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

Identification and transcriptome analysis of a photosynthesis deficient mutant of Populus davidiana Dode

Photosynthesis is the main source of energy for plants to sustain growth and development. Abnormalities in photosynthesis may cause defects in plant development. The elaborate regulatory mechanism underlying photosynthesis remains unclear. In this study, we identified a natural mutant from the Greater Khingan Mountains and named it as "1-T". This mutant had variegated leaf with irregular distribution of yellow and green. Chlorophyll contents and photosynthetic capacity of 1-T were significantly reduced compared to other poplar genotypes. Furthermore, a transcriptome analysis revealed 3269 differentially expressed genes (DEGs) in 1-T. The products of the DEGs were enriched in photosystem I and photosystem II. Three motifs were significantly enriched in the promoters of these DEGs. Yeast one-hybrid, Electrophoretic mobility shift assays and tobacco transient transformation experiments indicated that PdGLKs may bind to the three motifs. Further analysis indicated that these photosystem related genes were also significantly down-regulated in PdGLK-RNAi poplars. Therefore, we preliminarily concluded that the down-regulation of PdGLKs in 1-T may affect the expression of photosystem-related genes, resulting in abnormal photosystem development and thus affecting the growth and development. Our results provide new insights into the molecular mechanism of photosynthesis regulating plant growth.

Intra-chloroplast proteases: A holistic network view of chloroplast proteolysis

Different proteases and peptidases are present within chloroplasts and nonphotosynthetic plastids to process precursor proteins and to degrade cleaved chloroplast transit peptides and damaged, misfolded, or otherwise unwanted proteins. Collectively, these proteases and peptidases form a proteolysis network, with complementary activities and hierarchies, and build-in redundancies. Furthermore, this network is distributed across the different intra-chloroplast compartments (lumen, thylakoid, stroma, envelope). The challenge is to determine the contributions of each peptidase (system) to this network in chloroplasts and nonphotosynthetic plastids. This will require an understanding of substrate recognition mechanisms, degrons, substrate, and product size limitations, as well as the capacity and degradation kinetics of each protease. Multiple extra-plastidial degradation pathways complement these intra-chloroplast proteases. This review summarizes our current understanding of these intra-chloroplast proteases in Arabidopsis and crop plants with an emphasis on considerations for building a qualitative and quantitative network view.

REDUCED CHLOROPLAST COVERAGE proteins are required for plastid proliferation and carotenoid accumulation in tomato

Increasing the amount of cellular space allocated to plastids will lead to increases in the quality and yield of crop plants. However, mechanisms that allocate cellular space to plastids remain poorly understood. To test whether the tomato (Solanum lycopersicum L.) REDUCED CHLOROPLAST COVERAGE (SlREC) gene products serve as central components of the mechanism that allocates cellular space to plastids and contribute to the quality of tomato fruit, we knocked out the 4-member SlREC gene family. We found that slrec mutants accumulated lower levels of chlorophyll in leaves and fruits, accumulated lower levels of carotenoids in flowers and fruits, allocated less cellular space to plastids in leaf mesophyll and fruit pericarp cells, and developed abnormal plastids in flowers and fruits. Fruits produced by slrec mutants initiated ripening later than wild type and produced abnormal levels of ethylene and abscisic acid (ABA). Metabolome and transcriptome analyses of slrec mutant fruits indicated that the SlREC gene products markedly influence plastid-related gene expression, primary and specialized metabolism, and the response to biotic stress. Our findings and previous work with distinct species indicate that REC proteins help allocate cellular space to plastids in diverse species and cell types and, thus, play a central role in allocating cellular space to plastids. Moreover, the SlREC proteins are required for the high-level accumulation of chlorophyll and carotenoids in diverse organs, including fruits, promote the development of plastids and influence fruit ripening by acting both upstream and downstream of ABA biosynthesis in a complex network.

