Altered chloroplast development and delayed fruit ripening caused by mutations in a zinc metalloprotease at the lutescent2 locus of tomato

Plastid dynamism integrates development and environment

In land plants plastid type differentiation occurs concomitantly with cellular differentiation and the transition from one type to another is under developmental and environmental control. Plastid dynamism is based on a bilateral communication between plastids and nucleus through anterograde and retrograde signaling. Signaling occurs through the interaction with specific phytohormones (abscisic acid, strigolactones, jasmonates, gibberellins, brassinosteroids, ethylene, salicylic acid, cytokinin and auxin). The review is focused on the modulation of plastid capabilities at both transcriptional and post-translational levels at the crossroad between development and stress, with a particular attention to the chloroplast, because the most studied plastid type. The role of plastid-encoded and nuclear-encoded proteins for plastid development and stress responses, and the changes of plastid fate through the activity of stromules and plastoglobules, are discussed. Examples of plastid dynamism in response to soil stress agents (salinity, lead, cadmium, arsenic, and chromium) are described. Albinism and root greening are described based on the modulation activities of auxin and cytokinin. The physiological and functional responses of the sensory epidermal and vascular plastids to abiotic and biotic stresses along with their specific roles in stress sensing are described together with their potential modulation of retrograde signaling pathways. Future research perspectives include an in-depth study of sensory plastids to explore their potential for establishing a transgenerational memory to stress. Suggestions about anterograde and retrograde pathways acting at interspecific level and on the lipids of plastoglobules as a novel class of plastid morphogenic agents are provided.

Chloroplast metalloproteinase SlL2 reduces the thermotolerance of tomato by decreasing the content of SlCDJ1

Chloroplast is one of the most sensitive organelles to heat stress in plants. In chloroplasts, various proteases affect photosynthesis by degrading proteins under stress conditions. Tomato Lutescent2 (SlL2), a chloroplast zinc metalloprotease, was previously reported to alter chloroplast development and delay fruit ripening. However, its enzyme activity and roles in plant response to abiotic stress are still unclear. Here, we confirmed that the SlL2 protein which localized on thylakoid membrane was an ATP-independent hydrolase, and SlL2 gene responded to heat stress. Phenotype analysis showed that SlL2 plays a negative role in the heat-response mechanism. Under heat stress, the transgenic plants overexpressing SlL2 (OE) grew worse than the wild type (WT), as reflected by their decreased membrane stability, osmotic-regulating substance, and antioxidative enzyme activities, as well as increased reactive oxygen species (ROS) accumulation. By contrast, l2 mutant line showed the opposite phenotype and corresponding physiological indices under heat stress. In addition, overexpression of SlL2 decreased the photosynthetic activities, especially photosystem II. Moreover, SlL2 was found to interact with chloroplast-located chaperone protein SlCDJ1, decreasing its content under heat stress. These results indicate that SlL2 reduces the thermotolerance of tomato by reducing the content of SlCDJ1.

Analysis of morphological traits and regulatory mechanism of a semi-dwarf, albino, and blue grain wheat line

The plant height and leaf color are important traits in crops since they contribute to the production of grains and biomass. Progress has been made in mapping the genes that regulate plant height and leaf color in wheat (Triticum aestivum L.) and other crops. Wheat line DW-B (dwarfing, white leaves, and blue grains) with semi-dwarfing and albinism at the tillering stage and re-greening at the jointing stage was created using Lango and Indian Blue Grain. Transcriptomic analyses of the three wheat lines at the early jointing stages indicated that the genes of gibberellin (GA) signaling pathway and chlorophyll (Chl) biosynthesis were expressed differently in DW-B and its parents. Furthermore, the response to GA and Chl contents differed between DW-B and its parents. The dwarfing and albinism in DW-B were owing to defects in the GA signaling pathway and abnormal chloroplast development. This study can improve understanding of the regulation of plant height and leaf color.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-023-01379-z.

Escaping drought: The pectin methylesterase inhibitor gene Slpmei27 can significantly change drought resistance in tomato

Drought stress will lead to a decrease in tomato yield and poor flavour, yield and quality, resulting in economic losses in agricultural production. Mining the key genes regulating tomato drought resistance is of great significance to improve the drought resistance of tomato plants. The cell wall can directly participate in the plant drought stress response as one of the main components of the cell wall, and the regulation of pectin content in plant drought resistance is still unclear. Here, the candidate gene Solyc08g006690 (Slpmei27) was obtained by fine mapping based on genome sequencing technology (BSA-seq) of late-maturing stress-resistant tomato mutants found in the field. Slpmei27 is expressed in the cell wall. The transient silencing of Slpmei27 by VIGS significantly improved the drought resistance of tomato. Meanwhile, Slpmei27 silencing could significantly change the cell wall structure of plants, change the stomatal pass rate, reduce the water loss rate of plants, improve the scavenging ability of reactive oxygen species, change the redox balance in plants, and thus improve the drought resistance of tomato. The promoter region of this gene contains a large number of hormone-response and stress-response binding sites. The promoter region of the Slpmei27 gene in the mutant could lower the expression of downstream genes. Through this study, the mechanism by which Slpmei27 improves tomato drought resistance was revealed, and the relationship between pectin methyl ester metabolism and plant drought resistance was established, providing a theoretical basis for the production of high-quality tomato materials with high drought resistance.

A mutation in SlCHLH encoding a magnesium chelatase H subunit is involved in the formation of yellow stigma in tomato (Solanum lycopersicum L.)

Chlorophylls are ubiquitous pigments responsible for the green color in plants. Changes in the chlorophyll content have a significant impact on photosynthesis, plant growth and development. In this study, we used a yellow stigma mutant (ys) generated from a green stigma tomato WT by using ethylmethylsulfone (EMS)-induced mutagenesis. Compared with WT, the stigma of ys shows low chlorophyll content and impaired chloroplast ultrastructure. Through map-based cloning, the ys gene is localized to a 100 kb region on chromosome 4 between dCAPS596 and dCAPS606. Gene expression analysis and nonsynonymous SNP determination identified the Solyc04g015750, as the potential candidate gene, which encodes a magnesium chelatase H subunit (CHLH). In ys mutant, a single base C to T substitution in the SlCHLH gene results in the conversion of Serine into Leucine (Ser92Leu) at the N-terminal region. The functional complementation test shows that the SlCHLH from WT can rescue the green stigma phenotype of ys. In contrast, knockdown of SlCHLH in green stigma tomato AC, observed the yellow stigma phenotype at the stigma development stage. Overexpression of the mutant gene Slys in green stigma tomato AC results in the light green stigma. These results indicate that the mutation of the N-terminal S92 to Leu in SlCHLH is the main reason for the formation of the yellow stigma phenotype. Characterization of the ys mutant enriches the current knowledge of the tomato chlorophyll mutant library and provides a novel and effective tool for understanding the function of CHLH in tomato.

Organelles and phytohormones: a network of interactions in plant stress responses

Phytohormones are major signaling components that contribute to nearly all aspects of plant life. They constitute an interconnected communication network to fine-tune growth and development in response to the ever-changing environment. To this end, they have to coordinate with other signaling components, such as reactive oxygen species and calcium signals. On the one hand, the two endosymbiotic organelles, plastids and mitochondria, control various aspects of phytohormone signaling and harbor important steps of hormone precursor biosynthesis. On the other hand, phytohormones have feedback actions on organellar functions. In addition, organelles and phytohormones often act in parallel in a coordinated matter to regulate cellular functions. Therefore, linking organelle functions with increasing knowledge of phytohormone biosynthesis, perception, and signaling will reveal new aspects of plant stress tolerance. In this review, we highlight recent work on organelle-phytohormone interactions focusing on the major stress-related hormones abscisic acid, jasmonates, salicylic acid, and ethylene.

