DELAYED GREENING 238, a Nuclear-Encoded Chloroplast Nucleoid Protein, Is Involved in the Regulation of Early Chloroplast Development and Plastid Gene Expression in Arabidopsis thaliana

Genetic mapping and molecular characterization of the delayed green gene dg in watermelon (Citrullus lanatus)

Leaf color mutants are common in higher plants that can be used as markers in crop breeding and are important tools in understanding regulatory mechanisms of chlorophyll biosynthesis and chloroplast development. Genetic analysis was performed by evaluating F₁, F₂ and BC₁ populations derived from two parental lines (Charleston gray with green leaf color and Houlv with delayed green leaf color), suggesting that a single recessive gene controls the delayed green leaf color. In this study, the delayed green mutant showed a conditional pale green leaf color at the early leaf development but turned to green as the leaf development progressed. Delayed green leaf plants showed reduced pigment content, photosynthetic, chlorophyll fluorescence parameters, and impaired chloroplast development compared with green leaf plants. The delayed green (dg) locus was mapped to 7.48 Mb on chromosome 3 through bulk segregant analysis approach, and the gene controlling delayed green leaf color was narrowed to 53.54 kb between SNP130 and SNP135 markers containing three candidate genes. Sequence alignment of the three genes indicated that there was a single SNP mutation (G/A) in the coding region of ClCG03G010030 in the Houlv parent, which causes an amino acid change from Arginine to Lysine. The ClCG03G010030 gene encoded FtsH extracellular protease protein family is involved in early delayed green leaf development. The expression level of ClCG03G010030 was significantly reduced in delayed green leaf plants than in green leaf plants. These results indicated that the ClCG03G010030 might control watermelon green leaf color and the single SNP variation in ClCG03G010030 may result in early delayed green leaf color development during evolutionary process.

Plastid Deficient 1 Is Essential for the Accumulation of Plastid-Encoded RNA Polymerase Core Subunit β and Chloroplast Development in Arabidopsis

Plastid-encoded RNA polymerase (PEP)-dependent transcription is an essential process for chloroplast development and plant growth. It is a complex event that is regulated by numerous nuclear-encoded proteins. In order to elucidate the complex regulation mechanism of PEP activity, identification and characterization of PEP activity regulation factors are needed. Here, we characterize Plastid Deficient 1 (PD1) as a novel regulator for PEP-dependent gene expression and chloroplast development in Arabidopsis. The PD1 gene encodes a protein that is conserved in photoautotrophic organisms. The Arabidopsis pd1 mutant showed albino and seedling-lethal phenotypes. The plastid development in the pd1 mutant was arrested. The PD1 protein localized in the chloroplasts, and it colocalized with nucleoid protein TRXz. RT-quantitative real-time PCR, northern blot, and run-on analyses indicated that the PEP-dependent transcription in the pd1 mutant was dramatically impaired, whereas the nuclear-encoded RNA polymerase-dependent transcription was up-regulated. The yeast two-hybrid assays and coimmunoprecipitation experiments showed that the PD1 protein interacts with PEP core subunit β (PEP-β), which has been verified to be essential for chloroplast development. The immunoblot analysis indicated that the accumulation of PEP-β was barely detected in the pd1 mutant, whereas the accumulation of the other essential components of the PEP complex, such as core subunits α and β′, were not affected in the pd1 mutant. These observations suggested that the PD1 protein is essential for the accumulation of PEP-β and chloroplast development in Arabidopsis, potentially by direct interaction with PEP-β.

Identification of a delayed leaf greening gene from a mutation of pummelo

Delayed greening of young leaves is an unusual phenomenon of plants in nature. Citrus are mostly evergreen tree species. Here, a natural mutant of "Guanxi" pummelo (Citrus maxima), which shows yellow leaves at the young stage, was characterized to identify the genes underlying the trait of delayed leaf greening in plants. A segregating population with this mutant as the seed parent and a normal genotype as the pollen parent was generated. Two DNA pools respectively from the leaves of segregating seedlings with extreme phenotypes of normal leaf greening and delayed leaf greening were collected for sequencing. Bulked segregant analysis (BSA) and InDel marker analysis demonstrated that the delayed leaf greening trait is governed by a 0.3 Mb candidate region on chromosome 6. Gene expression analysis further identified a key candidate gene (Citrus Delayed Greening gene 1, CDG1) in the 0.3 Mb region, which showed significantly differential expression between the genotypes with delayed and normal leaf greening phenotypes. There was a 67 bp InDel region difference in the CDG1 promoter and the InDel region contains a TATA-box element. Confocal laser-scanning microscopy revealed that the CDG1-GFP fusion protein signals were co-localized with the chloroplast signals in the protoplasts. Overexpression of CDG1 in tobacco and Arabidopsis led to the phenotype of delayed leaf greening. These results suggest that the CDG1 gene is involved in controlling the delayed leaf greening phenotype with important functions in chloroplast development.

