Nonessential plastid-encoded ribosomal proteins in tobacco: a developmental role for plastid translation and implications for reductive genome evolution

Function of plastid translation in plant temperature acclimation: Retrograde signalling or extraribosomal 'moonlighting' functions?

Specific components of the plastid ribosome could act as pivotal limiting factors in plant temperature acclimation. We endeavour to elucidate the molecular nexus between plastid translation and temperature acclimation by incorporating the concept of extraribosomal ‘moonlighting’ functions of plastid ribosome proteins.

Removal of the large inverted repeat from the plastid genome reveals gene dosage effects and leads to increased genome copy number

The chloroplast genomes of most plants and algae contain a large inverted repeat (IR) region that separates two single-copy regions and harbours the ribosomal RNA operon. We have addressed the functional importance of the IR region by removing an entire copy of the 25.3-kb IR from the tobacco plastid genome. Using plastid transformation and subsequent selectable marker gene elimination, we precisely excised the IR, thus generating plants with a substantially reduced plastid genome size. We show that the lack of the IR results in a mildly reduced plastid ribosome number, suggesting a gene dosage benefit from the duplicated presence of the ribosomal RNA operon. Moreover, the IR deletion plants contain an increased number of plastid genomes, suggesting that genome copy number is regulated by measuring total plastid DNA content rather than by counting genomes. Together, our findings (1) demonstrate that the IR can enhance the translation capacity of the plastid, (2) reveal the relationship between genome size and genome copy number, and (3) provide a simplified plastid genome structure that will facilitate future synthetic biology applications.

Chloroplast Ribosome Biogenesis Factors

The formation of chloroplasts can be traced back to an ancient event in which a eukaryotic host cell containing mitochondria ingested a cyanobacterium. Since then, chloroplasts have retained many characteristics of their bacterial ancestor, including their transcription and translation machinery. In this review, recent research on the maturation of rRNA and ribosome assembly in chloroplasts is explored, along with their crucial role in plant survival and their implications for plant acclimation to changing environments. A comparison is made between the ribosome composition and auxiliary factors of ancient and modern chloroplasts, providing insights into the evolution of ribosome assembly factors. Although the chloroplast contains ancient proteins with conserved functions in ribosome assembly, newly evolved factors have also emerged to help plants acclimate to changes in their environment and internal signals. Overall, this review offers a comprehensive analysis of the molecular mechanisms underlying chloroplast ribosome assembly and highlights the importance of this process in plant survival, acclimation and adaptation.

Comparative chloroplast genomes of Argentina species: genome evolution and phylogenomic implications

The genus Argentina Hill belongs to the tribe Potentilleae Sweet and contains approximately 75 species predominantly distributed in the Sino-Himalayan region and the Malesian archipelago. So far we have less knowledge on the phylogenetic relationships within Argentina owing to limited sampling of Argentina taxa or gene fragments in previous studies. Moreover, to date there is no phylogenetic study on Argentina from the perspective of comparative chloroplast (cp) genomics. Here we performed comparative genomic analyses on the cp genomes of 39 accessions representing 18 taxa of Argentina. The Argentina cp genomes presented the typical quadripartite structure, with the sizes ranging from 155 096 bp to 157 166 bp. The 39 Argentina cp genomes contained a set of 112 unique genes, comprising four ribosomal RNA (rRNA) genes, 30 transfer RNA (tRNA) genes, as well as 78 protein-coding genes (PCGs). The cp genome organization, gene content and order in Argentina were highly conserved, but some visible divergences were present in IR/SC boundary regions. Ten regions (trnH-GUG-psbA, trnG-GCC-trnfM-CAU, trnD-GUC-trnY-GUA, rpl32-trnL-UAG, atpH-atpI, rps16-trnQ-UUG, trnS-GCU-trnG-UCC, ndhF-rpl32, trnR-UCU-atpA, and accD-psaI) were identified as excellent candidate DNA markers for future studies on species identification, population genetics and phylogeny of Argentina. Our results indicated that Argentina is monophyletic. In the current sampling, the A. smithiana - A. anserina clade was sister to the remainder of Argentina. Our results corroborated the previous taxonomic treatments to transfer A. phanerophlebia and A. micropetala from the genus Sibbaldia L. to Argentina. Our results showed close relationships among A. stenophylla, A. microphylla, A. taliensis, and A. tatsienluensis, congruent with previous studies based on the morphology of these species. Twenty-six genes (rps3, rps15, rps16, rps19, rpl16, rpl20, rpl22, rpoA, rpoB, rpoC1, rpoC2, atpA, atpF, psbB, psbF, ndhA, ndhB, ndhC, ndhD, ndhF, rbcL, accD, ccsA, matK, ycf1, ycf2) were with sites under positive selection, and adaptive evolution of these genes might have played crucial roles in Argentina species adaptation to the harsh mountain environment. This study will facilitate future work on taxonomy, phylogenetics, and adaptive evolution of Argentina.

Complete Chloroplast Genome of Alternanthera sessilis and Comparative Analysis with Its Congeneric Invasive Weed Alternanthera philoxeroides

Alternanthera sessilis is considered the closest relative to the invasive weed Alternanthera philoxeroides in China, making it an important native species for studying the invasive mechanisms and adaptations of A. philoxeroides. Chloroplasts play a crucial role in a plant's environmental adaptation, with their genomes being pivotal in the evolution and adaptation of both invasive and related species. However, the chloroplast genome of A. sessilis has remained unknown until now. In this study, we sequenced and assembled the complete chloroplast genome of A. sessilis using high-throughput sequencing. The A. sessilis chloroplast genome is 151,935 base pairs long, comprising two inverted repeat regions, a large single copy region, and a small single copy region. This chloroplast genome contains 128 genes, including 8 rRNA-coding genes, 37 tRNA-coding genes, 4 pseudogenes, and 83 protein-coding genes. When compared to the chloroplast genome of the invasive weed A. philoxeroides and other Amaranthaceae species, we observed significant variations in the ccsA, ycf1, and ycf2 regions in the A. sessilis chloroplast genome. Moreover, two genes, ccsA and accD, were found to be undergoing rapid evolution due to positive selection pressure. The phylogenetic trees were constructed for the Amaranthaceae family, estimating the time of independent species formation between A. philoxeroides and A. sessilis to be approximately 3.5186-8.8242 million years ago. These findings provide a foundation for understanding the population variation within invasive species among the Alternanthera genus.

Variations and reduction of plastome are associated with the evolution of parasitism in Convolvulaceae

Parasitic lifestyle can often relax the constraint on the plastome, leading to gene pseudogenization and loss, and resulting in diverse genomic structures and rampant genome degradation. Although several plastomes of parasitic Cuscuta have  been reported, the evolution of parasitism in the family Convolvulaceae which is linked to structural variations and reduction of plastome has not been well investigated. In this study, we assembled and collected 40 plastid genomes belonging to 23 species representing four subgenera of Cuscuta and ten species of autotrophic Convolvulaceae. Our findings revealed nine types of structural variations and six types of inverted repeat (IR) boundary variations in the plastome of Convolvulaceae spp. These structural variations were associated with the shift of parasitic lifestyle, and IR boundary shift, as well as the abundance of long repeats. Overall, the degradation of Cuscuta plastome proceeded gradually, with one clade exhibiting an accelerated degradation rate. We observed five stages of gene loss in Cuscuta, including NAD(P)H complex → PEP complex → Photosynthesis-related → Ribosomal protein subunits → ATP synthase complex. Based on our results, we speculated that the shift of parasitic lifestyle in early divergent time promoted relaxed selection on plastomes, leading to the accumulation of microvariations, which ultimately resulted in the plastome reduction. This study provides new evidence towards a better understanding of plastomic evolution, variation, and reduction in the genus Cuscuta.

White and green striate leaves 1, predicted to encode a 16S rRNA processing protein, plays a critical role in the processing of chloroplast ribosomes in maize (Zea mays L.)

Ribosomes play a crucial role in protein biosynthesis and are linked to plant growth and development. The RimM protein has been shown to be involved in the maturation of 30S ribosomal subunits, but its exact function in plants is still unknown. In this study, we discovered a maize mutant with white and green striate leaves (wgsl1) and reduced chlorophyll content. Genetic analysis showed that the wgsl1 mutation was recessive and controlled by a single nuclear gene. Map-based cloning of ZmWGSL1 identified a base substitution (G to A) that generated a missense mutation within the Zm00001d039036 gene in the wgsl1 mutant. Zm00001d039036 encodes a 16S rRNA processing protein containing the RimM motif. Further analysis of transcriptomic data showed that the transcript levels of many ribosomal proteins involved in the small and big ribosomal subunits were dramatically up-regulated in the wgsl1 mutant. Moreover, the level of ribosomal multimers was decreased. This suggests that ZmWGSL1 plays a crucial role in the maturation of the ribosome, leading to abnormal plant growth and development. In addition, subcellular localization results indicate that WGSL1 is localized in chloroplasts. Therefore, we suggest that WGSL1 is a nuclear-encoded protein, is transported to the chloroplast to drive functions, and affects the processing of ribosomes in the chloroplast.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-023-01407-y.