Slym1 control the color etiolation of leaves by facilitating the decomposition of chlorophyll in tomato

Photosynthesis, as an important biological process of plants, produces organic substances for plant growth and development. Although the molecular mechanisms of photosynthesis had been well investigated, the relationship between chlorophyll synthesis and photosynthesis remains largely unknown. The leaf-color mutant was an ideal material for studying photosynthesis and chlorophyll synthesis, which had been seldom investigated in tomato. Here, we obtained a yellow leaf tomato mutant ym (The mutant plants from the line of zs4) in field. Transmission electron microscopy (TEM) and photosynthetic parameters results demonstrated that chloroplast's structure was obviously destroyed and photosynthetic capacity gets weak. The mutant was hybridized with the control to construct the F₂ segregation population for sequencing. Slym1 gene, controlling yellow mutant trait, was identified using Bulked Segregation Analysis. Slym1 was up-regulated in the mutant and Slym1 was located in the nucleus. The genes associated with photosynthesis and chlorophyll synthesis were down-regulated in Slym1-OE transgenic tomato plants. The results suggested that Slym1 negatively regulate photosynthesis. Photosynthetic pigment synthesis related genes HEMA, HEMB1, CHLG and CAO were up-regulated in Slym1 silencing plants. The redundant Slym1 binding the intermediate proteins MP resulting in hindering the interaction between MP and HY5 due to the Slym1 with a high expression level in ym mutant, lead to lots of the HY5 with unbound state accumulates in cells, that could accelerate the decomposition of chlorophyll. Therefore, the yellow leaf-color mutant ym could be used as an ideal material for further exploring the relationship between leaf color mutant and photosynthesis and the specific mechanism.

A Tomato EMS-Mutagenized Population Provides New Valuable Resources for Gene Discovery and Breeding of Developmental Traits

Tomato (Solanum lycopersicum L.) is a major horticultural crop and a model species among eudicots, especially for traits related to reproductive development. Although considerable progress has been made since the tomato genome sequence project was completed, most of the genes identified remain predictions with an unknown or hypothetical function. This lack of functional characterization hampers the use of the huge amount of genomic information available to improve the quality and productivity of this crop. Reverse genetics strategies such as artificial mutagenesis and next-generation sequencing approaches build the perfect tandem for increasing knowledge on functional annotation of tomato genes. This work reports the phenotypic characterization of a tomato mutant collection generated from an EMS chemical mutagenesis program aimed to identify interesting agronomic mutants and novel gene functions. Tomato mutants were grouped into fourteen phenotypic classes, including vegetative and reproductive development traits, and the inheritance pattern of the identified mutations was studied. In addition, causal mutation of a selected mutant line was isolated through a mapping-by-sequencing approach as a proof of concept of this strategy’s successful implementation. Results support tomato mutagenesis as an essential tool for functional genomics in this fleshy-fruited model species and a highly valuable resource for future breeding programs of this crop species aimed at the development of more productive and resilient new varieties under challenging climatic and production scenarios.

SlMIR164A regulates fruit ripening and quality by controlling SlNAM2 and SlNAM3 in tomato

MiRNAs are important posttranscriptional regulators of plant development. Many miRNAs, such as the conserved miR164 species, are encoded by families of MIRNA genes, but the specific roles of individual MIRNA genes are largely undefined. Here, we characterize the functions and regulatory mechanisms of SlMIR164A, one of the primary genes of Sly-miR164, in tomato. We show that SlMIR164A is preferentially expressed at late stages of fruit development and plays a vital role in controlling fruit ripening and quality. Loss of function of SlMIR164A by CRISPR/Cas9-mediated mutagenesis results in accelerated fruit ripening and enhanced chloroplast development, which leads to altered sugar and organic acid contents and affects the nutritional quality of fruits. We also show that SlMIR164A modulates fruit ripening and quality through specific target genes, SlNAM2 and SlNAM3, which control key regulators of chloroplast function and fruit ripening processes. MIR164 genes have been shown to play conserved roles in regulating organ ageing, such as leaf senescence and fruit ripening, in a variety of plants, but whether and how their family members in tomato exert the same function remain to be elucidated. Our results reveal a previously undiscovered role of SlMIR164A in ripening control, which will further our understanding of the actions of MIR164 family, as well as the mechanisms of fruit ripening and quality control in tomato. Moreover, as loss of SlMIR164A exhibits minor impacts on organ morphology, our results can be leveraged in tomato breeding for specific manipulation of fruit ripening and quality to facilitate tomato improvement in agriculture.

A Chimeric TGA Repressor Slows Down Fruit Maturation and Ripening in Tomato

The bZIP transcription factor (TF) SlTGA2.2 was previously highlighted as a possible hub in a network regulating fruit growth and transition to ripening (maturation phase). It belongs to a clade of TFs well known for their involvement in the regulation of the salicylic acid-dependent systemic acquired resistance. To investigate if this TGA TF plays a role in tomato fruit growth and maturation, we took advantage of the fruit-specific SlPPC2 promoter (PPC2pro) to target the expression of a SlTGA2.2-SRDX chimeric repressor in a developmental window restricted to early fruit growth and maturation. Here, we show that this SlTGA2.2-SRDX repressor alters early fruit development and metabolism, including chloroplast number and structure, considerably extends the time necessary to reach the mature green stage and slows down fruit ripening. RNA sequencing and plant hormone analyses reveal that PPC2pro:SlTGA2.2-SRDX fruits are maintained in an immature stage as long as PPC2pro is active, through early modifications of plant hormonal signaling and down-regulation of MADS-RIN and NAC-NOR ripening regulators. Once PPC2pro becomes inactive and therefore SlTGA2.2-SRDX expression is reduced, ripening can proceed, albeit at a slower pace than normal. Altogether, this work emphasizes the developmental continuum between fruit growth, maturation and ripening and provides a useful tool to alter and study the molecular bases of tomato fruit transition to ripening.

Identification of Differentially Expressed Genes and Pathways Involved in Growth and Development of Mesona chinensis Benth Under Red- and Blue-Light Conditions

Mesona chinensis Benth (MCB) is an important Chinese herbal medicine. The plant factories might be one of the ways to solve the shortage of MCB supply. In this study, the MCB seedlings were treated under the red (R) and blue (B) lights in the plant factory. Results showed that the red light promoted the growth and development of MCB in comparison with the blue light. Under the red-light condition, the biomass, plant height, and root characteristics were significantly higher than those under blue-light condition, while the soil and plant analyzer development (SPAD) under the red-light treatment was significantly lower than that under the blue-light treatment. Red light also significantly promoted the content of soluble sugar and pectin of MCB compared with blue light. Transcriptome analysis showed that a total of 4,165 differentially expressed genes (DEGs) were detected including 2,034 upregulated and 2,131 downregulated. Of these, 1,112 DEGs including 410 upregulated and 702 downregulated genes were associated with 111 pathways. Moreover, a total of 8,723 differentially expressed transcription factors (TFs) were identified in R vs. B, and these TFs were distributed in 56 gene families. Metabonomic results revealed that a total of 184 metabolites and 99 differentially expressed metabolites (DEMs) (42 upregulated and 57 downregulated) were identified in the red- and blue-light treatments. Integrative analysis of transcriptome and metabolome unveiled that a total of 24 pathways included 70 compounds (metabolites) and were associated with 28 unigenes. In particular, these pathways included starch and sucrose metabolism, phenylpropanoid biosynthesis, cysteine and methionine metabolism, glycolysis/gluconeogenesis, and pentose and glucuronate interconversions. The unigenes included asparagine synthetase (AS), thymidine kinase (TK), alpha, alpha-trehalose-phosphate synthase (TPS), phosphatase IMPL1 (IMPL1), dihydroflavonol 4-reductase (D4R), and 4-coumarate-CoA ligase-like 6 (4CL6), bifunctional aspartokinase-homoserine dehydrogenase 1 (thrA), and abscisic acid 8'-hydroxylase 2 isoform X1 (ABA8). It was indicated that these pathways and genes might play important roles in the growth and development of MCB. This study laid a foundation for the future research of MCB.

Plant protease as regulator and signaling molecule for enhancing environmental stress-tolerance

Proteases are ubiquitous in prokaryotes and eukaryotes. Plant proteases are key regulators of various physiological processes, including protein homeostasis, organelle development, senescence, seed germination, protein processing, environmental stress response, and programmed cell death. Proteases are involved in the breakdown of peptide bonds resulting in irreversible posttranslational modification of the protein. Proteases act as signaling molecules that specifically regulate cellular function by cleaving and triggering receptor molecules. Peptides derived from proteolysis regulate ROS signaling under oxidative stress in the plant. It degrades misfolded and abnormal proteins into amino acids to repair the cell damage and regulates the biological process in response to environmental stress. Proteases modulate the biogenesis of phytohormones which control plant growth, development, and environmental stresses. Protein homeostasis, the overall balance between protein synthesis and proteolysis, is required for plant growth and development. Abiotic and biotic stresses are major factors that negatively impact cellular survivability, biomass production, and reduced crop yield potentials. Therefore, the identification of various stress-responsive proteases and their molecular functions may elucidate valuable information for the development of stress-resilient crops with higher yield potentials. However, the understanding of molecular mechanisms of plant protease remains unexplored. This review provides an overview of proteases related to development, signaling, and growth regulation to acclimatize environmental stress in plants.

