Ribosomes required for normal chloroplast development especially under low

Ribosomes are large ribonucleoprotein complexes that are essential for protein synthesis in all living cells. Plastid protein synthesis utilizes a bacterial-type 70S ribosome that comprises one small (30S) and one large (50S) ribosomal subunit (Tiller and Bock, 2014). Plastid ribosomal proteins (PRPs) have been shown to play versatile roles in plant growth and development (Fleischmann et al. 2011; Tiller et al. 2012). Song et al. 2014 has described a white leaf and panicles 1 (wlp1) mutant in rice where its leaf pigmentation were due to altered chlorophyll content and chloroplast development. The expression level of WLP1 was induced by low temperatures. Furthermore, positional cloning of WLP1, complementation studies and knock-down experiments revealed that it encodes a 50S ribosome L13 protein, and that WLP1 was mainly transcribed in green tissues and is particularly abundant in the early seedling leaves. All these showed clearly that WLP1 is required for normal chloroplast development especially under low temperatures (Song et al. 2014).

            A very recent study of a novel thermo-sensitive chlorophyll-deficient mutant 11 (TCD11) in rice by Wang et al 2017, demonstrated the important role of this gene in chloroplast development at low temperatures. TCD11 has a distinct albino phenotype and shows lethality at 20oC, while displaying normal phenotype at 32oC. By performing map-based cloning and molecular complementation they have showed that TCD11 encodes a ribosomal small subunit protein S6 in chloroplasts (RPS6). More important, the disruption of TCD11 reduced the transcription levels of certain chloroplast associated genes and prevented the assembly of ribosome in chloroplasts at low temperature (Wang et al. 2017).

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Recently, Lin et al 2017, demonstrated through map-based cloning that a thermos-sensitive virescent mutant (TSV3) is essential for chloroplast development during early leaf growth under cold stress. TSV3 is localized to the chloroplast. After performing map-based cloning and complementation they showed that TSV3 encoded a small GTP-binding protein. TSV3 display an albino phenotype and its expression is high in leaves and almost absent in other tissues, meaning it has a tissue specific expression. Under cold stress the chloroplast biogenesis associated genes were severely decreased at the 3-leaf stage. All these observations suggested that TSV3 plays a crucial role in chloroplast development under cold stress.