The complete chloroplast genome of Ulmus mianzhuensis with insights into structural variations, adaptive evolution, and phylogenetic relationships of Ulmus (Ulmaceae)

BACKGROUND

Ulmus mianzhuensis is an endemic tree species in China with high ornamental and economic value. Currently, little is known regarding its genomic architecture, phylogenetic position, or adaptive evolution. Here, we sequenced the complete chloroplast genome (cp genome) of U. mianzhuensis and further compared the variations in gene organization and structure within Ulmus species to define their genomic evolution, then reconstructed the phylogenomic relationship of 31 related Ulmus species to explore the systematic position of U. mianzhuensis and the utility of cp genome for resolving phylogenetics among Ulmus species.

RESULTS

Our results revealed that all the Ulmus species exhibited a typical quadripartite structure, with a large single copy (LSC) region of 87,170 - 88,408 bp, a small single copy (SSC) region of 18,650 - 19,038 bp and an inverted repeat (IR) region of 26,288 - 26,546 bp. Within Ulmus species, gene structure and content of cp genomes were highly conserved, although slight variations were found in the boundary of SC/IR regions. Moreover, genome-wide sliding window analysis uncovered the variability of ndhC-trnV-UAC, ndhF-rpl32, and psbI-trnS-GCU were higher among 31 Ulmus that may be useful for the population genetics and potential DNA barcodes. Two genes (rps15 and atpF) were further detected under a positive selection of Ulmus species. Comparative phylogenetic analysis based on the cp genome and protein-coding genes revealed consistent topology that U. mianzhuensis is a sister group to U. parvifolia (sect. Microptelea) with a relatively low-level nucleotide variation of the cp genome. Additionally, our analyses also found that the traditional taxonomic system of five sections in Ulmus is not supported by the current phylogenomic topology with a nested evolutionary relationship between sections.

CONCLUSIONS

Features of the cp genome length, GC content, organization, and gene order were highly conserved within Ulmus. Furthermore, molecular evidence from the low variation of the cp genome suggested that U. mianzhuensis should be merged into U. parvifolia and regarded as a subspecies of U. parvifolia. Overall, we demonstrated that the cp genome provides valuable information for understanding the genetic variation and phylogenetic relationship in Ulmus.

Interspecific variation and phylogenetic relationship between mangrove and non-mangrove species of a same family (Meliaceae)-insights from comparative analysis of complete chloroplast genome

The mahogany family, Meliaceae, contains 58 genera with only one mangrove genus: Xylocarpus. Two of the three species of the genus Xylocarpus are true mangroves (X. granatum and X. moluccensis), and one is a non-mangrove (X. rumphii). In order to resolve the phylogenetic relationship between the mangrove and non-mangrove species, we sequenced chloroplast genomes of these Xylocarpus species along with two non-mangrove species of the Meliaceae family (Carapa guianensis and Swietenia macrophylla) and compared the genome features and variations across the five species. The five Meliaceae species shared 130 genes (85 protein-coding genes, 37 tRNA, and eight rRNA) with identical direction and order, with a few variations in genes and intergenic spacers. The repetitive sequences identified in the rpl22 gene region only occurred in Xylocarpus, while the repetitive sequences in accD were found in X. moluccensis and X. rumphii. The TrnH-GUG and rpl32 gene regions and four non-coding gene regions showed high variabilities between X. granatum and the two non-mangrove species (S. macrophylla and C. guianensis). In addition, among the Xylocarpus species, only two genes (accD and clpP) showed positive selection. Carapa guianensis and S. macrophylla owned unique RNA editing sites. The above genes played an important role in acclimation to different stress factors like heat, low temperature, high UV light, and high salinity. Phylogenetic analysis with 22 species in the order Sapindales supported previous studies, which revealed that the non-mangrove species X. rumphii is closer to X. moluccensis than X. granatum. Overall, our results provided important insights into the variation of genetic structure and adaptation mechanism at interspecific (three Xylocarpus species) and intergeneric (mangrove and non-mangrove genera) levels.

The chloroplast genome of black pepper (Piper nigrum L.) and its comparative analysis with related Piper species

Piper nigrum, also known as black pepper, is an economically and ecologically important crop of the genus Piper. It has been titled as the king of spices due to its wide consumption throughout the world. In the present investigation, the chloroplast genome of P. nigrum has been assembled from a whole genome sequence by integrating the short and long reads generated through Illumina and PacBio platforms, respectively. The chloroplast genome was observed to be 161,522 bp in size, having a quadripartite structure with a large single copy (LSC) region of 89,153 bp and a small single copy (SSC) region of 18,255 bp separated by a copy of inverted repeats (IRs), each 27,057 bp in length. Taking into consideration all the duplicated genes, a total of 131 genes were observed, which included 81 protein-coding genes, 37 tRNAs, 4 rRNAs, and 1 pseudogene. Individually, the LSC region consisted of 83 genes, the SSC region had 13 genes, and 18 genes were present in each IR region. Additionally, 216 SSRs were detected and 11 of these were validated through amplification in 12 species of Piper. The features of the chloroplast genome have been compared with those of the genus Piper. Our results provide useful insights into evolutionary and molecular studies of black pepper which will contribute to its further genetic improvement and breeding.

Comparison of Boraginales Plastomes: Insights into Codon Usage Bias, Adaptive Evolution, and Phylogenetic Relationships

The Boraginales (Boraginaceae a.l.) comprise more than 2450 species worldwide. However, little knowledge exists of the characteristics of the complete plastid genome. In this study, three new sequences representing the first pt genome of Heliotropiaceae and Cordiaceae were assembled and compared with other Boraginales species. The pt genome sizes of Cordia dichotoma, Heliotropium arborescens, and Tournefortia montana were 151,990 bp, 156,243 bp, and 155,891 bp, respectively. Multiple optimal codons were identified, which may provide meaningful information for enhancing the gene expression of Boraginales species. Furthermore, codon usage bias analyses revealed that natural selection and other factors may dominate codon usage patterns in the Boraginales species. The boundaries of the IR/LSC and IR/SSC regions were significantly different, and we also found a signal of obvious IR region expansion in the pt genome of Nonea vesicaria and Arnebia euchroma. Genes with high nucleic acid diversity (pi) values were also calculated, which may be used as potential DNA barcodes to investigate the phylogenetic relationships in Boraginales. psaI, rpl33, rpl36, and rps19 were found to be under positive selection, and these genes play an important role in our understanding of the adaptive evolution of the Boraginales species. Phylogenetic analyses implied that Boraginales can be divided into two groups. The existence of two subfamilies (Lithospermeae and Boragineae) in Boraginaceae is also strongly supported. Our study provides valuable information on pt genome evolution and phylogenetic relationships in the Boraginales species.

Unveiling the functions of plastid ribosomal proteins in plant development and abiotic stress tolerance

Translation of mRNAs into proteins is a universal process and ribosomes are the molecular machinery that carries it out. In eukaryotic cells, ribosomes can be found in the cytoplasm, mitochondria, and also in the chloroplasts of photosynthetic organisms. A number of genetic studies have been performed to determine the function of plastid ribosomal proteins (PRPs). Tobacco has been frequently used as a system to study the ribosomal proteins encoded by the chloroplast genome. In contrast, Arabidopsis thaliana and rice are preferentially used models to study the function of nuclear-encoded PRPs by using direct or reverse genetics approaches. The results of these works have provided a relatively comprehensive catalogue of the roles of PRPs in different plant biology aspects, which highlight that some PRPs are essential, while others are not. The latter ones are involved in chloroplast biogenesis, lateral root formation, leaf morphogenesis, plant growth, photosynthesis or chlorophyll synthesis. Furthermore, small gene families encode some PRPs. In the last few years, an increasing number of findings have revealed a close association between PRPs and tolerance to adverse environmental conditions. Sometimes, the same PRP can be involved in both developmental processes and the response to abiotic stress. The aim of this review is to compile and update the findings hitherto published on the functional analysis of PRPs. The study of the phenotypic effects caused by the disruption of PRPs from different species reveals the involvement of PRPs in different biological processes and highlights the significant impact of plastid translation on plant biology.