Biochemical and transcriptomic analyses reveal that critical genes involved in pigment biosynthesis influence leaf color changes in a new sweet osmanthus cultivar 'Qiannan Guifei'

Background

Osmanthus fragrans (Oleaceae) is one of the most important ornamental plant species in China. Many cultivars with different leaf color phenotypes and good ornamental value have recently been developed. For example, a new cultivar ‘Qiannan Guifei’, presents a rich variety of leaf colors, which change from red to yellow-green and ultimately to green as leaves develop, making this cultivar valuable for landscaping. However, the biochemical characteristics and molecular mechanisms underlying leaf color changes of these phenotypes have not been elucidated. It has been hypothesized that the biosynthesis of different pigments in O. fragrans might change during leaf coloration. Here, we analyzed transcriptional changes in genes involved in chlorophyll (Chl), flavonoid, and carotenoid metabolic pathways and identified candidate genes responsible for leaf coloration in the new cultivar ‘Qiannan Guifei’.

Methods

Leaf samples were collected from ‘Qiannan Guifei’ plants at the red (R), yellow-green (YG) and green (G) leaf stages. We compared the different-colored leaves via leaf pigment concentrations, chloroplast ultrastructure, and transcriptomic data. We further analyzed differentially expressed genes (DEGs) involved in the Chl, flavonoid, and carotenoid metabolic pathways. In addition, we used qRT-PCR to validate expression patterns of the DEGs at the three stages.

Results

We found that, compared with those at the G stage, chloroplasts at the R and YG stages were less abundant and presented abnormal morphologies. Pigment analyses revealed that the leaves had higher flavonoid and anthocyanin levels at the R stage but lower Chl and carotenoid concentrations. Similarly, Chl and carotenoid concentrations were lower at the YG stage than at the G stage. By using transcriptomic sequencing, we further identified 61 DEGs involved in the three pigment metabolic pathways. Among these DEGs, seven structural genes (OfCHS, OfCHI, OfF3H, OfDFR, OfANS, OfUGT andOf3AT) involved in the flavonoid biosynthesis pathway were expressed at the highest level at the R stage, thereby increasing the biosynthesis of flavonoids, especially anthocyanins. Six putativeOfMYB genes, including three flavonoid-related activators and three repressors, were also highly expressed at the R stage, suggesting that they might coordinately regulate the accumulation of flavonoids, including anthocyanins. Additionally, expressions of the Chl biosynthesis-related genes OfHEMA, OfCHLG and OfCAO and the carotenoid biosynthesis-related genes OfHYB and OfZEP were upregulated from the R stage to the G stage, which increased the accumulation of Chl and carotenoids throughout leaf development. In summary, we screened the candidate genes responsible for the leaf color changes of ‘Qiannan Guifei’, improved current understanding of the regulatory mechanisms underlying leaf coloration and provided potential targets for future leaf color improvement in O. fragrans.

L2, a chloroplast metalloproteinase, regulates fruit ripening by participating in ethylene autocatalysis under the control of ethylene response factors

Although autocatalytic ethylene biosynthesis plays an important role in the ripening of climacteric fruits, our knowledge of the network that promotes it remains limited. We identified white fruit (wf), a tomato mutant that produces immature fruit that are white and that ripen slowly. We found that an inversion on chromosome 10 disrupts the LUTESCENT2 (L2) gene, and that white fruit is allelic to lutescent2. Using CRISPR/Cas9 technology we knocked out L2 in wild type tomato and found that the l2-cr mutants produced phenotypes that were very similar to white fruit (lutescent2). In the l2-cr fruit, chloroplast development was impaired and the accumulation of carotenoids and lycopene occurred more slowly than in wild type. During fruit ripening in l2-cr mutants, the peak of ethylene release was delayed, less ethylene was produced, and the expression of ACO genes was significantly suppressed. We also found that exogenous ethylene induces the expression of L2 and that ERF.B3, an ethylene response factor, binds to the promoter of the L2 gene and activates its transcription. Thus, the expression of L2 is regulated by exogenous ethylene. Taken together, our results indicate that ethylene may affect the expression of L2 gene and that L2 participates in autocatalytic ethylene biosynthesis during tomato fruit ripening.

Introgression of the sesquiterpene biosynthesis from Solanum habrochaites to cultivated tomato offers insights into trichome morphology and arthropod resistance

Cultivated tomatoes harboring the plastid-derived sesquiterpenes from S. habrochaites have altered type-VI trichome morphology and unveil additional genetic components necessary for piercing-sucking pest resistance. Arthropod resistance in the tomato wild relative Solanum habrochaites LA1777 is linked to specific sesquiterpene biosynthesis. The Sesquiterpene synthase 2 (SsT2) gene cluster on LA1777 chromosome 8 controls plastid-derived sesquiterpene synthesis. The main genes at SsT2 are Z-prenyltransferase (zFPS) and Santalene and Bergamotene Synthase (SBS), which produce α-santalene, β-bergamotene, and α-bergamotene in LA1777 round-shaped type-VI glandular trichomes. Cultivated tomatoes have mushroom-shaped type-VI trichomes with much smaller glands that contain low levels of monoterpenes and cytosolic-derived sesquiterpenes, not presenting the same pest resistance as in LA1777. We successfully transferred zFPS and SBS from LA1777 to cultivated tomato (cv. Micro-Tom, MT) by a backcrossing approach. The trichomes of the MT-Sst2 introgressed line produced high levels of the plastid-derived sesquiterpenes. The type-VI trichome internal storage-cavity size increased in MT-Sst2, probably as an effect of the increased amount of sesquiterpenes, although it was not enough to mimic the round-shaped LA1777 trichomes. The presence of high amounts of plastid-derived sesquiterpenes was also not sufficient to confer resistance to various tomato piercing-sucking pests, indicating that the effect of the sesquiterpenes found in the wild S. habrochaites can be insect specific. Our results provide for a better understanding of the morphology of S. habrochaites type-VI trichomes and paves the way to obtain insect-resistant tomatoes.

Arabidopsis EGY1 Is Critical for Chloroplast Development in Leaf Epidermal Guard Cells

In Arabidopsis thaliana, the Ethylene-dependent Gravitropism-deficient and Yellow-green 1 (EGY1) gene encodes a thylakoid membrane-localized protease involved in chloroplast development in leaf mesophyll cells. Recently, EGY1 was also found to be crucial for the maintenance of grana in mesophyll chloroplasts. To further explore the function of EGY1 in leaf tissues, we examined the phenotype of chloroplasts in the leaf epidermal guard cells and pavement cells of two 40Ar17+ irradiation-derived mutants, Ar50-33-pg1 and egy1-4. Fluorescence microscopy revealed that fully expanded leaves of both egy1 mutants showed severe chlorophyll deficiency in both epidermal cell types. Guard cells in the egy1 mutant exhibited permanent defects in chloroplast formation during leaf expansion. Labeling of plastids with CaMV35S or Protodermal Factor1 (PDF1) promoter-driven stroma-targeted fluorescent proteins revealed that egy1 guard cells contained the normal number of plastids, but with moderately reduced size, compared with wild-type guard cells. Transmission electron microscopy further revealed that the development of thylakoids was impaired in the plastids of egy1 mutant guard mother cells, guard cells, and pavement cells. Collectively, these observations demonstrate that EGY1 is involved in chloroplast formation in the leaf epidermis and is particularly critical for chloroplast differentiation in guard cells.

