Arabidopsis ANGULATA10 is required for thylakoid biogenesis and mesophyll development

CXIP4 depletion causes early lethality and pre-mRNA missplicing in Arabidopsis

Zinc knuckle (ZCCHC) motif-containing proteins are present in unicellular and multicellular eukaryotes and most ZCCHC proteins with known functions participate in the metabolism of various classes of RNA, such as mRNAs, ribosomal RNAs, and microRNAs. The Arabidopsis (Arabidopsis thaliana) genome encodes 69 ZCCHC-containing proteins, but the functions of most remain unclear. One of these proteins is CAX-INTERACTING PROTEIN 4 (CXIP4), which has been classified as a PTHR31437 family member, along with human SREK1-interacting protein 1 (SREK1IP1), which is thought to function in pre-mRNA splicing and RNA methylation. Metazoan SREK1IP1-like and plant CXIP4-like proteins only share a ZCCHC motif, and their functions remain almost entirely unknown. We studied two loss-of-function alleles of Arabidopsis CXIP4, the first mutations in PTHR31437 family genes described to date: cxip4-1 is likely null and shows early lethality, and cxip4-2 is hypomorphic and viable, with pleiotropic morphological defects. The cxip4-2 mutant exhibited deregulation of defense genes and upregulation of transcription factor encoding genes, some of which might explain its developmental defects. This mutant also exhibited increased intron retention events, and the specific functions of misspliced genes, such as those involved in "gene silencing by DNA methylation" and "mRNA polyadenylation factor" suggest that CXIP4 has additional functions. The CXIP4 protein localizes to the nucleus in a pattern resembling nuclear speckles, which are rich in splicing factors. Therefore, CXIP4 is required for plant survival and proper development, and mRNA maturation.

Transcriptomic analyses reveal variegation-induced metabolic changes leading to high L-theanine levels in albino sectors of variegated tea (Camellia sinensis)

Camellia sinensis cv. 'Yanling Huayecha' (YHC) is an albino-green chimaeric tea mutant with stable genetic traits. Here, we analysed the cell ultrastructure, photosynthetic pigments, amino acids, and transcriptomes of the albino, mosaic, and green zones of YHC. Well-organized thylakoids were found in chloroplasts in mesophyll cells of the green zone but not the albino zone. The albino zone of the leaves contained almost no photosynthetic pigment. However, the levels of total amino acids and theanine were higher in the albino zone than in the mosaic and green zones. A transcriptomic analysis showed that carbon metabolism, nitrogen metabolism and amino acid biosynthesis showed differences among the different zones. Metabolite and transcriptomic analyses revealed that (1) downregulation of CsPPOX1 and damage to thylakoids in the albino zone may block chlorophyll synthesis; (2) downregulation of CsLHCB6, CsFdC2 and CsSCY1 influences chloroplast biogenesis and thylakoid membrane formation, which may contribute to the appearance of variegated tea leaves; and (3) tea plant variegation disrupts the balance between carbon and nitrogen metabolism and promotes the accumulation of amino acids, and upregulation of CsTSⅠ and CsAlaDC may enhance L-theanine synthesis. In summary, our study provides a theoretical basis and valuable insights for elucidating the molecular mechanisms and promoting the economic utilization of variegation in tea.

Members of the DEAL subfamily of the DUF1218 gene family are required for bilateral symmetry but not for dorsoventrality in Arabidopsis leaves

Most plant leaves exhibit bilateral symmetry, which has been hypothesized as an inevitable consequence of the existence of the proximodistal and dorsoventral axes. No gene has been described that affects leaf bilateral symmetry but not dorsoventrality in Arabidopsis thaliana. We screened for viable insertional mutations that affect leaf morphology, and out of more than 700 mutants found only one, desigual1-1 (deal1-1), that exhibited bilateral symmetry breaking but no obvious defects in dorsoventrality. We found that deal1-1 is an allele of VASCULATURE COMPLEXITY AND CONNECTIVITY (VCC). Several overlapping regulatory pathways establish the interspersed lobes and indentations along the margin of Arabidopsis thaliana leaves. These pathways involve feedback loops of auxin, the PIN-FORMED1 (PIN1) auxin efflux carrier, and the CUP-SHAPED COTYLEDON2 (CUC2) transcriptional regulator. Early vcc (deal1) leaf primordia fail to acquire bilateral symmetry and instead form ectopic lobes and sinuses. The vcc leaves show aberrant recruitment of marginal cells expressing properly polarized PIN1, resulting in misplaced auxin maxima. Normal PIN1 polarization requires CUC2 expression and CUC2 genetically interacts with VCC; VCC also affects CUC2 expression. VCC has a domain of unknown function, DUF1218, and localizes to the endoplasmic reticulum membrane. VCC acts partially redundantly with its two closest paralogs, DEAL2 and DEAL3, in early leaf margin patterning and is required for bilateral symmetry, but its loss of function does not visibly affect dorsoventrality.

Retrograde Signals Navigate the Path to Chloroplast Development

Complex signaling networks between the chloroplast and the nucleus mediate the emergence of the seedling into the light and the establishment of photosynthesis.