Plastome-based phylogeny and biogeography of Lactuca L. (Asteraceae) support revised lettuce gene pool categories

This study generated and analyzed complete plastome and internal transcribed spacer (ITS) data of 46 Lactuca species, 13 African endemic (AE) Lactuca species, and 15 species from eight related genera in Lactucinae. The new plastome and nuclear ITS sequences were then used to reconstruct the phylogenetic relationships of Lactuca species. The whole-plastome data were used to estimate divergence time and ancestral area reconstruction of the identified major Lactuca lineages. The results showed that Lactuca species are generally similar in plastome size, Guanine and Cytosine (GC) content, gene structure, and categories, although crop lettuce (Lactuca sativa L.) and its gene pool relatives were found to have one unique pseudogene (ψ ndhF), and accD, atpF, cemA, clpP, and rpl22 showed signs of positive selection. Our phylogenomic analysis demonstrated that Lactuca is monophyletic after excluding Lactuca alatipes Collett and Hemsl and AE Lactuca species. AE Lactuca species are morphologically distinct from core Lactuca lineage and need to be excluded from Lactua. The core Lactuca species most likely originated from Asia-Temperate W ~6.82 Mya and then dispersed globally and formed nine clades. Finally, the lettuce gene pool concept was amended according to the phylogenetic and historical biogeographic analyses. This study revised the circumscription of Lactuca, revealed robust phylogenetic relationships within the genus, and provided insights into Lactucinae phylogeny. The lettuce gene pool species could be used as potential genetic resources for lettuce breeding.

The nuclear-encoded plastid ribosomal protein L18s are essential for plant development

Plastid ribosomal proteins (PRPs) are necessary components for plastid ribosome biogenesis, playing essential roles in plastid development. The ribosomal protein L18 involved in the assemble of 5S rRNA and 23S rRNA, is vital for E. coli viability, but the functions of its homologs in plant plastid remain elusive. Here, we characterized the functions of the plant plastid ribosomal protein L18s (PRPL18s) in Arabidopsis and rice. AtPRPL18 was ubiquitously expressed in most of the plant tissues, but with higher expression levels in seedling shoots, leaves, and flowers. AtPRPL18 was localized in chloroplast. Genetic and cytological analyses revealed that a loss of function of AtPRPL18 resulted in embryo development arrest at globular stage. However, overexpression of AtPRPL18 did not show any visible phenotypical changes in Arabidopsis. The rice OsPRPL18 was localized in chloroplast. In contrast to AtPRPL18, knockout of OsPRPL18 did not affect embryo development, but led to an albino lethal phenotype at the seedling stage. Cytological analyses showed that chloroplast development was impaired in the osprpl18-1 mutant. Moreover, a loss-function of OsPRPL18 led to defects in plastid ribosome biogenesis and a serious reduction in the efficiency of plastid intron splicing. In all, these results suggested that PRPL18s play critical roles in plastid ribosome biogenesis, plastid intron splicing, and chloroplast development, and are essential for plant survival.

Plastid ribosome protein L5 is essential for post-globular embryo development in Arabidopsis thaliana

Plastid ribosomal proteins (PRPs) can play essential roles in plastid ribosome functioning that affect plant function and development. However, the roles of many PRPs remain unknown, including elucidation of which PRPs are essential or display redundancy. Here, we report that the nuclear-encoded PLASTID RIBOSOMAL PROTEIN L5 (PRPL5) is essential for early embryo development in A. thaliana, as homozygous loss-of-function mutations in the PRPL5 gene impairs chloroplast development and leads to embryo failure to develop past the globular stage. We confirmed the prpl5 embryo-lethal phenotype by generating a mutant CRISPR/Cas9 line and by genetic complementation. As PRPL5 underwent transfer to the nuclear genome early in the evolution of Embryophyta, PRPL5 can be expected to have acquired a chloroplast transit peptide. We identify and validate the presence of an N-terminal chloroplast transit peptide, but unexpectedly also confirm the presence of a conserved and functional Nuclear Localization Signal on the protein C-terminal end. This study highlights the fundamental role of the plastid translation machinery during the early stages of embryo development in plants and raises the possibility of additional roles of plastid ribosomal proteins in the nucleus.

AtNusG, a chloroplast nucleoid protein of bacterial origin linking chloroplast transcriptional and translational machineries, is required for proper chloroplast gene expression in Arabidopsis thaliana

In Escherichia coli, transcription-translation coupling is mediated by NusG. Although chloroplasts are descendants of endosymbiotic prokaryotes, the mechanism underlying this coupling in chloroplasts remains unclear. Here, we report transcription-translation coupling through AtNusG in chloroplasts. AtNusG is localized in chloroplast nucleoids and is closely associated with the chloroplast PEP complex by interacting with its essential component PAP9. It also comigrates with chloroplast ribosomes and interacts with their two components PRPS5 (uS5c) and PRPS10 (uS10c). These data suggest that the transcription and translation machineries are coupled in chloroplasts. In the atnusg mutant, the accumulation of chloroplast-encoded photosynthetic gene transcripts, such as psbA, psbB, psbC and psbD, was not obviously changed, but that of their proteins was clearly decreased. Chloroplast polysomic analysis indicated that the decrease in these proteins was due to the reduced efficiency of their translation in this mutant, leading to reduced photosynthetic efficiency and enhanced sensitivity to cold stress. These data indicate that AtNusG-mediated coupling between transcription and translation in chloroplasts ensures the rapid establishment of photosynthetic capacity for plant growth and the response to environmental changes. Therefore, our study reveals a conserved mechanism of transcription-translation coupling between chloroplasts and E. coli, which perhaps represents a regulatory mechanism of chloroplast gene expression. This study provides insights into the underlying mechanisms of chloroplast gene expression in higher plants.

Regulation of Chloroplast Development and Function at Adverse Temperatures in Plants

The chloroplast is essential for photosynthesis, plant growth and development. As semiautonomous organelles, the biogenesis and development of chloroplasts need to be well-regulated during plant growth and stress responses. Low or high ambient temperatures are adverse environmental stresses that affect crop growth and productivity. As sessile organisms, plants regulate the development and function of chloroplasts in a fluctuating temperature environment to maintain normal photosynthesis. This review focuses on the molecular mechanisms and regulatory factors required for chloroplast biogenesis and development under cold or heat stress conditions and highlights the importance of chloroplast gene transcription, RNA metabolism, ribosome function and protein homeostasis essential for chloroplast development under adverse temperature conditions.

Chloroplast translational regulation uncovers nonessential photosynthesis genes as key players in plant cold acclimation

Plants evolved efficient multifaceted acclimation strategies to cope with low temperatures. Chloroplasts respond to temperature stimuli and participate in temperature sensing and acclimation. However, very little is known about the involvement of chloroplast genes and their expression in plant chilling tolerance. Here we systematically investigated cold acclimation in tobacco seedlings over 2 days of exposure to low temperatures by examining responses in chloroplast genome copy number, transcript accumulation and translation, photosynthesis, cell physiology, and metabolism. Our time-resolved genome-wide investigation of chloroplast gene expression revealed substantial cold-induced translational regulation at both the initiation and elongation levels, in the virtual absence of changes at the transcript level. These cold-triggered dynamics in chloroplast translation are widely distinct from previously described high light-induced effects. Analysis of the gene set responding significantly to the cold stimulus suggested nonessential plastid-encoded subunits of photosynthetic protein complexes as novel players in plant cold acclimation. Functional characterization of one of these cold-responsive chloroplast genes by reverse genetics demonstrated that the encoded protein, the small cytochrome b6f complex subunit PetL, crucially contributes to photosynthetic cold acclimation. Together, our results uncover an important, previously underappreciated role of chloroplast translational regulation in plant cold acclimation.

Chloroplast translational regulation uncovers nonessential photosynthesis genes as key players in plant cold acclimation

Plants evolved efficient multifaceted acclimation strategies to cope with low temperatures. Chloroplasts respond to temperature stimuli and participate in temperature sensing and acclimation. However, very little is known about the involvement of chloroplast genes and their expression in plant chilling tolerance. Here we systematically investigated cold acclimation in tobacco seedlings over 2 days of exposure to low temperatures by examining responses in chloroplast genome copy number, transcript accumulation and translation, photosynthesis, cell physiology, and metabolism. Our time-resolved genome-wide investigation of chloroplast gene expression revealed substantial cold-induced translational regulation at both the initiation and elongation levels, in the virtual absence of changes at the transcript level. These cold-triggered dynamics in chloroplast translation are widely distinct from previously described high light-induced effects. Analysis of the gene set responding significantly to the cold stimulus suggested nonessential plastid-encoded subunits of photosynthetic protein complexes as novel players in plant cold acclimation. Functional characterization of one of these cold-responsive chloroplast genes by reverse genetics demonstrated that the encoded protein, the small cytochrome b6f complex subunit PetL, crucially contributes to photosynthetic cold acclimation. Together, our results uncover an important, previously underappreciated role of chloroplast translational regulation in plant cold acclimation.