SlRCM1, which encodes tomato Lutescent1, is required for chlorophyll synthesis and chloroplast development in fruits

In plants, chloroplasts are the sites at which photosynthesis occurs, and an increased abundance of chloroplasts increases the nutritional quality of plants and the resultant color of fruits. However, the molecular mechanisms underlying chlorophyll synthesis and chloroplast development in tomato fruits remain unknown. In this study, we isolated a chlorophyll-deficient mutant, reduced chlorophyll mutant 1 (rcm1), by ethylmethanesulfonate mutagenesis; this mutant produced yellowish fruits with altered chloroplast development. MutMap revealed that Solyc08g005010 is the causal gene underlying the rcm1 mutant phenotype. A single-nucleotide base substitution in the second exon of SlRCM1 results in premature termination of its translated protein. SlRCM1 encodes a chloroplast-targeted metalloendopeptidase that is orthologous to the BCM1 protein of Arabidopsis and the stay-green G protein of soybean (Glycine max L. Merr.). Notably, the yellowish phenotype of the lutescent1 mutant can be restored with the allele of SlRCM1 from wild-type tomato. In contrast, knockout of SlRCM1 by the CRISPR/Cas9 system in Alisa Craig yielded yellowish fruits at the mature green stage, as was the case for lutescent1. Amino acid sequence alignment and functional complementation assays showed that SlRCM1 is indeed Lutescent1. These findings provide new insights into the regulation of chloroplast development in tomato fruits.

An Argon-Ion-Induced Pale Green Mutant of Arabidopsis Exhibiting Rapid Disassembly of Mesophyll Chloroplast Grana

Argon-ion beam is an effective mutagen capable of inducing a variety of mutation types. In this study, an argon ion-induced pale green mutant of Arabidopsis thaliana was isolated and characterized. The mutant, designated Ar50-33-pg1, exhibited moderate defects of growth and greening and exhibited rapid chlorosis in photosynthetic tissues. Fluorescence microscopy confirmed that mesophyll chloroplasts underwent substantial shrinkage during the chlorotic process. Genetic and whole-genome resequencing analyses revealed that Ar50-33-pg1 contained a large 940 kb deletion in chromosome V that encompassed more than 100 annotated genes, including 41 protein-coding genes such as TYRAAt1/TyrA1, EGY1, and MBD12. One of the deleted genes, EGY1, for a thylakoid membrane-localized metalloprotease, was the major contributory gene responsible for the pale mutant phenotype. Both an egy1 mutant and F1 progeny of an Ar50-33-pg1 × egy1 cross-exhibited chlorotic phenotypes similar to those of Ar50-33-pg1. Furthermore, ultrastructural analysis of mesophyll cells revealed that Ar50-33-pg1 and egy1 initially developed wild type-like chloroplasts, but these were rapidly disassembled, resulting in thylakoid disorganization and fragmentation, as well as plastoglobule accumulation, as terminal phenotypes. Together, these data support the utility of heavy-ion mutagenesis for plant genetic analysis and highlight the importance of EGY1 in the structural maintenance of grana in mesophyll chloroplasts.

Tomato SlBL4 plays an important role in fruit pedicel organogenesis and abscission

Abscission, a cell separation process, is an important trait that influences grain and fruit yield. We previously reported that BEL1-LIKE HOMEODOMAIN 4 (SlBL4) is involved in chloroplast development and cell wall metabolism in tomato fruit. In the present study, we showed that silencing SlBL4 resulted in the enlargement and pre-abscission of the tomato (Solanum lycopersicum cv. Micro-TOM) fruit pedicel. The anatomic analysis showed the presence of more epidermal cell layers and no obvious abscission zone (AZ) in the SlBL4 RNAi lines compared with the wild-type plants. RNA-seq analysis indicated that the regulation of abscission by SlBL4 was associated with the altered abundance of genes related to key meristems, auxin transporters, signaling components, and cell wall metabolism. Furthermore, SlBL4 positively affected the auxin concentration in the abscission zone. A dual-luciferase reporter assay revealed that SlBL4 activated the transcription of the JOINTLESS, OVATE, PIN1, and LAX3 genes. We reported a novel function of SlBL4, which plays key roles in fruit pedicel organogenesis and abscission in tomatoes.

Manipulation of ZDS in tomato exposes carotenoid- and ABA-specific effects on fruit development and ripening

Spontaneous mutations in fruit-specific carotenoid biosynthetic genes of tomato (Solanum lycopersicum) have led to improved understanding of ripening-associated carotenogenesis. Here, we confirm that ZDS is encoded by a single gene in tomato transcriptionally regulated by ripening transcription factors RIN, NOR and ethylene. Manipulation of ZDS was achieved through transgenic repression and heterologous over-expression in tomato. CaMV 35S-driven RNAi repression inhibited carotenoid biosynthesis in all aerial tissues examined resulting in elevated levels of ζ-carotene isomers and upstream carotenoids, while downstream all trans-lycopene and subsequent photoprotective carotenes and xanthophylls were diminished. Consequently, immature fruit displayed photo-bleaching consistent with reduced levels of the photoprotective carotenes and developmental phenotypes related to a reduction in the carotenoid-derived phytohormone abscisic acid (ABA). ZDS-repressed ripe fruit was devoid of the characteristic red carotenoid, all trans-lycopene and displayed brilliant yellow pigmentation due to elevated 9,9' di-cis-ζ-carotene. Over-expression of the Arabidopsis thaliana ZDS (AtZDS) gene bypassed endogenous co-suppression and revealed ZDS as an additional bottleneck in ripening-associated carotenogenesis of tomato. Quantitation of carotenoids in addition to multiple ripening parameters in ZDS-altered lines and ABA-deficient fruit-specific carotenoid mutants was used to separate phenotypic consequences of ABA from other effects of ZDS manipulation and reveal a unique and dynamic ζ-carotene isomer profile in ripe fruit.

BEL1-LIKE HOMEODOMAIN4 regulates chlorophyll accumulation, chloroplast development, and cell wall metabolism in tomato fruit

Tomato (Solanum lycopersicum) is a model plant for studying fruit development and ripening. In this study, we found that down-regulation of a tomato bell-like homeodomain 4 (SlBL4) resulted in a slightly darker-green fruit phenotype and increased accumulation of starch, fructose, and glucose. Analysis of chlorophyll content and TEM observations was consistent with these phenotypes, indicating that SlBL4 was involved in chlorophyll accumulation and chloroplast formation. Ripened fruit of SlBL4-RNAi plants had noticeably decreased firmness, larger intercellular spaces, and thinner cell walls than the wild-type. RNA-seq identified differentially expressed genes involved in chlorophyll metabolism, chloroplast development, cell wall metabolism, and carotenoid metabolism. ChIP-seq identified (G/A) GCCCA (A/T/C) and (C/A/T) (C/A/T) AAAAA (G/A/T) (G/A) motifs. SlBL4 directly inhibited the expression of protoporphyrinogen oxidase (SlPPO), magnesium chelatase H subunit (SlCHLD), pectinesterase (SlPE), protochlorophyllide reductase (SlPOR), chlorophyll a/b binding protein 3B (SlCAB-3B), and homeobox protein knotted 2 (TKN2). In contrast, it positively regulated the expression of squamosa promoter binding protein-like colorless non-ripening (LeSPL-CNR). Our results indicate that SlBL4 is involved in chlorophyll accumulation, chloroplast development, cell wall metabolism, and the accumulation of carotenoids during tomato fruit ripening, and provide new insights for the transcriptional regulation mechanism of BELL-mediated fruit growth and ripening.

The chloroplast metalloproteases VAR2 and EGY1 act synergistically to regulate chloroplast development in Arabidopsis

Chloroplast development and photosynthesis require the proper assembly and turnover of photosynthetic protein complexes. Chloroplasts harbor a repertoire of proteases to facilitate proteostasis and development. We have previously used an Arabidopsis leaf variegation mutant, yellow variegated2 (var2), defective in thylakoid FtsH protease complexes, as a tool to dissect the genetic regulation of chloroplast development. Here, we report a new genetic enhancer mutant of var2, enhancer of variegation3-1 (evr3-1). We confirm that EVR3 encodes a chloroplast metalloprotease, reported previously as ethylene-dependent gravitropism-deficient and yellow-green1 (EGY1)/ammonium overly sensitive1 (AMOS1). We observed that mutations in EVR3/EGY1/AMOS1 cause more severe leaf variegation in var2-5 and synthetic lethality in var2-4 Using a modified blue-native PAGE system, we reveal abnormal accumulations of photosystem I, photosystem II, and light-harvesting antenna complexes in EVR3/EGY1/AMOS1 mutants. Moreover, we discover distinct roles of VAR2 and EVR3/EGY1/AMOS1 in the turnover of photosystem II reaction center under high light stress. In summary, our findings indicate that two chloroplast metalloproteases, VAR2/AtFtsH2 and EVR3/EGY1/AMOS1, function coordinately to regulate chloroplast development and reveal new roles of EVR3/EGY1/AMOS1 in regulating chloroplast proteostasis in Arabidopsis.

