Identification of Nuclear Genes Encoding Chloroplast-Localized Proteins Required for Embryo Development in Arabidopsis    

Abstract

We describe here the diversity of chloroplast proteins required for embryo development in Arabidopsis (Arabidopsis thaliana). Interfering with certain chloroplast functions has long been known to result in embryo lethality. What has not been reported before is a comprehensive screen for embryo-defective (emb) mutants altered in chloroplast proteins. From a collection of transposon and T-DNA insertion lines at the RIKEN chloroplast function database (http://rarge.psc.riken.jp/chloroplast/) that initially appeared to lack homozygotes and segregate for defective seeds, we identified 23 additional examples of EMB genes that likely encode chloroplast-localized proteins. Fourteen gene identities were confirmed with allelism tests involving duplicate mutant alleles. We then queried journal publications and the SeedGenes database (www.seedgenes.org) to establish a comprehensive dataset of 381 nuclear genes encoding chloroplast proteins of Arabidopsis associated with embryo-defective (119 genes), plant pigment (121 genes), gametophyte (three genes), and alternate (138 genes) phenotypes. Loci were ranked based on the level of certainty that the gene responsible for the phenotype had been identified and the protein product localized to chloroplasts. Embryo development is frequently arrested when amino acid, vitamin, or nucleotide biosynthesis is disrupted but proceeds when photosynthesis is compromised and when levels of chlorophyll, carotenoids, or terpenoids are reduced. Chloroplast translation is also required for embryo development, with genes encoding chloroplast ribosomal and pentatricopeptide repeat proteins well represented among EMB datasets. The chloroplast accD locus, which is necessary for fatty acid biosynthesis, is essential in Arabidopsis but not in Brassica napus or maize (Zea mays), where duplicated nuclear genes compensate for its absence or loss of function.

Organelle–nucleus cross-talk regulates plant intercellular communication via plasmodesmata

“…The plastid genome encodes products with critical functions in photosynthesis, plastid gene expression, and fatty acid synthesis (43,44). Table 4 reveals that several plastid transcripts are differentially down-regulated in both mutants.…”

Repression of Essential Chloroplast Genes Reveals New Signaling Pathways and Regulatory Feedback Loops inChlamydomonas

“…In this case, plastid growth and division are maintained in the absence of plastid protein synthesis. Moreover, many mutations that disrupt the splicing of chloroplast tRNAs or ribosomal protein mRNAs in maize lead to a complete loss of plastid ribosomes without affecting embryo development (Bryant et al, 2011;Lloyd and Meinke, 2012). Many additional albino mutants of maize and rice (Oryza sativa) lack plastid ribosomes, although the underlying mechanisms are not yet clear.…”

“…In contrast with the grasses, embryo development is compromised in Arabidopsis mutants deficient in plastid translation. This difference between monocotyledonous and dicotyledonous plants appears to be due to the chloroplast accD locus involved in fatty acid biosynthesis, which is essential in Arabidopsis but not in maize, and rapeseed where nuclear genes compensate for its absence (Bryant et al, 2011).…”

“…This is in contrast with the situations in tobacco in which transplastomic plants with a homoplasmic disruption of the rpoB gene are still viable (Allison et al, 1996) and in barley (Hordeum vulgare) mutants in which plastid translation is fully deficient (Hess et al, 1994a). Similarly, in maize (Zea mays) or rapeseed (Brassica napus), embryo development is not arrested in the absence of chloroplast translation (Bryant et al, 2011;Lloyd and Meinke, 2012), whereas loss of plastid translation results in embryo lethality in Arabidopsis thaliana (Lloyd and Meinke, 2012). We show that, similar to the case of land plants, abrogation of either chloroplast protein synthesis or transcription in C. reinhardtii activates plastid-to-nucleus signaling pathways, causing the induction of several nuclear-encoded stress-responsive plastid proteins prior to cell death.…”

Modified Clp Protease Complex in the ClpP3 Null Mutant and Consequences for Chloroplast Development and Function in Arabidopsis

“…During seed development, embryo plastids begin to differentiate into green chloroplasts at the torpedo stage and then become colorless during seed ripening, as chloroplasts dedifferentiate to nonphotosynthetic plastids by losing their thylakoids and associated chlorophylls. Upon germination, these dedifferentiated plastids are converted into chloroplasts in the cotyledons (Ciamporova and Pretova, 1980; for review of the significance and biogenesis of plastids in embryogenesis, see Hsu et al, 2010;Bryant et al, 2011).…”