AtRsmD Is Required for Chloroplast Development and Chloroplast Function in Arabidopsis thaliana

AtRsmD was recently demonstrated to be a chloroplast 16S rRNA methyltransferase (MTase) for the m²G915 modification in Arabidopsis. Here, its function of AtRsmD for chloroplast development and photosynthesis was further analyzed. The AtRsmD gene is highly expressed in green photosynthetic tissues. AtRsmD is associated with the thylakoid in chloroplasts. The atrsmd-2 mutant exhibited impaired photosynthetic efficiency in emerging leaves under normal growth conditions. A few thylakoid lamellas could be observed in the chloroplast from the atrsmd-2 mutant, and these thylakoids were loosely organized. Knockout of the AtRsmD gene had minor effects on chloroplast ribosome biogenesis and RNA loading on chloroplast ribosomes, but it reduced the amounts of chloroplast-encoded photosynthesis-related proteins in the emerging leaves, for example, D1, D2, CP43, and CP47, which reduced the accumulation of the photosynthetic complex. Nevertheless, knockout of the AtRsmD gene did not cause a general reduction in chloroplast-encoded proteins in Arabidopsis grown under normal growth conditions. Additionally, the atrsmd-2 mutant exhibited more sensitivity to lincomycin, which specifically inhibits the elongation of nascent polypeptide chains. Cold stress exacerbated the effect on chloroplast ribosome biogenesis in the atrsmd-2 mutant. All these data suggest that the AtRsmD protein plays distinct regulatory roles in chloroplast translation, which is required for chloroplast development and chloroplast function.

WHIRLIES Are Multifunctional DNA-Binding Proteins With Impact on Plant Development and Stress Resistance

WHIRLIES are plant-specific proteins binding to DNA in plastids, mitochondria, and nucleus. They have been identified as significant components of nucleoids in the organelles where they regulate the structure of the nucleoids and diverse DNA-associated processes. WHIRLIES also fulfil roles in the nucleus by interacting with telomers and various transcription factors, among them members of the WRKY family. While most plants have two WHIRLY proteins, additional WHIRLY proteins evolved by gene duplication in some dicot families. All WHIRLY proteins share a conserved WHIRLY domain responsible for ssDNA binding. Structural analyses revealed that WHIRLY proteins form tetramers and higher-order complexes upon binding to DNA. An outstanding feature is the parallel localization of WHIRLY proteins in two or three cell compartments. Because they translocate from organelles to the nucleus, WHIRLY proteins are excellent candidates for transducing signals between organelles and nucleus to allow for coordinated activities of the different genomes. Developmental cues and environmental factors control the expression of WHIRLY genes. Mutants and plants with a reduced abundance of WHIRLY proteins gave insight into their multiple functionalities. In chloroplasts, a reduction of the WHIRLY level leads to changes in replication, transcription, RNA processing, and DNA repair. Furthermore, chloroplast development, ribosome formation, and photosynthesis are impaired in monocots. In mitochondria, a low level of WHIRLIES coincides with a reduced number of cristae and a low rate of respiration. The WHIRLY proteins are involved in the plants' resistance toward abiotic and biotic stress. Plants with low levels of WHIRLIES show reduced responsiveness toward diverse environmental factors, such as light and drought. Consequently, because such plants are impaired in acclimation, they accumulate reactive oxygen species under stress conditions. In contrast, several plant species overexpressing WHIRLIES were shown to have a higher resistance toward stress and pathogen attacks. By their multiple interactions with organelle proteins and nuclear transcription factors maybe a comma can be inserted here? and their participation in organelle-nucleus communication, WHIRLY proteins are proposed to serve plant development and stress resistance by coordinating processes at different levels. It is proposed that the multifunctionality of WHIRLY proteins is linked to the plasticity of land plants that develop and function in a continuously changing environment.

Comparative Analysis and Phylogenetic Relationships of Ceriops Species (Rhizophoraceae) and Avicennia lanata (Acanthaceae): Insight into the Chloroplast Genome Evolution between Middle and Seaward Zones of Mangrove Forests

Ceriops and Avicennia are true mangroves in the middle and seaward zones of mangrove forests, respectively. The chloroplast genomes of Ceriops decandra, Ceriops zippeliana, and Ceriops tagal were assembled into lengths of 166,650, 166,083 and 164,432 bp, respectively, whereas Avicennia lanata was 148,264 bp in length. The gene content and gene order are highly conserved among these species. The chloroplast genome contains 125 genes in A. lanata and 129 genes in Ceriops species. Three duplicate genes (rpl2, rpl23, and trnM-CAU) were found in the IR regions of the three Ceriops species, resulting in expansion of the IR regions. The rpl32 gene was lost in C. zippeliana, whereas the infA gene was present in A. lanata. Short repeats (<40 bp) and a lower number of SSRs were found in A. lanata but not in Ceriops species. The phylogenetic analysis supports that all Ceriops species are clustered in Rhizophoraceae and A. lanata is in Acanthaceae. In a search for genes under selective pressures of coastal environments, the rps7 gene was under positive selection compared with non-mangrove species. Finally, two specific primer sets were developed for species identification of the three Ceriops species. Thus, this finding provides insightful genetic information for evolutionary relationships and molecular markers in Ceriops and Avicennia species.

The plastome of Melocactus glaucescens Buining & Brederoo reveals unique evolutionary features and loss of essential tRNA genes

The plastome of Melocactus glaucescens shows unique rearrangements, IR expansion, and unprecedented gene losses in Cactaceae. Our data indicate tRNA import from the cytosol to the plastids in this species. Cactaceae represents one of the richest families in keystone species of arid and semiarid biomes. This family shows various specific features comprehending morphology, anatomy, and metabolism, which allow them to grow under unfavorable environmental conditions. The subfamily Cactoideae contains the most divergence of species, which are highly variable in growth habit and morphology. This subfamily includes the endangered species Melocactus glaucescens (tribe Cereeae), which is a cactus endemic to the biome Caatinga in Brazil. Aiming to analyze the plastid evolution and develop molecular markers, we sequenced and analyzed in detail the plastome of M. glaucescens. Our analyses revealed that the M. glaucescens plastome is the most divergent among the species of the family Cactaceae sequenced so far. We characterized here unique rearrangements, expanded IRs containing an unusual set of genes, and several gene losses. Some genes related to the ndh complex were lost during the plastome evolution, while others have lost their functionality. Additionally, the loss of three tRNA genes (trnA-UGC, trnV-UAC, and trnV-GAC) suggests tRNA import from the cytosol to the plastids in M. glaucescens. Moreover, we identified high gene divergence, several putative positive signatures, and possible unique RNA-editing sites. Furthermore, we mapped 169 SSRs in the plastome of M. glaucescens, which are helpful to access the genetic diversity of natural populations and conservation strategies. Finally, our data provide new insights into the evolution of plastids in Cactaceae, which is an outstanding lineage adapted to extreme environmental conditions and a notorious example of the atypical evolution of plastomes.

Proteomic analysis of a plastid gene encoding RPS4 mutant in Chinese cabbage (Brassica campestris L. ssp. pekinensis)

Plastids are important plant cell organelles containing a genome and bacterial-type 70S ribosomes-primarily composed of plastid ribosomal proteins and ribosomal RNAs. In this study, a chlorophyll-deficient mutant (cdm) obtained from double-haploid Chinese cabbage 'FT' was identified as a plastome mutant with an A-to-C base substitution in the plastid gene encoding the ribosomal protein RPS4. To further elucidate the function and regulatory mechanisms of RPS4, a comparative proteomic analysis was conducted between cdm and its wild-type 'FT' plants by isobaric tags and a relative and absolute quantitation (iTRAQ)-based strategy. A total of 6,245 proteins were identified, 540 of which were differentially abundant proteins (DAPs) in the leaves of cdm as compared to those of 'FT'-including 233 upregulated and 307 downregulated proteins. Upregulated DAPs were mainly involved in translation, organonitrogen compound biosynthetic process, ribosomes, and spliceosomes. Meanwhile, downregulated DAPs were mainly involved in photosynthesis, photosynthetic reaction centres, photosynthetic light harvesting, carbon fixation, and chlorophyll binding. These results indicated an important role of RPS4 in the regulation of growth and development of Chinese cabbage, possibly by regulating plastid translation activity by affecting the expression of specific photosynthesis- and cold stress-related proteins. Moreover, a multiple reaction monitoring (MRM) test and quantitative real-time polymerase chain reaction analysis confirmed our iTRAQ results. Quantitative proteomic analysis allowed us to confirm diverse changes in the metabolic pathways between cdm and 'FT' plants. This work provides new insights into the regulation of chlorophyll biosynthesis and photosynthesis in Chinese cabbage.

Complete chloroplast genome sequences of five Bruguiera species (Rhizophoraceae): comparative analysis and phylogenetic relationships

Bruguiera is a genus of true mangroves that are mostly distributed in the Indo-West Pacific region. However, the number of published whole chloroplast genome sequences of Bruguiera species are limited. Here, the complete chloroplast sequences of five Bruguiera species were sequenced and assembled using Illumina data. The chloroplast genomes of B. gymnorhiza, B. hainesii, B. cylindrica, B. parviflora and B. sexangula were assembled into 161,195, 164,295, 164,297, 163,228 and 164,170 bp, respectively. All chloroplast genomes contain 37 tRNA and eight rRNA genes, with either 84 or 85 protein-coding genes. A comparative analysis of these genomes revealed high similarity in gene structure, gene order and boundary position of the LSC, SSC and two IR regions. Interestingly, B. gymnorhiza lost a rpl32 gene in the SSC region. In addition, a ndhF gene in B. parviflora straddles both the SSC and IRB boundary regions. These genes reveal differences in chloroplast evolution among Bruguiera species. Repeats and SSRs in the chloroplast genome sequences were found to be highly conserved between B. cylindrica and B. hainesii as well as B. gymnorhiza and B. sexangula indicating close genetic relationships based on maternal inheritance. Notably, B. hainesii, which is considered a hybrid between B. gymnorhiza and B. cylindrica, appears to have inherited the chloroplast from B. cylindrica. Investigating the effects of selection events on shared protein-coding genes showed a positive selection in rps7 and rpl36 genes in all species compared to land-plant species. A phylogenetic analysis, based on 59 conserved chloroplast protein-coding genes, showed strong support that all Bruguiera species are in the clade Rhizophoraceae. This study provides valuable genetic information for the study of evolutionary relationships and population genetics in Bruguiera and other mangrove species.