Deficient glutamate biosynthesis triggers a concerted upregulation of ribosomal protein genes in Arabidopsis

Biomass production requires the coordination between growth and metabolism. In a large-scale screen for mutants affected in leaf morphology, we isolated the orbiculata1 (orb1) mutants, which exhibit a pale green phenotype and reduced growth. The combination of map-based cloning and next-generation sequencing allowed us to establish that ORB1 encodes the GLUTAMATE SYNTHASE 1 (GLU1) enzyme, also known as FERREDOXIN-DEPENDENT GLUTAMINE OXOGLUTARATE AMINOTRANSFERASE 1 (Fd-GOGAT1). We performed an RNA-seq analysis to identify global gene expression changes in the orb1–3 mutant. We found altered expression levels of genes encoding enzymes involved in nitrogen assimilation and amino acid biosynthesis, such as glutamine synthetases, asparagine synthetases and glutamate dehydrogenases, showing that the expression of these genes depends on the levels of glutamine and/or glutamate. In addition, we observed a concerted upregulation of genes encoding subunits of the cytosolic ribosome. A gene ontology (GO) analysis of the differentially expressed genes between Ler and orb1–3 showed that the most enriched GO terms were ‘translation’, ‘cytosolic ribosome’ and ‘structural constituent of ribosome’. The upregulation of ribosome-related functions might reflect an attempt to keep protein synthesis at optimal levels even when the pool of glutamate is reduced.

The ANGULATA7 gene encodes a DnaJ-like zinc finger-domain protein involved in chloroplast function and leaf development in Arabidopsis

The characterization of mutants with altered leaf shape and pigmentation has previously allowed the identification of nuclear genes that encode plastid-localized proteins that perform essential functions in leaf growth and development. A large-scale screen previously allowed us to isolate ethyl methanesulfonate-induced mutants with small rosettes and pale green leaves with prominent marginal teeth, which were assigned to a phenotypic class that we dubbed Angulata. The molecular characterization of the 12 genes assigned to this phenotypic class should help us to advance our understanding of the still poorly understood relationship between chloroplast biogenesis and leaf morphogenesis. In this article, we report the phenotypic and molecular characterization of the angulata7-1 (anu7-1) mutant of Arabidopsis thaliana, which we found to be a hypomorphic allele of the EMB2737 gene, which was previously known only for its embryonic-lethal mutations. ANU7 encodes a plant-specific protein that contains a domain similar to the central cysteine-rich domain of DnaJ proteins. The observed genetic interaction of anu7-1 with a loss-of-function allele of GENOMES UNCOUPLED1 suggests that the anu7-1 mutation triggers a retrograde signal that leads to changes in the expression of many genes that normally function in the chloroplasts. Many such genes are expressed at higher levels in anu7-1 rosettes, with a significant overrepresentation of those required for the expression of plastid genome genes. Like in other mutants with altered expression of plastid-encoded genes, we found that anu7-1 exhibits defects in the arrangement of thylakoidal membranes, which appear locally unappressed.

Plants grow with a little help from their organelle friends

Chloroplasts and mitochondria are indispensable for plant development. They not only provide energy and carbon sources to cells, but also have evolved to become major players in a variety of processes such as amino acid metabolism, hormone biosynthesis and cellular signalling. As semi-autonomous organelles, they contain a small genome that relies largely on nuclear factors for its maintenance and expression. An intensive crosstalk between the nucleus and the organelles is therefore essential to ensure proper functioning, and the nuclear genes encoding organellar proteins involved in photosynthesis and oxidative phosphorylation are obviously crucial for plant growth. Organ growth is determined by two main cellular processes: cell proliferation and cell expansion. Here, we review how plant growth is affected in mutants of organellar proteins that are differentially expressed during leaf and root development. Our findings indicate a clear role for organellar proteins in plant organ growth, primarily during cell proliferation. However, to date, the role of the nuclear-encoded organellar proteins in the cellular processes driving organ growth has not been investigated in much detail. We therefore encourage researchers to extend their phenotypic characterization beyond macroscopic features in order to get a better view on how chloroplasts and mitochondria regulate the basic processes of cell proliferation and cell expansion, essential to driving growth.

Specific interaction of IM30/Vipp1 with cyanobacterial and chloroplast membranes results in membrane remodeling and eventually in membrane fusion

The photosynthetic light reaction takes place within the thylakoid membrane system in cyanobacteria and chloroplasts. Besides its global importance, the biogenesis, maintenance and dynamics of this membrane system are still a mystery. In the last two decades, strong evidence supported the idea that these processes involve IM30, the inner membrane-associated protein of 30kDa, a protein also known as the vesicle-inducing protein in plastids 1 (Vipp1). Even though we just only begin to understand the precise physiological function of this protein, it is clear that interaction of IM30 with membranes is crucial for biogenesis of thylakoid membranes. Here we summarize and discuss forces guiding IM30-membrane interactions, as the membrane properties as well as the oligomeric state of IM30 appear to affect proper interaction of IM30 with membrane surfaces. Interaction of IM30 with membranes results in an altered membrane structure and can finally trigger fusion of adjacent membranes, when Mg²⁺ is present. Based on recent results, we finally present a model summarizing individual steps involved in IM30-mediated membrane fusion. This article is part of a Special Issue entitled: Lipid order/lipid defects and lipid-control of protein activity edited by Dirk Schneider.

