast development.ConclusionsThe ASL3 gene encoded a novel chloroplast-targeted PPR protein with 10 tandem PPR motifs in rice. Disruption of the ASL3 would lead to a defective chloroplast and seedling lethality, and affected expression levels of genes associated with chloroplast development and photosynthesis at early leaf stage of rice.Electronic supplementary materialThe online version of this article (doi:10.1186/s12284-015-0050-9) contains supplementary material, which is available to authorized users.