Comparative Chloroplast Genomes of Zosteraceae Species Provide Adaptive Evolution Insights Into Seagrass

Seagrasses are marine flowering plants found in tropical and sub-tropical areas that live in coastal regions between the sea and land. All seagrass species evolved from terrestrial monocotyledons, providing the opportunity to study plant adaptation to sea environments. Here, we sequenced the chloroplast genomes (cpGenomes) of three Zostera species, then analyzed and compared their cpGenome structures and sequence variations. We also performed a phylogenetic analysis using published seagrass chloroplasts and calculated the selection pressure of 17 species within seagrasses and nine terrestrial monocotyledons, as well as estimated the number of shared genes of eight seagrasses. The cpGenomes of Zosteraceae species ranged in size from 143,877 bp (Zostera marina) to 152,726 bp (Phyllospadix iwatensis), which were conserved and displayed similar structures and gene orders. Additionally, we found 17 variable hotspot regions as candidate DNA barcodes for Zosteraceae species, which will be helpful for studying the phylogenetic relationships and interspecies differences between seagrass species. Interestingly, nine genes had positive selection sites, including two ATP subunit genes (atpA and atpF), two ribosome subunit genes (rps4 and rpl20), two DNA-dependent RNA polymerase genes (rpoC1 and rpoC2), as well as accD, clpP, and ycf2. These gene regions may have played key roles in the seagrass adaptation to diverse environments. The Branch model analysis showed that seagrasses had a higher rate of evolution than terrestrial monocotyledons, suggesting that seagrasses experienced greater environmental pressure. Moreover, a branch-site model identified positively selected sites (PSSs) in ccsA, suggesting their involvement in the adaptation to sea environments. These findings are valuable for further investigations on Zosteraceae cpGenomes and will serve as an excellent resource for future studies on seagrass adaptation to sea environments.

A Repertory of Rearrangements and the Loss of an Inverted Repeat Region in Passiflora Chloroplast Genomes

Chloroplast genomes (cpDNA) in angiosperms are usually highly conserved. Although rearrangements have been observed in some lineages, such as Passiflora, the mechanisms that lead to rearrangements are still poorly elucidated. In the present study, we obtained 20 new chloroplast genomes (18 species from the genus Passiflora, and Dilkea retusa and Mitostemma brevifilis from the family Passifloraceae) in order to investigate cpDNA evolutionary history in this group. Passiflora cpDNAs vary in size considerably, with ∼50 kb between shortest and longest. Large inverted repeat (IR) expansions were identified, and at the extreme opposite, the loss of an IR was detected for the first time in Passiflora, a rare event in angiosperms. The loss of an IR region was detected in Passiflora capsularis and Passiflora costaricensis, a species in which occasional biparental chloroplast inheritance has previously been reported. A repertory of rearrangements such as inversions and gene losses were detected, making Passiflora one of the few groups with complex chloroplast genome evolution. We also performed a phylogenomic study based on all the available cp genomes and our analysis implies that there is a need to reconsider the taxonomic classifications of some species in the group.

An Insight Into the Mechanism of Plant Organelle Genome Maintenance and Implications of Organelle Genome in Crop Improvement: An Update

Besides the nuclear genome, plants possess two small extra chromosomal genomes in mitochondria and chloroplast, respectively, which contribute a small fraction of the organelles’ proteome. Both mitochondrial and chloroplast DNA have originated endosymbiotically and most of their prokaryotic genes were either lost or transferred to the nuclear genome through endosymbiotic gene transfer during the course of evolution. Due to their immobile nature, plant nuclear and organellar genomes face continuous threat from diverse exogenous agents as well as some reactive by-products or intermediates released from various endogenous metabolic pathways. These factors eventually affect the overall plant growth and development and finally productivity. The detailed mechanism of DNA damage response and repair following accumulation of various forms of DNA lesions, including single and double-strand breaks (SSBs and DSBs) have been well documented for the nuclear genome and now it has been extended to the organelles also. Recently, it has been shown that both mitochondria and chloroplast possess a counterpart of most of the nuclear DNA damage repair pathways and share remarkable similarities with different damage repair proteins present in the nucleus. Among various repair pathways, homologous recombination (HR) is crucial for the repair as well as the evolution of organellar genomes. Along with the repair pathways, various other factors, such as the MSH1 and WHIRLY family proteins, WHY1, WHY2, and WHY3 are also known to be involved in maintaining low mutation rates and structural integrity of mitochondrial and chloroplast genome. SOG1, the central regulator in DNA damage response in plants, has also been found to mediate endoreduplication and cell-cycle progression through chloroplast to nucleus retrograde signaling in response to chloroplast genome instability. Various proteins associated with the maintenance of genome stability are targeted to both nuclear and organellar compartments, establishing communication between organelles as well as organelles and nucleus. Therefore, understanding the mechanism of DNA damage repair and inter compartmental crosstalk mechanism in various sub-cellular organelles following induction of DNA damage and identification of key components of such signaling cascades may eventually be translated into strategies for crop improvement under abiotic and genotoxic stress conditions. This review mainly highlights the current understanding as well as the importance of different aspects of organelle genome maintenance mechanisms in higher plants.

Highly degenerate plastomes in two hemiparasitic dwarf mistletoes: Arceuthobium chinense and A. pini (Viscaceae)

The leafless and endophytic habitat may significantly relax the selection pressure on photosynthesis, and plastid transcription and translation, causing the loss/pseudogenization of several essential plastid-encoding genes in dwarf mistletoes. Dwarf mistletoes (Arceuthobium spp., Viscaceae) are the most destructive plant parasites to numerous conifer species worldwide. In this study, the plastid genomes (plastomes) of Arceuthobium chinense Lecomte and A. pini Hawksworth and Wiens were sequenced and characterized. Although dwarf mistletoes are hemiparasites capable of photosynthesis, their plastomes were highly degenerated, as indicated by the smallest plastome size, the lowest GC content, and relatively very few intact genes among the Santalales hemiparasites. Unexpectedly, several essential housekeeping genes (rpoA, rpoB, rpoC1, and rpoC2) and some core photosynthetic genes (psbZ and petL), as well as the rpl33 gene, that is indispensable for plants under stress conditions, were deleted or pseudogenized in the Arceuthobium plastomes. Our data suggest that the leafless and endophytic habit, which heavily relies on the coniferous hosts for nutrients and carbon requirement, may largely relax the selection pressure on photosynthesis, as well as plastid transcription and translation, thus resulting in the loss/pseudogenization of such essential plastid-encoding genes in dwarf mistletoes. Therefore, the higher level of plastome degradation in Arceuthobium species than other Santalales hemiparasites is likely correlated with the evolution of leafless and endophytic habit. A higher degree of plastome degradation in Arceuthobium. These findings provide new insights into the plastome degeneration associated with parasitism in Santalales and deepen our understanding of the biology of dwarf mistletoes.

Acclimation in plants - the Green Hub consortium

Acclimation is the capacity to adapt to environmental changes within the lifetime of an individual. This ability allows plants to cope with the continuous variation in ambient conditions to which they are exposed as sessile organisms. Because environmental changes and extremes are becoming even more pronounced due to the current period of climate change, enhancing the efficacy of plant acclimation is a promising strategy for mitigating the consequences of global warming on crop yields. At the cellular level, the chloroplast plays a central role in many acclimation responses, acting both as a sensor of environmental change and as a target of cellular acclimation responses. In this Perspective article, we outline the activities of the Green Hub consortium funded by the German Science Foundation. The main aim of this research collaboration is to understand and strategically modify the cellular networks that mediate plant acclimation to adverse environments, employing Arabidopsis, tobacco (Nicotiana tabacum) and Chlamydomonas as model organisms. These efforts will contribute to 'smart breeding' methods designed to create crop plants with improved acclimation properties. To this end, the model oilseed crop Camelina sativa is being used to test modulators of acclimation for their potential to enhance crop yield under adverse environmental conditions. Here we highlight the current state of research on the role of gene expression, metabolism and signalling in acclimation, with a focus on chloroplast-related processes. In addition, further approaches to uncovering acclimation mechanisms derived from systems and computational biology, as well as adaptive laboratory evolution with photosynthetic microbes, are highlighted.