Integrative analysis of the methylome and transcriptome of tomato fruit (Solanum lycopersicum L.) induced by postharvest handling

Tomato fruit ripening is triggered by the demethylation of key genes, which alters their transcriptional levels thereby initiating and propagating a cascade of physiological events. What is unknown is how these processes are altered when fruit are ripened using postharvest practices to extend shelf-life, as these practices often reduce fruit quality. To address this, postharvest handling-induced changes in the fruit DNA methylome and transcriptome, and how they correlate with ripening speed, and ripening indicators such as ethylene, abscisic acid, and carotenoids, were assessed. This study comprehensively connected changes in physiological events with dynamic molecular changes. Ripening fruit that reached 'Turning' (T) after dark storage at 20°C, 12.5°C, or 5°C chilling (followed by 20°C rewarming) were compared to fresh-harvest fruit 'FHT'. Fruit stored at 12.5°C had the biggest epigenetic marks and alterations in gene expression, exceeding changes induced by postharvest chilling. Fruit physiological and chronological age were uncoupled at 12.5°C, as the time-to-ripening was the longest. Fruit ripening to Turning at 12.5°C was not climacteric; there was no respiratory or ethylene burst, rather, fruit were high in abscisic acid. Clear differentiation between postharvest-ripened and 'FHT' was evident in the methylome and transcriptome. Higher expression of photosynthetic genes and chlorophyll levels in 'FHT' fruit pointed to light as influencing the molecular changes in fruit ripening. Finally, correlative analyses of the -omics data putatively identified genes regulated by DNA methylation. Collectively, these data improve our interpretation of how tomato fruit ripening patterns are altered by postharvest practices, and long-term are expected to help improve fruit quality.

Leaky mutations in the zeaxanthin epoxidase in Capsicum annuum result in bright-red fruit containing a high amount of zeaxanthin

Fruit color is one of the most important traits in peppers due to its esthetic value and nutritional benefits and is determined by carotenoid composition, resulting from diverse mutations of carotenoid biosynthetic genes. The EMS204 line, derived from an EMS mutant population, presents bright-red color, compared with the wild type Yuwolcho cultivar. HPLC analysis indicates that EMS204 fruit contains more zeaxanthin and less capsanthin and capsorubin than Yuwolcho. MutMap was used to reveal the color variation of EMS204 using an F3 population derived from a cross of EMS204 and Yuwolcho, and the locus was mapped to a 2.5-Mbp region on chromosome 2. Among the genes in the region, a missense mutation was found in ZEP (zeaxanthin epoxidase) that results in an amino acid sequence alteration (V291 → I). A color complementation experiment with Escherichia coli and ZEP in vitro assay using thylakoid membranes revealed decreased enzymatic activity of EMS204 ZEP. Analysis of endogenous plant hormones revealed a significant reduction in abscisic acid content in EMS204. Germination assays and salinity stress experiments corroborated the lower ABA levels in the seeds. Virus-induced gene silencing showed that ZEP silencing also results in bright-red fruit containing less capsanthin but more zeaxanthin than control. A germplasm survey of red color accessions revealed no similar carotenoid profiles to EMS204. However, a breeding line containing a ZEP mutation showed a very similar carotenoid profile to EMS204. Our results provide a novel breeding strategy to develop red pepper cultivars containing high zeaxanthin contents using ZEP mutations.

Genome-wide identification of GA2ox genes family and analysis of PbrGA2ox1-mediated enhanced chlorophyll accumulation by promoting chloroplast development in pear

BACKGROUND

Chlorophyll (Chl) is an agronomic trait associated with photosynthesis and yield. Gibberellin 2-oxidases (GA2oxs) have previously been shown to be involved in Chl accumulation. However, whether and how the PbrGA2ox proteins (PbrGA2oxs) mediate Chl accumulation in pear (Pyrus spp.) is scarce.

RESULTS

Here, we aimed to elucidate the role of the pear GA2ox gene family in Chl accumulation and the related underlying mechanisms. We isolated 13 PbrGA2ox genes (PbrGA2oxs) from the pear database and identified PbrGA2ox1 as a potential regulator of Chl accumulation. We found that transiently overexpressing PbrGA2ox1 in chlorotic pear leaves led to Chl accumulation, and PbrGA2ox1 silencing in normal pear leaves led to Chl degradation, as evident by the regreening and chlorosis phenomenon, respectively. Meanwhile, PbrGA2ox1-overexpressing (OE) tobacco plants discernably exhibited Chl built-up, as evidenced by significantly higher Pn and Fv/Fm. In addition, RNA sequencing (RNA-seq), physiological and biochemical investigations revealed an increase in abscisic acid (ABA), methyl jasmonate (MeJA), and salicylic acid (SA) concentrations and signaling pathways; a marked elevation in reducing and soluble sugar contents; and a marginal decline in the starch and sucrose levels in OE plants. Interestingly, PbrGA2ox1 overexpression did not prominently affect Chl synthesis. However, it indeed facilitated chloroplast development by increasing chloroplast number per cell and compacting the thylakoid granum stacks. These findings might jointly contribute to Chl accumulation in OE plants.