The novel protein DELAYED PALE-GREENING1 is required for early chloroplast biogenesis in Arabidopsis thaliana

Chloroplast biogenesis is one of the most important subjects in plant biology. In this study, an Arabidopsis early chloroplast biogenesis mutant with a delayed pale-greening phenotype (dpg1) was isolated from a T-DNA insertion mutant collection. Both cotyledons and true leaves of dpg1 mutants were initially albino but gradually became pale green as the plant matured. Transmission electron microscopic observations revealed that the mutant displayed a delayed proplastid-to-chloroplast transition. Sequence and transcription analyses showed that AtDPG1 encodes a putatively chloroplast-localized protein containing three predicted transmembrane helices and that its expression depends on both light and developmental status. GUS staining for AtDPG1::GUS transgenic lines showed that this gene was widely expressed throughout the plant and that higher expression levels were predominantly found in green tissues during the early stages of Arabidopsis seedling development. Furthermore, quantitative real-time RT-PCR analyses revealed that a number of chloroplast- and nuclear-encoded genes involved in chlorophyll biosynthesis, photosynthesis and chloroplast development were substantially down-regulated in the dpg1 mutant. These data indicate that AtDPG1 plays an essential role in early chloroplast biogenesis, and its absence triggers chloroplast-to-nucleus retrograde signalling, which ultimately down-regulates the expression of nuclear genes encoding chloroplast-localized proteins.

Plastids and Carotenoid Accumulation

Plastids are ubiquitously present in plants and are the organelles for carotenoid biosynthesis and storage. Based on their morphology and function, plastids are classified into various types, i.e. proplastids, etioplasts, chloroplasts, amyloplasts, and chromoplasts. All plastids, except proplastids, can synthesize carotenoids. However, plastid types have a profound effect on carotenoid accumulation and stability. In this chapter, we discuss carotenoid biosynthesis and regulation in various plastids with a focus on carotenoids in chromoplasts. Plastid transition related to carotenoid biosynthesis and the different capacity of various plastids to sequester carotenoids and the associated effect on carotenoid stability are described in light of carotenoid accumulation in plants.

Insights into chloroplast biogenesis and development

In recent years many advances have been made to obtain insight into chloroplast biogenesis and development. In plants several plastids types exist such as the proplastid (which is the progenitor of all plastids), leucoplasts (group of colourless plastids important for storage including elaioplasts (lipids), amyloplasts (starch) or proteinoplasts (proteins)), chromoplasts (yellow to orange-coloured due to carotenoids, in flowers or in old leaves as gerontoplasts), and the green chloroplasts. Chloroplasts are indispensable for plant development; not only by performing photosynthesis and thus rendering the plant photoautotrophic, but also for biochemical processes (which in some instances can also take place in other plastids types), such as the synthesis of pigments, lipids, and plant hormones and sensing environmental stimuli. Although we understand many aspects of these processes there are gaps in our understanding of the establishment of functional chloroplasts and their regulation. Why is that so? Even though chloroplast function is comparable in all plants and most of the algae, ferns and moss, detailed analyses have revealed many differences, specifically with respect to its biogenesis. As an update to our prior review on the genetic analysis of chloroplast biogenesis and development [1] herein we will focus on recent advances in Angiosperms (monocotyledonous and dicotyledonous plants) that provide novel insights and highlight the challenges and prospects for unravelling the regulation of chloroplast biogenesis specifically during the establishment of the young plants. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Rapid identification of angulata leaf mutations using next-generation sequencing

Map-based (positional) cloning has traditionally been the preferred strategy for identifying the causal genes underlying the phenotypes of mutants isolated in forward genetic screens. Massively parallel sequencing technologies are enabling the rapid cloning of genes identified in such screens. We have used a combination of linkage mapping and whole-genome re-sequencing to identify the causal mutations in four loss-of-function angulata (anu) mutants. These mutants were isolated in a screen for mutants with defects in leaf shape and leaf pigmentation. Our results show that the anu1-1, anu4-1, anu9-1 and anu12-1 mutants carry new alleles of the previously characterized SECA2, TRANSLOCON AT THE OUTER MEMBRANE OF CHLOROPLASTS 33 (TOC33), NON-INTRINSIC ABC PROTEIN 14 (NAP14) and CLP PROTEASE PROTEOLYTIC SUBUNIT 1 (CLPR1) genes. Re-sequencing the genomes of fine mapped mutants is a feasible approach that has allowed us to identify a moderate number of candidate mutations, including the one that causes the mutant phenotype, in a nonstandard genetic background. Our results indicate that anu mutations specifically affect plastid-localized proteins involved in diverse processes, such as the movement of peptides through chloroplast membranes (ANU1 and ANU4), metal homeostasis (ANU9) and protein degradation (ANU12).