Advancing organelle genome transformation and editing for crop improvement

Plant cells contain three organelles that harbor DNA: the nucleus, plastids, and mitochondria. Plastid transformation has emerged as an attractive platform for the generation of transgenic plants, also referred to as transplastomic plants. Plastid genomes have been genetically engineered to improve crop yield, nutritional quality, and resistance to abiotic and biotic stresses, as well as for recombinant protein production. Despite many promising proof-of-concept applications, transplastomic plants have not been commercialized to date. Sequence-specific nuclease technologies are widely used to precisely modify nuclear genomes, but these tools have not been applied to edit organelle genomes because the efficient homologous recombination system in plastids facilitates plastid genome editing. Unlike plastid transformation, successful genetic transformation of higher plant mitochondrial genome transformation was tested in several research group, but not successful to date. However, stepwise progress has been made in modifying mitochondrial genes and their transcripts, thus enabling the study of their functions. Here, we provide an overview of advances in organelle transformation and genome editing for crop improvement, and we discuss the bottlenecks and future development of these technologies.

Deconvoluting apocarotenoid-mediated retrograde signaling networks regulating plastid translation and leaf development

Signals originating within plastids modulate organelle differentiation by transcriptionally regulating nuclear-encoded genes. These retrograde signals are also integral regulators of plant development, including leaf morphology. The clb5 mutant displays severe leaf morphology defects due to Apocarotenoid Signal 1 (ACS1) accumulation in the developmentally arrested plastid. Transcriptomic analysis of clb5 validates that ACS1 accumulation deregulates hundreds of nuclear genes, including the suppression of most genes encoding plastid ribosomal proteins. Herein, we order the molecular events causing the leaf phenotype associated with the accumulation of ACS1, which includes two consecutive retrograde signaling cascades. Firstly, ACS1 originating in the plastid drives inhibition of plastid translation (IPT) via nuclear transcriptome remodeling of chlororibosomal proteins, requiring light as an essential component. Subsequently, IPT results in leaf morphological defects via a GUN1-dependent pathway shared with seedlings undergoing chemical IPT treatments and is restricted to an early window of the leaf development. Collectively, this work advances our understanding of the complexity within plastid retrograde signaling exemplified by sequential signal exchange and consequences that in a particular temporal and spatial context contribute to the modulation of leaf development.

Plastid Genomes of Flowering Plants: Essential Principles

The plastid genome (plastome ) has proved a valuable source of data for evaluating evolutionary relationships among angiosperms. Through basic and applied approaches, plastid transformation technology offers the potential to understand and improve plant productivity, providing food, fiber, energy, and medicines to meet the needs of a burgeoning global population. The growing genomic resources available to both phylogenetic and biotechnological investigations is allowing novel insights and expanding the scope of plastome research to encompass new species. In this chapter, we present an overview of some of the seminal and contemporary research that has contributed to our current understanding of plastome evolution and attempt to highlight the relationship between evolutionary mechanisms and the tools of plastid genetic engineering.

The PPR-SMR Protein ATP4 Is Required for Editing the Chloroplast rps8 mRNA in Rice and Maize

Chloroplast gene expression involves the participation of hundreds of pentatricopeptide repeat (PPR) RNA binding proteins, and proteins in the PLS subfamily typically specify sites of RNA editing, whereas those in the P-subfamily typically stabilize RNA, activate translation, or promote intron splicing. Several P-type PPR proteins include a small MutS-related (SMR) domain, but the biochemical contribution of the SMR domain remains enigmatic. Here, we describe a rice (Oryza sativa) mutant, osatp4, lacking the ortholog of ATP4, a PPR-SMR protein in maize (Zea mays). osatp4 mutants were chlorotic and had a plastid-ribosome deficiency when grown in the cold. Like maize ATP4, OsATP4 was required for the accumulation of dicistronic rpl16-rpl14 transcripts. Surprisingly, OsATP4 was also required for the editing of a specific nucleotide in the ribosomal protein S8 transcripts, rps8, and this function was conserved in maize. By contrast, rps8 RNA was edited normally in the maize PROTON gradient regulation3 mutant, pgr3, which also lacks rpl16-rpl14 transcripts, indicating that the editing defect in atp4 mutants is not a secondary effect of altered rpl16-rpl14 RNA metabolism. Expression of the edited rps8 isoform in transgenic osatp4 mutants complemented the cold-sensitive phenotype, indicating that a rps8 expression defect accounts for the cold-sensitivity. We suggest that ATP4 stimulates rps8 editing by facilitating access of a previously characterized PLS-type RNA editing factor to its cognate cis-element upstream of the edited nucleotide.

Analysis of six chloroplast genomes provides insight into the evolution of Chrysosplenium (Saxifragaceae)

BACKGROUND

Chrysosplenium L. (Saxifragaceae) is a genus of plants widely distributed in Northern Hemisphere and usually found in moist, shaded valleys and mountain slopes. This genus is ideal for studying plant adaptation to low light conditions. Although some progress has been made in the systematics and biogeography of Chrysosplenium, its chloroplast genome evolution remains to be investigated.

RESULTS

To fill this gap, we sequenced the chloroplast genomes of six Chrysosplenium species and analyzed their genome structure, GC content, and nucleotide diversity. Moreover, we performed a phylogenetic analysis and calculated non-synonymous (Ka) /synonymous (Ks) substitution ratios using the combined protein-coding genes of 29 species within Saxifragales and two additional species as outgroups, as well as a pair-wise estimation for each gene within Chrysosplenium. Compared with the outgroups in Saxifragaceae, the six Chrysosplenium chloroplast genomes had lower GC contents; they also had conserved boundary regions and gene contents, as only the rpl32 gene was lost in four of the Chrysosplenium chloroplast genomes. Phylogenetic analyses suggested that the Chrysosplenium separated to two major clades (the opposite group and the alternate group). The selection pressure estimation (Ka/Ks ratios) of genes in the Chrysosplenium species showed that matK and ycf2 were subjected to positive selection.

CONCLUSION

This study provides genetic resources for exploring the phylogeny of Chrysosplenium and sheds light on plant adaptation to low light conditions. The lower average GC content and the lacking gene of rpl32 indicated selective pressure in their unique habitats. Different from results previously reported, our selective pressure estimation suggested that the genes related to photosynthesis (such as ycf2) were under positive selection at sites in the coding region.

The Role of Chloroplast Gene Expression in Plant Responses to Environmental Stress

Chloroplasts are plant organelles that carry out photosynthesis, produce various metabolites, and sense changes in the external environment. Given their endosymbiotic origin, chloroplasts have retained independent genomes and gene-expression machinery. Most genes from the prokaryotic ancestors of chloroplasts were transferred into the nucleus over the course of evolution. However, the importance of chloroplast gene expression in environmental stress responses have recently become more apparent. Here, we discuss the emerging roles of the distinct chloroplast gene expression processes in plant responses to environmental stresses. For example, the transcription and translation of psbA play an important role in high-light stress responses. A better understanding of the connection between chloroplast gene expression and environmental stress responses is crucial for breeding stress-tolerant crops better able to cope with the rapidly changing environment.

Phylogenetic and evolutionary features of the plastome of Tropaeolum pentaphyllum Lam. (Tropaeolaceae)

Complete plastome sequence of Tropaeolum pentaphyllum revealed molecular markers, hotspots of nucleotide polymorphism, RNA editing sites and phylogenetic aspects Tropaeolaceae Juss. ex DC. comprises approximately 95 species across North and South Americas. Tropaeolum pentaphyllum Lam. is an unconventional and endangered species with occurrence in some countries of South America. Although this species presents nutritional, medicinal and ornamental uses, genetic studies involving natural populations or promising genotypes are practically non-existent. Here, we report the nucleotide sequence of T. pentaphyllum plastome. It represents the first complete plastome sequence of the family Tropaeolaceae to be fully sequenced and analyzed in detail. The sequencing data revealed that the T. pentaphyllum plastome is highly similar to the plastomes of other Brassicales. Notwithstanding, our analyses detected some specific features concerning events of IR expansion and structural changes in some genes such as matK, rpoA, and rpoC2. We also detected 251 SSR loci, nine hotspots of nucleotide polymorphism, and two specific RNA editing sites in the plastome of T. pentaphyllum. Moreover, plastid phylogenomic inference indicated a closed relationship between the families Tropaeolaceae and Akaniaceae, which formed a sister group to Moringaceae-Caricaceae. Finally, our data bring new molecular markers and evolutionary features to be applied in the natural population, germplasm collection, and genotype selection aiming conservation, genetic diversity evaluation, and exploitation of this endangered species.