A two-step mutation process in the double WS1 homologs drives the evolution of burley tobacco, a special chlorophyll-deficient mutant with abnormal chloroplast development

MAIN CONCLUSION

The functional homologs WS1A and WS1B, identified by map-based cloning, control the burley character by affecting chloroplast development in tobacco, contributing to gene isolation and genetic improvement in polyploid crops. Burley represents a special type of tobacco (Nicotiana tabacum L.) cultivar that is characterized by a white stem with a high degree of chlorophyll deficiency. Although important progress in the research of burley tobacco has been made, the molecular mechanisms underlying this character remain unclear. Here, on the basis of our previous genetic analyses and preliminary mapping results, we isolated the White Stem 1A (WS1A) and WS1B genes using a map-based cloning approach. WS1A and WS1B are functional homologs with completely identical biological functions and highly similar expression patterns that control the burley character in tobacco. WS1A and WS1B are derived from Nicotiana sylvestris and Nicotiana tomentosiformis, the diploid ancestors of Nicotiana tabacum, respectively. The two genes encode zinc metalloproteases of the M50 family that are highly homologous to the Ethylene-dependent Gravitropism-deficient and Yellow-green 1 (EGY1) protein of Arabidopsis and the Lutescent 2 (L2) protein of tomato. Transmission electron microscopic examinations indicated that WS1A and WS1B are involved in the development of chloroplasts by controlling the formation of thylakoid membranes, very similar to that observed for EGY1 and L2. The genotyping of historical tobacco varieties revealed that a two-step mutation process occurred in WS1A and WS1B during the evolution of burley tobacco. We also discussed the strategy for gene map-based cloning in polyploid plants with complex genomes. This study will facilitate the identification of agronomically important genes in tobacco and other polyploid crops and provide insights into crop improvement via molecular approaches.

The zinc-finger transcription factor CcLOL1 controls chloroplast development and immature pepper fruit color in Capsicum chinense and its function is conserved in tomato

Chloroplast development and chlorophyll content in the immature fruit has a major impact on the morphology and quality in pepper (Capsicum spp.) fruit. Two major quantitative trait loci (QTLs), pc1 and pc10 that affect chlorophyll content in the pepper fruit by modulation of chloroplast compartment size were previously identified in chromosomes 1 and 10, respectively. The pepper homolog of GOLDEN2-LIKE transcription factor (CaGLK2) has been found as underlying pc10, similar to its effect on tomato chloroplast development. In the present study, we identified the pepper homolog of the zinc-finger transcription factor LOL1 (LSD ONE LIKE1; CcLOL1) as the gene underlying pc1. LOL1 has been identified in Arabidopsis as a positive regulator of programmed cell death and we report here on its role in controlling fruit development in the Solanaceae in a fruit-specific manner. The light-green C. chinense parent used for QTL mapping was found to carry a null mutation in CcLOL1. Verification of the function of the gene was done by generating CRISPR/Cas9 knockout mutants of the orthologous tomato gene resulting in light-green tomato fruits, indicating functional conservation of the orthologous genes in controlling chlorophyll content in the Solanaceae. Transcriptome profiling of light and dark-green bulks differing for pc1, showed that the QTL affects multiple photosynthesis and oxidation-reduction associated genes in the immature green fruit. Allelic diversity of three known genes CcLOL1, CaGLK2, and CcAPRR2 that influence pepper immature fruit color, was found to be associated with variation in chlorophyll content primarily in C. chinense.

Mutation Breeding in Tomato: Advances, Applicability and Challenges

Induced mutagenesis is one of the most effective strategies for trait improvement without altering the well-optimized genetic background of the cultivars. In this review, several currently accessible methods such as physical, chemical and insertional mutagenesis have been discussed concerning their efficient exploration for the tomato crop improvement. Similarly, challenges for the adaptation of genome-editing, a newly developed technique providing an opportunity to induce precise mutation, have been addressed. Several efforts of genome-editing have been demonstrated in tomato and other crops, exploring its effectiveness and convenience for crop improvement. Descriptive data compiled here from such efforts will be helpful for the efficient exploration of technological advances. However, uncertainty about the regulation of genome-edited crops is still a significant concern, particularly when timely trait improvement in tomato cultivars is needed. In this regard, random approaches of induced mutagenesis are still promising if efficiently explored in breeding applications. Precise identification of casual mutation is a prerequisite for the molecular understanding of the trait development as well as its utilization for the breeding program. Recent advances in sequencing techniques provide an opportunity for the precise detection of mutagenesis-induced sequence variations at a large scale in the genome. Here, we reviewed several novel next-generation sequencing based mutation mapping approaches including Mutmap, MutChromeSeq, and whole-genome sequencing-based mapping which has enormous potential to accelerate the mutation breeding in tomato. The proper utilization of the existing well-characterized tomato mutant resources combined with novel mapping approaches would inevitably lead to rapid enhancement of tomato quality and yield. This article provides an overview of the principles and applications of mutagenesis approaches in tomato and discusses the current progress and challenges involved in tomato mutagenesis research.

Identification of genes involved in fruit development/ripening in Capsicum and development of functional markers

The molecular mechanism of the underlying genes involved in the process of fruit ripening in Capsicum (family Solanaceae) is not clearly known. In the present study, we identified orthologs of 32 fruit development/ripening genes of tomato in Capsicum, and validated their expression in fruit development stages in C. annuum, C. frutescens, and C. chinense. In silico expression analysis using transcriptome data identified a total of 12 out of 32 genes showing differential expression during different stages of fruit development in Capsicum. Real time expression identified gene LOC107847473 (ortholog of MADS-RIN) had substantially higher expression (>500 folds) in breaker and mature fruits, which suggested the non-climacteric ripening behaviour of Capsicum. However, differential expression of Ehtylene receptor 2-like (LOC107873245) gene during fruit maturity supported the climacteric behaviour of only C. frutescens (hot pepper). Furthermore, development of 49 gene based simple sequence repeat (SSR) markers would help in selection of identified genes in Capsicum breeding.

Ectopic expression of ORANGE promotes carotenoid accumulation and fruit development in tomato

Carotenoids are critically important to plants and humans. The ORANGE (OR) gene is a key regulator for carotenoid accumulation, but its physiological roles in crops remain elusive. In this study, we generated transgenic tomato ectopically overexpressing the Arabidopsis wild-type OR (AtOR^WT ) and a 'golden SNP'-containing OR (AtOR^His ). We found that AtOR^His initiated chromoplast formation in very young fruit and stimulated carotenoid accumulation at all fruit developmental stages, uncoupled from other ripening activities. The elevated levels of carotenoids in the AtOR lines were distributed in the same subplastidial fractions as in wild-type tomato, indicating an adaptive response of plastids to sequester the increased carotenoids. Microscopic analysis revealed that the plastid sizes were increased in both AtOR^WT and AtOR^His lines at early fruit developmental stages. Moreover, AtOR overexpression promoted early flowering, fruit set and seed production. Ethylene production and the expression of ripening-associated genes were also significantly increased in the AtOR transgenic fruit at ripening stages. RNA-Seq transcriptomic profiling highlighted the primary effects of OR overexpression on the genes in the processes related to RNA, protein and signalling in tomato fruit. Taken together, these results expand our understanding of OR in mediating carotenoid accumulation in plants and suggest additional roles of OR in affecting plastid size as well as flower and fruit development, thus making OR a target gene not only for nutritional biofortification of agricultural products but also for alteration of horticultural traits.

Time-Course Transcriptome Analysis of Arabidopsis Siliques Discloses Genes Essential for Fruit Development and Maturation

Fruits protect the developing seeds of angiosperms and actively contribute to seed dispersion. Furthermore, fruit and seed development are highly synchronized and require exchange of information between the mother plant and the developing generations. To explore the mechanisms controlling fruit formation and maturation, we performed a transcriptomic analysis on the valve tissue of the Arabidopsis (Arabidopsis thaliana) silique using RNA sequencing. In doing so, we have generated a data set of differentially regulated genes that will help to elucidate the molecular mechanisms that underpin the initial phase of fruit growth and, subsequently, trigger fruit maturation. The robustness of our data set has been tested by functional genomic studies. Using a reverse genetics approach, we selected 10 differentially expressed genes and explored the consequences of their disruption for both silique growth and senescence. We found that genes contained in our data set play essential roles in different stages of silique development and maturation, indicating that our transcriptome-based gene list is a powerful tool for the elucidation of the molecular mechanisms controlling fruit formation in Arabidopsis.