Arabidopsis Mitochondrial Transcription Termination Factor mTERF2 Promotes Splicing of Group IIB Introns

Plastid gene expression (PGE) is essential for chloroplast biogenesis and function and, hence, for plant development. However, many aspects of PGE remain obscure due to the complexity of the process. A hallmark of nuclear-organellar coordination of gene expression is the emergence of nucleus-encoded protein families, including nucleic-acid binding proteins, during the evolution of the green plant lineage. One of these is the mitochondrial transcription termination factor (mTERF) family, the members of which regulate various steps in gene expression in chloroplasts and/or mitochondria. Here, we describe the molecular function of the chloroplast-localized mTERF2 in Arabidopsis thaliana. The complete loss of mTERF2 function results in embryo lethality, whereas directed, microRNA (amiR)-mediated knockdown of MTERF2 is associated with perturbed plant development and reduced chlorophyll content. Moreover, photosynthesis is impaired in amiR-mterf2 plants, as indicated by reduced levels of photosystem subunits, although the levels of the corresponding messenger RNAs are not affected. RNA immunoprecipitation followed by RNA sequencing (RIP-Seq) experiments, combined with whole-genome RNA-Seq, RNA gel-blot, and quantitative RT-PCR analyses, revealed that mTERF2 is required for the splicing of the group IIB introns of ycf3 (intron 1) and rps12.

A novel tetratricopeptide repeat protein, WHITE TO GREEN1, is required for early chloroplast development and affects RNA editing in chloroplasts

The chloroplast is essential for plant photosynthesis and production, but the regulatory mechanism of chloroplast development is still elusive. Here, a novel gene, WHITE TO GREEN1 (WTG1), was identified to have a function in chloroplast development and plastid gene expression by screening Arabidopsis leaf coloration mutants. WTG1 encodes a chloroplast-localized tetratricopeptide repeat protein that is expressed widely in Arabidopsis cells. Disruption of WTG1 suppresses plant growth, retards leaf greening and chloroplast development, and represses photosynthetic gene expression, but complemented expression of WTG1 restored a normal phenotype. Moreover, WTG1 protein is associated with the organelle RNA editing factors MORF8 and MORF9, and RNA editing of the plastid petL-5 and ndhG-50 transcripts was affected in wtg1 mutants. These results indicate that WTG1 affects both transcriptional and posttranscriptional regulation of plastid gene expression, and provide evidence for the involvement of a tetratricopeptide repeat protein in chloroplast RNA editing in Arabidopsis.

White Leaf and Panicle 2, encoding a PEP-associated protein, is required for chloroplast biogenesis under heat stress in rice

The plastid-encoded RNA polymerase (PEP) plays an important role in the transcription machinery of mature chloroplasts, yet details of its function remain elusive in rice. Here, we identified a novel PEP-associated protein (PAP), WLP2, based on its two allelic white leaf and panicle mutants, wlp2s and wlp2w. The two mutants were albino lethal at high temperatures and showed decreased chlorophyll accumulation, abnormal chloroplast ultrastructure, and attenuated photosynthetic activity. Map-based cloning suggested that WLP2 encodes a putative pfkB-type carbohydrate kinase family protein, which is homologous to fructokinase-like 1 (AtFLN1) in Arabidopsis. WLP2 is mainly expressed in green tissues and its protein localizes in chloroplasts. Expression levels of PEP-encoded genes, chloroplast development genes and photosynthesis-related genes were compromised in wlp2 mutants, indicating that WLP2 is essential for normal chloroplast biogenesis. Moreover, WLP2 and its paralog OsFLN2 can physically interact with thioredoxin OsTRXz to form a TRX-FLN regulatory module, which not only regulates transcription of the PEP-encoded genes but also maintains the redox balance in chloroplasts under heat stress. Furthermore, the wlp2w mutant gene represents a potential advantage in enhancing seed purity and high-throughput breeding. Our results strongly indicate that WLP2 protects chloroplast development from heat stress via a TRX-FLN regulatory module in rice.