Comparative Plastome Analysis of Root- and Stem-Feeding Parasites of Santalales Untangle the Footprints of Feeding Mode and Lifestyle Transitions

In plants, parasitism triggers the reductive evolution of plastid genomes (plastomes). To disentangle the molecular evolutionary associations between feeding on other plants below- or aboveground and general transitions from facultative to obligate parasitism, we analyzed 34 complete plastomes of autotrophic, root- and stem-feeding hemiparasitic, and holoparasitic Santalales. We observed inexplicable losses of housekeeping genes and tRNAs in hemiparasites and dramatic genomic reconfiguration in holoparasitic Balanophoraceae, whose plastomes have exceptionally low GC contents. Genomic changes are related primarily to the evolution of hemi- or holoparasitism, whereas the transition from a root- to a stem-feeding mode plays no major role. In contrast, the rate of molecular evolution accelerates in a stepwise manner from autotrophs to root- and then stem-feeding parasites. Already the ancestral transition to root-parasitism coincides with a relaxation of selection in plastomes. Another significant selectional shift in plastid genes occurs as stem-feeders evolve, suggesting that this derived form coincides with trophic specialization despite the retention of photosynthetic capacity. Parasitic Santalales fill a gap in our understanding of parasitism-associated plastome degeneration. We reveal that lifestyle-genome associations unfold interdependently over trophic specialization and feeding mode transitions, where holoparasitic Balanophoraceae provide a system for exploring the functional realms of plastomes.

The critical function of the plastid rRNA methyltransferase, CMAL, in ribosome biogenesis and plant development

Methylation of nucleotides in ribosomal RNAs (rRNAs) is a ubiquitous feature that occurs in all living organisms. The formation of methylated nucleotides is performed by a variety of RNA-methyltransferases. Chloroplasts of plant cells result from an endosymbiotic event and possess their own genome and ribosomes. However, enzymes responsible for rRNA methylation and the function of modified nucleotides in chloroplasts remain to be determined. Here, we identified an rRNA methyltransferase, CMAL (Chloroplast MraW-Like), in the Arabidopsis chloroplast and investigated its function. CMAL is the Arabidopsis ortholog of bacterial MraW/ RsmH proteins and accounts to the N4-methylation of C1352 in chloroplast 16S rRNA, indicating that CMAL orthologs and this methyl-modification nucleotide is conserved between bacteria and the endosymbiont-derived eukaryotic organelle. The knockout of CMAL in Arabidopsis impairs the chloroplast ribosome accumulation and accordingly reduced the efficiency of mRNA translation. Interestingly, the loss of CMAL leads not only to defects in chloroplast function, but also to abnormal leaf and root development and overall plant morphology. Further investigation showed that CMAL is involved in the plant development probably by modulating auxin derived signaling pathways. This study uncovered the important role of 16S rRNA methylation mediated by CMAL in chloroplast ribosome biogenesis and plant development.

Complete Chloroplast Genomes of 14 Mangroves: Phylogenetic and Comparative Genomic Analyses

Mangroves are a group of plant species that occupy the coastal intertidal zone and are major components of this ecologically important ecosystem. Mangroves belong to about twenty diverse families. Here, we sequenced and assembled chloroplast genomes of 14 mangrove species from eight families spanning five rosid orders and one asterid order: Fabales (Pongamia pinnata), Lamiales (Avicennia marina), Malpighiales (Excoecaria agallocha, Bruguiera sexangula, Kandelia obovata, Rhizophora stylosa, and Ceriops tagal), Malvales (Hibiscus tiliaceus, Heritiera littoralis, and Thespesia populnea), Myrtales (Laguncularia racemosa, Sonneratia ovata, and Pemphis acidula), and Sapindales (Xylocarpus moluccensis). These chloroplast genomes range from 149 kb to 168 kb in length. A conserved structure of two inverted repeats (IRa and IRb, ~25.8 kb), one large single-copy region (LSC, ~89.0 kb), and one short single-copy region (SSC, ~18.9 kb) as well as ~130 genes (85 protein-coding, 37 tRNAs, and 8 rRNAs) was observed. We found the lowest divergence in the IR regions among the four regions. We also identified simple sequence repeats (SSRs), which were found to be variable in numbers. Most chloroplast genes are highly conserved, with only four genes under positive selection or relaxed pressure. Combined with publicly available chloroplast genomes, we carried out phylogenetic analysis and confirmed the previously reported phylogeny within rosids, including the positioning of obscure families in Malpighiales. Our study reports 14 mangrove chloroplast genomes and illustrates their genome features and evolution.

The Functions of Chloroplast Glutamyl-tRNA in Translation and Tetrapyrrole Biosynthesis

The chloroplast glutamyl-tRNA (tRNA^Glu) is unique in that it has two entirely different functions. In addition to acting in translation, it serves as the substrate of glutamyl-tRNA reductase (GluTR), the enzyme catalyzing the committed step in the tetrapyrrole biosynthetic pathway. How the tRNA^Glu pool is distributed between the two pathways and whether tRNA^Glu allocation limits tetrapyrrole biosynthesis and/or protein biosynthesis remains poorly understood. We generated a series of transplastomic tobacco (Nicotiana tabacum) plants to alter tRNA^Glu expression levels and introduced a point mutation into the plastid trnE gene, which has been reported to uncouple protein biosynthesis from tetrapyrrole biosynthesis in chloroplasts of the protist Euglena gracilis We show that, rather than comparable uncoupling of the two pathways, the trnE mutation is lethal in tobacco because it inhibits tRNA processing, thus preventing translation of Glu codons. Ectopic expression of the mutated trnE gene uncovered an unexpected inhibition of glutamyl-tRNA reductase by immature tRNA^Glu We further demonstrate that whereas overexpression of tRNA^Glu does not affect tetrapyrrole biosynthesis, reduction of GluTR activity through inhibition by tRNA^Glu precursors causes tetrapyrrole synthesis to become limiting in early plant development when active photosystem biogenesis provokes a high demand for de novo chlorophyll biosynthesis. Taken together, our findings provide insight into the roles of tRNA^Glu at the intersection of protein biosynthesis and tetrapyrrole biosynthesis.

Severe Plastid Genome Size Reduction in a Mycoheterotrophic Orchid, Danxiaorchis singchiana, Reveals Heavy Gene Loss and Gene Relocations

Danxiaorchis singchiana (Orchidaceae) is a leafless mycoheterotrophic orchid in the subfamily Epidendroideae. We sequenced the complete plastome of D. singchiana. The plastome has a reduced size of 87,931 bp, which includes a pair of inverted repeat (IR) regions of 13,762 bp each that are separated by a large single copy (LSC) region of 42,575 bp and a small single copy (SSC) region of 17,831 bp. When compared to its sister taxa, Cremastra appendiculata and Corallorhiza striata var. involuta, D. singchiana showed an inverted gene block in the LSC and SSC regions. A total of 61 genes were predicted, including 21 tRNA, 4 rRNA, and 36 protein-coding genes. While most of the housekeeping genes were still intact and seem to be protein-coding, only four photosynthesis-related genes appeared presumably intact. The majority of the presumably intact protein-coding genes seem to have undergone purifying selection (dN/dS < 1), and only the psaC gene was positively selected (dN/dS > 1) when compared to that in Cr. appendiculata. Phylogenetic analysis of 26 complete plastome sequences from 24 species of the tribe Epidendreae had revealed that D. singchiana diverged after Cr. appendiculata and is sister to the genus Corallorhiza with strong bootstrap support (100%).

Chloroplast genome of an extremely endangered conifer Thuja sutchuenensis Franch.: gene organization, comparative and phylogenetic analysis

Thuja sutchuenensis is a critically endangered tertiary relict species of Cupressaceae from southwestern China. We sequenced the complete chloroplast (cp) genome of T. sutchuenensis, showing the genome content of 129,776 bp, 118 unique genes including 82 unique protein-coding genes, 32 tRNA genes, and 4 rRNA genes. The genome structures, gene order, and GC content are similar to other typical gymnosperm cp genomes. Thirty-eight simple sequence repeats were identified in the T. sutchuenensis cp genome. We also found an apparent inversion between trnT and psbK between genera Thuja and Thujopsis. In addition, positive selection signals were detected in seven genes with high Ka/Ks ratios. The reconstructed phylogeny based on locally collinear blocks of cp genomes among 21 gymnosperms species is similar to previous inferences. We also inferred a Late-Miocene divergence between T. sutchuenensis and T. standishii, according to the dating of ~ 11.05 Mya by cp genomes. These results will be helpful for future studies of Cupressaceae phylogeny as well as studies in population genetics, systematics, and cp genetic engineering.