BEL1-LIKE HOMEODOMAIN 11 regulates chloroplast development and chlorophyll synthesis in tomato fruit

Chloroplast development and chlorophyll(Chl)metabolism in unripe tomato contribute to the growth and quality of the fruit, however these mechanisms are poorly understood. In this study, we initially investigated seven homeobox-containing transcription factors (TFs) with specific ripening-associated expression patterns using virus-induced gene silencing (VIGS) technology and found that inhibiting the expression of one of these TFs, BEL1-LIKE HOMEODOMAIN11 (SlBEL11), significantly increased Chl levels in unripe tomato fruit. This enhanced Chl accumulation was further validated by generating stable RNA interference (RNAi) transgenic lines. RNA sequencing (RNA-seq) of RNAi-SlBEL11 fruit at the mature green (MG) stage showed that 48 genes involved in Chl biosynthesis, photosynthesis and chloroplast development were significantly upregulated compared with the wild type (WT) fruit. Genomic global scanning for Homeobox TF binding sites combined with RNA-seq differential gene expression analysis showed that 22 of these 48 genes were potential target genes of SlBEL11 protein. These genes included Chl biosynthesis-related genes encoding for protochlorophyllide reductase (POR), magnesium chelatase H subunit (CHLH) and chlorophyllide a oxygenase (CAO), and chloroplast development-related genes encoding for chlorophyll a/b binding protein (CAB), homeobox protein knotted 2 (TKN2) and ARABIDOPSIS PSEUDO RESPONSE REGULATOR 2-LIKE (APRR2-like). Electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation quantitative polymerase chain reaction (PCR) (ChIP-qPCR) assays were employed to verify that SlBEL11 protein could bind to the promoters for TKN2, CAB and POR. Taken together, our findings demonstrated that SlBEL11 plays an important role in chloroplast development and Chl synthesis in tomato fruit.

AtPDS overexpression in tomato: exposing unique patterns of carotenoid self-regulation and an alternative strategy for the enhancement of fruit carotenoid content

The regulation of plant carotenogenesis is an active research area for both biological discovery and practical implementation. In tomato (Solanum lycopersicum), we demonstrate additional bottlenecks exist in the poly-cis-transformation of phytoene to lycopene in the context of ripening-induced PSY1 expression and activity and reveal phytoene desaturase (PDS), as a target for manipulation towards elevated lycopene content in maturing tomato fruit. Overexpression of Arabidopsis PDS, AtPDS, elevated PDS transcript abundance in all aerial tissues resulting in both altered carotenoid accumulation and associated pathway gene expression in a tissue-specific manner. Significant increases in downstream carotenoids (all-trans-lycopene and β-carotene) and minimal changes in carotenogenic gene expression (carotenoid isomerase-like 1, CRTIL1) suggest overexpression of heterologous AtPDS in tomato circumvents endogenous regulatory mechanism observed with previous strategies. In transgenic leaves, depletion of the PDS substrate, phytoene, was accompanied by minor, but significant increases in xanthophyll production. Alterations in the leaf carotenogenic transcript profile, including the upstream MEP pathway, were observed revealing unique feedback and feedforward regulatory mechanisms in response to AtPDS overexpression. AtPDS overexpression in the background of the tangerine (carotenoid isomerase, CRTISO) mutant exposes its potential in elevating downstream cis-lycopene accumulation in ripe tomato fruit, as cis-lycopene is more bioavailable yet less abundant than all-trans-lycopene in the wild-type control. In summary, we demonstrate the limitation of PDS in ripening fruit, its utility in modifying carotenoid profiles towards improved quality, and reveal novel carotenoid pathway feedback regulation.

A reference genome for Nicotiana tabacum enables map-based cloning of homeologous loci implicated in nitrogen utilization efficiency

BACKGROUND

Tobacco (Nicotiana tabacum) is an important plant model system that has played a key role in the early development of molecular plant biology. The tobacco genome is large and its characterisation challenging because it is an allotetraploid, likely arising from hybridisation between diploid N. sylvestris and N. tomentosiformis ancestors. A draft assembly was recently published for N. tabacum, but because of the aforementioned genome complexities it was of limited utility due to a high level of fragmentation.

RESULTS

Here we report an improved tobacco genome assembly, which, aided by the application of optical mapping, achieves an N50 size of 2.17 Mb and enables anchoring of 64% of the genome to pseudomolecules; a significant increase from the previous value of 19%. We use this assembly to identify two homeologous genes that explain the differentiation of the burley tobacco market class, with potential for greater understanding of Nitrogen Utilization Efficiency and Nitrogen Use Efficiency in plants; an important trait for future sustainability of agricultural production.

CONCLUSIONS

Development of an improved genome assembly for N. tabacum enables what we believe to be the first successful map-based gene discovery for the species, and demonstrates the value of an improved assembly for future research in this model and commercially-important species.

Transcriptome and proteomic analyses reveal multiple differences associated with chloroplast development in the spaceflight-induced wheat albino mutant mta

Chloroplast development is an integral part of plant survival and growth, and occurs in parallel with chlorophyll biosynthesis. However, little is known about the mechanisms underlying chloroplast development in hexaploid wheat. Here, we obtained a spaceflight-induced wheat albino mutant mta. Chloroplast ultra-structural observation showed that chloroplasts of mta exhibit abnormal morphology and distribution compared to wild type. Photosynthetic pigments content was also significantly decreased in mta. Transcriptome and chloroplast proteome profiling of mta and wild type were done to identify differentially expressed genes (DEGs) and proteins (DEPs), respectively. In total 4,588 DEGs including 1,980 up- and 2,608 down-regulated, and 48 chloroplast DEPs including 15 up- and 33 down-regulated were identified in mta. Classification of DEGs revealed that most were involved in chloroplast development, chlorophyll biosynthesis, or photosynthesis. Besides, transcription factors such as PIF3, GLK and MYB which might participate in those pathways were also identified. The correlation analysis between DEGs and DEPs revealed that the transcript-to-protein in abundance was functioned into photosynthesis and chloroplast relevant groups. Real time qPCR analysis validated that the expression level of genes encoding photosynthetic proteins was significantly decreased in mta. Together, our results suggest that the molecular mechanism for albino leaf color formation in mta is a thoroughly regulated and complicated process. The combined analysis of transcriptome and proteome afford comprehensive information for further research on chloroplast development mechanism in wheat. And spaceflight provides a potential means for mutagenesis in crop breeding.

Essentials of Proteolytic Machineries in Chloroplasts

Plastids are unique organelles that can alter their structure and function in response to environmental and developmental stimuli. Chloroplasts are one type of plastid and are the sites for various metabolic processes, including photosynthesis. For optimal photosynthetic activity, the chloroplast proteome must be properly shaped and maintained through regulated proteolysis and protein quality control mechanisms. Enzymatic functions and activities are conferred by protein maturation processes involving consecutive proteolytic reactions. Protein abundances are optimized by the balanced protein synthesis and degradation, which is depending on the metabolic status. Malfunctioning proteins are promptly degraded. Twenty chloroplast proteolytic machineries have been characterized to date. Specifically, processing peptidases and energy-driven processive proteases are the major players in chloroplast proteome biogenesis, remodeling, and maintenance. Recently identified putative proteases are potential regulators of photosynthetic functions. Here we provide an updated, comprehensive overview of chloroplast protein degradation machineries and discuss their importance for photosynthesis. Wherever possible, we also provide structural insights into chloroplast proteases that implement regulated proteolysis of substrate proteins/peptides.

Silencing of the SlNAP7 gene influences plastid development and lycopene accumulation in tomato

Ripening is an important stage of fruit development. To screen the genes associated with pigment formation in tomato fruit, a suppression subtractive hybridization (SSH) cDNA library was constructed by using tomato fruit in the green ripe and break ripe stages, and 129 differential genes were obtained. Using redness as a screening marker, virus-induced gene silencing (VIGS) of the differential genes was performed with a sprout vacuum-infiltration system (SVI). The results showed that silencing the SlNAP7 gene affected the chloroplast development of tomato leaves, manifesting as a photo-bleaching phenotype, and silenced fruit significantly affected the accumulation of lycopene, manifested as a yellow phenotype. In our study, we found that silencing the SlNAP7 gene downregulates the expression of the POR and PORA genes and destroys the normal development of the chloroplast. The expression of related genes included in the lycopene biosynthesis pathway was not significantly changed, but lycopene accumulation was significantly reduced in tomato fruit. Perhaps it was caused by the destruction of the chromoplast, which leads to the oxidation of lycopene. The results show that the SlNAP7 gene influences chloroplast development and lycopene accumulation in tomato.

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.

Map-based cloning, identification and characterization of the w gene controlling white immature fruit color in cucumber (Cucumis sativus L.)

A single-nucleotide insertion resulted in a premature stop codon that is responsible for white immature fruit color in cucumber. Despite our previous progress in the mapping of the gene controlling white color in immature cucumber fruit and the identification of candidate genes, the specific gene that governs chlorophyll metabolism and its regulatory mechanism remains unknown. Here, we generated a mapping population consisting of 9497 F2 plants to delimit the controlling gene to an 8.2-kb physical interval that defines a sole candidate gene, APRR2. Sequencing the full-length DNA and cDNA of APRR2 allowed for identification of an allele, aprr2, encoding a truncated 101-amino acid protein due to a frameshift mutation and a premature stop codon. Gene structure prediction indicated that these 101 residues are located in a domain necessary for the function of the protein. The expression patterns of APRR2 were entirely consistent with the visual changes in green color intensity during fruit development. A microscopic observation of the fruit pericarp revealed fewer chloroplasts and a lower chloroplast chlorophyll storage capacity in Q24 (white) than in Q30 (green). A single-base insertion in the white color gene w, which leads to a premature stop codon, is hypothesized to have disabled the function of this gene in chlorophyll accumulation and chloroplast development. These findings contribute to basic research and the genetic improvement of fruit color.

Overexpression of SlRBZ Results in Chlorosis and Dwarfism through Impairing Chlorophyll, Carotenoid, and Gibberellin Biosynthesis in Tomato

ZFPs play important roles in many biological processes, including plant development, stress response, and phytohormone response. RanBP2-type zinc finger transcription factors have been characterized in animals and humans. However, their functions remain largely unknown in plants. In this study, we identified a RanBP2-type zinc finger protein gene (SlRBZ) in tomato. SlRBZ was constitutively expressed in roots, stems, leaves, flowers, and fruits. The SlRBZ-GFP fused protein was localized in the nucleus. Overexpression of SlRBZ resulted in chlorosis and dwarf phenotypes in tomato. Determination of physiological index showed that chlorophyll, carotenoid, and GAs contents were evidently decreased in transgenic plants. Furthermore, the qRT-PCR and RNA-Seq analyses demonstrated that the transcription of the genes involved in these biosynthesis pathways obviously decreased in SlRBZ-OE plants. In addition, ultrastructural observation by transmission electron microscopy indicated that plastids could not develop into mature chloroplasts with normal chloroplast membrane and thylakoid membrane system in SlRBZ-OE plants. The results suggest that overexpression of SlRBZ may impair the biosynthesis of chlorophyll, carotenoid, and gibberellin through blocking chloroplast development, resulting in chlorosis and dwarfism in tomato.

Oryza sativa Chloroplast Signal Recognition Particle 43 (OscpSRP43) Is Required for Chloroplast Development and Photosynthesis

A rice chlorophyll-deficient mutant w67 was isolated from an ethyl methane sulfonate (EMS)-induced IR64 (Oryza sativa L. ssp. indica) mutant bank. The mutant exhibited a distinct yellow-green leaf phenotype in the whole plant growth duration with significantly reduced levels of chlorophyll and carotenoid, impaired chloroplast development and lowered capacity of photosynthesis compared with the wild-type IR64. Expression of a number of genes associated with chlorophyll metabolism, chloroplast biogenesis and photosynthesis was significantly altered in the mutant. Genetic analysis indicated that the yellow-green phenotype was controlled by a single recessive nuclear gene located on the short arm of chromosome 3. Using map-based strategy, the mutation was isolated and predicted to encode a chloroplast signal recognition particle 43 KD protein (cpSRP43) with 388 amino acid residuals. A single base substitution from A to T at position 160 resulted in a premature stop codon. OscpSRP43 was constitutively expressed in various organs with the highest level in the leaf. Functional complementation could rescue the mutant phenotype and subcellular localization showed that the cpSRP43:GFP fusion protein was targeted to the chloroplast. The data suggested that Oryza sativa cpSRP43 (OscpSRP43) was required for the normal development of chloroplasts and photosynthesis in rice.

STATE TRANSITION7-Dependent Phosphorylation Is Modulated by Changing Environmental Conditions, and Its Absence Triggers Remodeling of Photosynthetic Protein Complexes

In plants and algae, the serine/threonine kinase STN7/STT7, orthologous protein kinases in Chlamydomonas reinhardtii and Arabidopsis (Arabidopsis thaliana), respectively, is an important regulator in acclimation to changing light environments. In this work, we assessed STT7-dependent protein phosphorylation under high light in C. reinhardtii, known to fully induce the expression of light-harvesting complex stress-related protein3 (LHCSR3) and a nonphotochemical quenching mechanism, in relationship to anoxia where the activity of cyclic electron flow is stimulated. Our quantitative proteomics data revealed numerous unique STT7 protein substrates and STT7-dependent protein phosphorylation variations that were reliant on the environmental condition. These results indicate that STT7-dependent phosphorylation is modulated by the environment and point to an intricate chloroplast phosphorylation network responding in a highly sensitive and dynamic manner to environmental cues and alterations in kinase function. Functionally, the absence of the STT7 kinase triggered changes in protein expression and photoinhibition of photosystem I (PSI) and resulted in the remodeling of photosynthetic complexes. This remodeling initiated a pronounced association of LHCSR3 with PSI-light harvesting complex I (LHCI)-ferredoxin-NADPH oxidoreductase supercomplexes. Lack of STT7 kinase strongly diminished PSII-LHCII supercomplexes, while PSII core complex phosphorylation and accumulation were significantly enhanced. In conclusion, our study provides strong evidence that the regulation of protein phosphorylation is critical for driving successful acclimation to high light and anoxic growth environments and gives new insights into acclimation strategies to these environmental conditions.

Fruits from ripening impaired, chlorophyll degraded and jasmonate insensitive tomato mutants have altered tocopherol content and composition

Since isoprenoids are precursors in chlorophyll, carotenoid and tocopherol pathways, the study of their metabolism is of fundamental importance in understanding the regulatory cross-talk that contributes to the nutritional quality of tomato fruits. By means of an integrated analysis of metabolite and gene expression profiles, isoprenoid metabolism was dissected in ripening-impaired (ripening inhibitor and non-ripening), senescence-related (lutescent1 and green flesh) and jasmonate insensitive (jasmonic acid insensitive 1-1) tomato mutants, all in the Micro-Tom genetic background. It was found that the more upstream the location of the mutated gene, the more extensive the effect on the transcriptional profiles of the isoprenoid-related genes. Although there was a distinct effect in the analyzed mutations on chlorophyll, carotenoid and tocopherol metabolism, a metabolic adjustment was apparent such the antioxidant capacity mostly remained constant. Transcriptional profiles from fruits of ripening and senescence-related tomato mutants suggested that maintenance of the de novo phytyl diphosphate synthesis might, in later ripening stages, compensate for the lack of chlorophyll-derived phytol used in tocopherol production. Interestingly, an impairment in jasmonate perception led to higher total tocopherol levels in ripe fruits, accompanied by an increase in antioxidant capacity, highlighting the contribution of tocopherols to this nutritionally important trait.

Protein maturation and proteolysis in plant plastids, mitochondria, and peroxisomes

Plastids, mitochondria, and peroxisomes are key organelles with dynamic proteomes in photosynthetic eukaryotes. Their biogenesis and activity must be coordinated and require intraorganellar protein maturation, degradation, and recycling. The three organelles together are predicted to contain ∼200 presequence peptidases, proteases, aminopeptidases, and specific protease chaperones/adaptors, but the substrates and substrate selection mechanisms are poorly understood. Similarly, lifetime determinants of organellar proteins, such as N-end degrons and tagging systems, have not been identified, but the substrate recognition mechanisms likely share similarities between organelles. Novel degradomics tools for systematic analysis of protein lifetime and proteolysis could define such protease-substrate relationships, degrons, and protein lifetime. Intraorganellar proteolysis is complemented by autophagy of whole organelles or selected organellar content, as well as by cytosolic protein ubiquitination and degradation by the proteasome. This review summarizes (putative) plant organellar protease functions and substrate-protease relationships. Examples illustrate key proteolytic events.