The plastomes of Astrocaryum aculeatum G. Mey. and A. murumuru Mart. show a flip-flop recombination between two short inverted repeats

The plastomes of Astrocaryum murumuru and A. aculeatum revealed a lineage-specific structural feature originated by flip-flop recombination, non-synonymous substitutions in conserved genes and several molecular markers. Astrocaryum murumuru Mart. and A. aculeatum G.Mey. are two palm species of Amazon forest that are economically important as source of food, oil and raw material for several applications. Genetic studies aiming to establish strategies for conservation and domestication of both species are still in the beginning given that the exploitation is mostly by extractive activity. The identification and characterization of molecular markers are essential to assess the genetic diversity of natural populations of both species. Therefore, we sequenced and characterized in detail the plastome of both species. We compared both species and identified 32 polymorphic SSR loci, 150 SNPs, 46 indels and eight hotspots of nucleotide diversity. Additionally, we reported a specific RNA editing site found in the ccsA gene, which is exclusive to A. murumuru. Moreover, the structural analysis in the plastomes of both species revealed a 4.6-kb inversion encompassing a set of genes involved in chlororespiration and plastid translation. This 4.6-kb inversion is a lineage-specific structural feature of the genus Astrocaryum originated by flip-flop recombination between two short inverted repeats. Furthermore, our phylogenetic analysis using whole plastomes of 39 Arecaceae species placed the Astrocaryum species sister to Acrocomia within the tribe Cocoseae. Finally, our data indicated substantial changes in the plastome structure and sequence of both species of the genus Astrocaryum, bringing new molecular markers, several structural and evolving features, which can be applied in several areas such as genetic, evolution, breeding, phylogeny and conservation strategies for both species.

The rpl23 gene and pseudogene are hotspots of illegitimate recombination in barley chloroplast mutator seedlings

Previously, through a TILLING (Targeting Induced Local Lesions in Genomes) approach applied on barley chloroplast mutator (cpm) seedlings a high frequency of polymorphisms in the rpl23 gene was detected. All the polymorphisms corresponded to five differences already known to exist in nature between the rpl23 gene located in the inverted repeats (IRs) and the rpl23 pseudogene located in the large single copy region (LSC). In this investigation, polymorphisms in the rpl23 gene were verified and besides, a similar situation was found for the pseudogene in cpm seedlings. On the other hand, no polymorphisms were found in any of those loci in 40 wild type barley seedlings. Those facts and the independent occurrence of polymorphisms in the gene and pseudogene in individual seedlings suggest that the detected polymorphisms initially arose from gene conversion between gene and pseudogene. Moreover, an additional recombination process involving small recombinant segments seems to occur between the two gene copies as a consequence of their location in the IRs. These and previous results support the hypothesis that the CPM protein is a component of the plastome mismatch repair (MMR) system, whose failure of the anti-recombination activity results in increased illegitimate recombination between the rpl23 gene and pseudogene.

Highly accelerated rates of genomic rearrangements and nucleotide substitutions in plastid genomes of Passiflora subgenus Decaloba

Plastid genomes (plastomes) of photosynthetic angiosperms are for the most part highly conserved in their organization, mode of inheritance and rates of nucleotide substitution. A small number of distantly related lineages share a syndrome of features that deviate from this general pattern, including extensive genomic rearrangements, accelerated rates of nucleotide substitution, biparental inheritance and plastome-genome incompatibility. Previous studies of plastomes in Passiflora with limited taxon sampling suggested that the genus exhibits this syndrome. To examine this phenomenon further, 15 new plastomes from Passiflora were sequenced and combined with previously published data to examine the phylogenetic relationships, genome organization and evolutionary rates across all five subgenera and the sister genus Adenia. Phylogenomic analyses using 68 protein-coding genes shared by Passiflora generated a fully resolved and strongly supported tree that is congruent with previous phylogenies based on a few plastid and nuclear loci. This phylogeny was used to examine the distribution of plastome rearrangements across Passiflora. Multiple gene and intron losses and inversions were identified in Passiflora with some occurring in parallel and others that extended across the Passifloraceae. Furthermore, extensive expansions and contractions of the inverted repeat (IR) were uncovered and in some cases this resulted in exclusion of all ribosomal RNA genes from the IR. The most highly rearranged lineage was subgenus Decaloba, which experienced extensive IR expansion that incorporated up to 25 protein-coding genes usually located in large single copy region. Nucleotide substitution rate analyses of 68 protein-coding genes across the genus showed lineage- and locus-specific acceleration. Significant increase in dS, dN and dN/dS was detected for clpP across the genus and for ycf4 in certain lineages. Significant increases in dN and dN/dS for ribosomal subunits and plastid-encoded RNA polymerase genes were detected in the branch leading to the expanded IR-clade in subgenus Decaloba. This subgenus displays the syndrome of unusual features, making it an ideal system to investigate the dynamic evolution of angiosperm plastomes.

The UAP56-Interacting Export Factors UIEF1 and UIEF2 Function in mRNA Export

In eukaryotes, the regulated transport of mRNAs from the nucleus to the cytosol through nuclear pore complexes represents an important step in the expression of protein-coding genes. In plants, the mechanism of nucleocytosolic mRNA transport and the factors involved are poorly understood. The Arabidopsis (Arabidopsis thaliana) genome encodes two likely orthologs of UAP56-interacting factor, which acts as mRNA export factor in mammalian cells. In yeast and plant cells, both proteins interact directly with the mRNA export-related RNA helicase UAP56 and the interaction was mediated by an N-terminal UAP56-binding motif. Accordingly, the two proteins were termed UAP56-INTERACTING EXPORT FACTOR1 and 2 (UIEF1/2). Despite lacking a known RNA-binding motif, recombinant UIEF1 interacted with RNA, and the C-terminal part of UIEF1 mainly contributed to the RNA interaction. Mutation of UIEF1, UIEF2, or both in the double-mutant 2xuief caused modest growth defects. A cross between the 2xuief and 4xaly (defective in the four ALY1-4 mRNA export factors) mutants produced the sextuple mutant 4xaly 2xuief, which displayed more severe growth impairment than the 4xaly plants. Developmental defects including delayed bolting and reduced seed set were observed in the 4xaly but not the 2xuief plants. Analysis of the cellular distribution of polyadenylated mRNAs revealed more pronounced nuclear mRNA accumulation in 4xaly 2xuief than in 2xuief and 4xaly cells. In conclusion, the results indicate that UIEF1 and UIEF2 act as mRNA export factors in plants and that they cooperate with ALY1-ALY4 to mediate efficient nucleocytosolic mRNA transport.

The Role of Plastidic Trigger Factor Serving Protein Biogenesis in Green Algae and Land Plants

Biochemical processes in chloroplasts are important for virtually all life forms. Tight regulation of protein homeostasis and the coordinated assembly of protein complexes, composed of both imported and locally synthesized subunits, are vital to plastid functionality. Protein biogenesis requires the action of cotranslationally acting molecular chaperones. One such chaperone is trigger factor (TF), which is known to cotranslationally bind most newly synthesized proteins in bacteria, thereby assisting their correct folding and maturation. However, how these processes are regulated in chloroplasts remains poorly understood. We report here functional investigation of chloroplast-localized TF (TIG1) in the green alga (Chlamydomonas reinhardtii) and the vascular land plant Arabidopsis (Arabidopsis thaliana). We show that chloroplastic TIG1 evolved as a specialized chaperone. Unlike other plastidic chaperones that are functionally interchangeable with their prokaryotic counterpart, TIG1 was not able to complement the broadly acting ortholog in Escherichia coli. Whereas general chaperone properties such as the prevention of aggregates or substrate recognition seems to be conserved between bacterial and plastidic TFs, plant TIG1s differed by associating with only a relatively small population of translating ribosomes. Furthermore, a reduction of plastidic TIG1 levels leads to deregulated protein biogenesis at the expense of increased translation, thereby disrupting the chloroplast energy household. This suggests a central role of TIG1 in protein biogenesis in the chloroplast.

Genetic, evolutionary and phylogenetic aspects of the plastome of annatto (Bixa orellana L.), the Amazonian commercial species of natural dyes

The plastome of B. orellana reveals specific evolutionary features, unique RNA editing sites, molecular markers and the position of Bixaceae within Malvales. Annatto (Bixa orellana L.) is a native species of tropical Americas with center of origin in Brazilian Amazonia. Its seeds accumulate the apocarotenoids, bixin and norbixin, which are only found in high content in this species. The seeds of B. orellana are commercially valued by the food industry because its dyes replace synthetic ones from the market due to potential carcinogenic risks. The increasing consumption of B. orellana seeds for dye extraction makes necessary the increase of productivity, which is possible accessing the genetic basis and searching for elite genotypes. The identification and characterization of molecular markers are essential to analyse the genetic diversity of natural populations and to establish suitable strategies for conservation, domestication, germplasm characterization and genetic breeding. Therefore, we sequenced and characterized in detail the plastome of B. orellana. The plastome of B. orellana is a circular DNA molecule of 159,708 bp with a typical quadripartite structure and 112 unique genes. Additionally, a total of 312 SSR loci were identified in the plastome of B. orellana. Moreover, we predicted in 23 genes a total of 57 RNA-editing sites of which 11 are unique for B. orellana. Furthermore, our plastid phylogenomic analyses, using the plastome sequences available in the plastid database belonging to species of order Malvales, indicate a closed relationship between Bixaceae and Malvaceae, which formed a sister group to Thymelaeaceae. Finally, our study provided useful data to be employed in several genetic and biotechnological approaches in B. orellana and related species of the family Bixaceae.