Plastid intramembrane proteolysis

Progress in the field of regulated intramembrane proteolysis (RIP) in recent years has not surpassed plant biology. Nevertheless, reports on RIP in plants, and especially in chloroplasts, are still scarce. Of the four different families of intramembrane proteases, only two have been linked to chloroplasts so far, rhomboids and site-2 proteases (S2Ps). The lack of chloroplast-located rhomboid proteases was associated with reduced fertility and aberrations in flower morphology, probably due to perturbations in jasmonic acid biosynthesis, which occurs in chloroplasts. Mutations in homologues of S2P resulted in chlorophyll deficiency and impaired chloroplast development, through a yet unknown mechanism. To date, the only known substrate of RIP in chloroplasts is a PHD transcription factor, located in the envelope. Upon proteolytic cleavage by an unknown protease, the soluble N-terminal domain of this protein is released from the membrane and relocates to the nucleus, where it activates the transcription of the ABA response gene ABI4. Continuing studies on these proteases and substrates, as well as identification of the genes responsible for different chloroplast mutant phenotypes, are expected to shed more light on the roles of intramembrane proteases in chloroplast biology.

Analysis of natural and induced variation in tomato glandular trichome flavonoids identifies a gene not present in the reference genome

Flavonoids are ubiquitous plant aromatic specialized metabolites found in a variety of cell types and organs. Methylated flavonoids are detected in secreting glandular trichomes of various Solanum species, including the cultivated tomato (Solanum lycopersicum). Inspection of the sequenced S. lycopersicum Heinz 1706 reference genome revealed a close homolog of Solanum habrochaites MOMT1 3'/5' myricetin O-methyltransferase gene, but this gene (Solyc06g083450) is missing the first exon, raising the question of whether cultivated tomato has a distinct 3' or 3'/5' O-methyltransferase. A combination of mining genome and cDNA sequences from wild tomato species and S. lycopersicum cultivar M82 led to the identification of Sl-MOMT4 as a 3' O-methyltransferase. In parallel, three independent ethyl methanesulfonate mutants in the S. lycopersicum cultivar M82 background were identified as having reduced amounts of di- and trimethylated myricetins and increased monomethylated myricetin. Consistent with the hypothesis that Sl-MOMT4 is a 3' O-methyltransferase gene, all three myricetin methylation defective mutants were found to have defects in MOMT4 sequence, transcript accumulation, or 3'-O-methyltransferase enzyme activity. Surprisingly, no MOMT4 sequence is found in the Heinz 1706 reference genome sequence, and this cultivar accumulates 3-methyl myricetin and is deficient in 3'-methyl myricetins, demonstrating variation in this gene among cultivated tomato varieties.

Increasing tomato fruit quality by enhancing fruit chloroplast function. A double-edged sword?

Fruits are generally regarded as photosynthate sinks as they rely on energy provided by sugars transported from leaves to carry out the highly demanding processes of development and ripening; eventually these imported photosynthates also contribute to the fruit organoleptic properties. Three recent reports have revealed, however, that transcriptional factors enhancing chloroplast development in fruit may result in higher contents not only of tomato fruit-specialized metabolites but also of sugars. In addition to suggesting new ways to improve fruit quality by fortifying fruit chloroplasts and plastids, these results prompted us to re-evaluate the importance of the contribution of chloroplasts/photosynthesis to fruit development and ripening.

KNOX genes influence a gradient of fruit chloroplast development through regulation of GOLDEN2-LIKE expression in tomato

The chlorophyll content of unripe fleshy fruits is positively correlated with the nutrient content and flavor of ripe fruit. In tomato (Solanum lycopersicum) fruit, the uniform ripening (u) locus, which encodes a GOLDEN 2-LIKE transcription factor (SlGLK2), influences a gradient of chloroplast development that extends from the stem end of the fruit surrounding the calyx to the base of the fruit. With the exception of the u locus, the factors that influence the formation of this developmental gradient are unknown. In this study, characterization and positional cloning of the uniform gray-green (ug) locus of tomato reveals a thus far unknown role for the Class I KNOTTED1-LIKE HOMEOBOX (KNOX) gene, TKN4, in specifying the formation of this chloroplast gradient. The involvement of KNOX in fruit chloroplast development was confirmed through characterization of the Curl (Cu) mutant, a dominant gain-of-function mutation of TKN2, which displays ectopic fruit chloroplast development that resembles SlGLK2 over-expression. TKN2 and TKN4 act upstream of SlGLK2 and the related gene ARABIDOPSIS PSEUDO RESPONSE REGULATOR 2-LIKE (SlAPRR2-LIKE) to establish their latitudinal gradient of expression across developing fruit that leads to a gradient of chloroplast development. Class I KNOX genes typically influence plant morphology through maintenance of meristem activity, but this study identifies a role for TKN2 and TKN4 in specifically influencing chloroplast development in fruit but not leaves, suggesting that this fundamental process is differentially regulated in these two organs.

The tomato mutation nxd1 reveals a gene necessary for neoxanthin biosynthesis and demonstrates that violaxanthin is a sufficient precursor for abscisic acid biosynthesis

Carotenoid pigments are indispensable for plant life. They are synthesized within plastids where they provide essential functions in photosynthesis. Carotenoids serve as precursors for the synthesis of the strigolactone phytohormones, which are made from β-carotene, and of abscisic acid (ABA), which is produced from certain xanthophylls. Despite the significant progress that has been made in our understanding of the carotenoid biosynthesis pathway, the synthesis of the xanthophyll neoxanthin has remained unknown. We report here on the isolation of a tomato (Solanum lycopersicum) mutant, neoxanthin-deficient 1 (nxd1), which lacks neoxanthin, and on the cloning of a gene that is necessary for neoxanthin synthesis in both tomato and Arabidopsis. The locus nxd1 encodes a gene of unknown function that is conserved in all higher plants. The activity of NXD1 is essential but cannot solely support neoxanthin synthesis. Lack of neoxanthin does not significantly reduce the fitness of tomato plants in cultivated field conditions and does not impair the synthesis of ABA, suggesting that in tomato violaxanthin is a sufficient precursor for ABA production in vivo.

Genetic regulation and structural changes during tomato fruit development and ripening

Fruits are an important evolutionary acquisition of angiosperms, which afford protection for seeds and ensure their optimal dispersal in the environment. Fruits can be divided into dry or fleshy. Dry fruits are the more ancient and provide for mechanical seed dispersal. In contrast, fleshy fruits develop soft tissues in which flavor compounds and pigments accumulate during the ripening process. These serve to attract animals that eat them and disseminate the indigestible seeds. Fruit maturation is accompanied by several striking cytological modifications. In particular, plastids undergo significant structural alterations, including the dedifferentiation of chloroplasts into chromoplasts. Chloroplast biogenesis, their remodeling in response to environmental constraints and their conversion into alternative plastid types are known to require communication between plastids and the nucleus in order to coordinate the expression of their respective genomes. In this review, we discuss the role of plastid modifications in the context of fruit maturation and ripening, and consider the possible involvement of organelle-nucleus crosstalk via retrograde (plastid to nucleus) and anterograde (nucleus to plastid) signaling in the process.

Emerging roles for diverse intramembrane proteases in plant biology

Progress in the field of regulated intramembrane proteolysis (RIP) in recent years has made its impact on plant biology as well. Although this field within plant research is still in its infancy, some interesting observations have started to emerge. Gene encoding orthologs of rhomboid proteases, site-2 proteases (S2P), presenilin/γ-secretases, and signal peptide peptidases are found in plant genomes and some of these gene products were identified in different plant cell membranes. The lack of chloroplast-located rhomboid proteases was associated with reduced fertility and aberrations in flower morphology. Mutations in homologues of S2P resulted in chlorophyll deficiency and impaired chloroplast development. An S2P was also implicated in the response to ER stress through cleavage of ER-membrane bZIP transcription factors, allowing their migration to the nucleus and activation of the transcription of BiP chaperones. Other membrane-bound transcription factors of the NAC and PHD families were also demonstrated to undergo RIP and relocalization to the nucleus. These and other new data are expected to shed more light on the roles of intramembrane proteases in plant biology in the future. This article is part of a Special Issue entitled: Intramembrane Proteases.