CONCLUSION

Overall, our results suggested that GA2oxs accelerate Chl accumulation by stimulating chloroplast development and proved the potential of PbrGA2ox1 as a candidate gene for genetically breeding biofortified pear plants with a higher yield.

Guide RNA scaffold variants enabled easy cloning of large gRNA cluster for multiplexed gene editing

Cas9 protein-mediated gene editing has revolutionized genetic manipulation in most organisms. There are many cases where multiplexed gene editing is needed. Cas9 is capable of multiplex gene editing when expressed with multiple guide RNAs. Conventional cloning methods for multiplexed gene editing vector is not efficient due to repeated use of a single-guide RNA scaffold and inefficient ligation. In this study, we conducted structure-guided mutagenesis and random mutagenesis on the original sgRNA scaffold and identified a large number of functional sgRNA scaffold variants. With these scaffold variants and different tRNAs, fusion polymerase chain reaction protocol was developed to rapidly synthesize spacer-scaffold-tRNA-spacer units with up to 9 targets. In conjunction with golden gate cloning, gene editing vectors with up to 24 target sites were efficiently cloned in one-step cloning. One such gene editing vector targeting 12 genes in tomato were tested in stable transformation and 10 out of the 12 genes were found mutated in a single transgenic line. To facilitate the application of multiplexed gene editing using these scaffold variants and tRNAs from different species, a webserver was created to generate primer sets and provide template sequences for the synthesis of large sgRNA expression units based on the user-supplied target sequences and species.

Morphology, photosynthetic and molecular mechanisms associated with powdery mildew resistance in Kentucky bluegrass

Kentucky bluegrass (Poa pratensis L.), one of the most widely used cool-season turfgrasses around the world, is sensitive to powdery mildew (PM; Blumeria graminis). The PM strain identification and regulation mechanisms of Kentucky bluegrass in response to pathogens still remain unclear. Through morphological and molecular analyses, we identified that the pathogen in Kentucky bluegrass was B. graminis f. sp. poae. The infection of B. graminis led to a reduction of the sclerenchyma area, expansion of vesicular cells and movement of chloroplasts. The infected leaves had significantly lower values in net photosynthesis, stomatal conductance and transpiration rate, maximal quantum yield of PSII photochemistry, photochemical quenching and non-regulated energy dissipation compared to mock-inoculated leaves. Expressions of light-harvesting antenna protein genes LHCA and LHCB and photosynthetic electron transport genes petE and petH decreased significantly in infected leaves. Furthermore, upregulations of genes involved in plant-pathogen interaction, such as HSP90, RBOH, and RPM and downregulations of EDS, RPS and WRKY were observed in infected leaves. The findings may help design a feasible approach to effectively control the PM disease in Kentucky bluegrass and other related perennial grass species.

Heritable epigenetic modification of BpPIN1 is associated with leaf shapes in Betula pendula

The new variety Betula pendula 'Dalecarlica', selected from Betula pendula, shows high ornamental value owing to its lobed leaf shape. In this study, to identify the genetic components of leaf shape formation, we performed bulked segregant analysis and molecular marker-based fine mapping to identify the causal gene responsible for lobed leaves in B. pendula 'Dalecarlica'. The most significant variations associated with leaf shape were identified within the gene BpPIN1 encoding a member of the PIN-FORMED family, responsible for the auxin efflux carrier. We further confirmed the hypomethylation at the promoter region promoting the expression level of BpPIN1, which causes stronger and longer veins and lobed leaf shape in B. pendula 'Dalecarlica'. These results indicated that DNA methylation at the BpPIN1 promoter region is associated with leaf shapes in B. pendula. Our findings revealed an epigenetic mechanism of BpPIN1 in the regulation of leaf shape in Betula  Linn. (birch), which could help in the molecular breeding of ornamental traits.

VvPL15 Is the Core Member of the Pectate Lyase Gene Family Involved in Grape Berries Ripening and Softening

The process of ripening and softening in grape begins at veraison and is closely related to the depolymerization of pectin components. A variety of enzymes are involved in pectin metabolism and one class of enzyme, pectin lyases (PLs), have been reported to play an important role in softening in many fruits; however, little information is available on the VvPL gene family in grape. In this study, 16 VvPL genes were identified in the grape genome using bioinformatics methods. Among them, VvPL5, VvPL9, and VvPL15 had the highest expression levels during grape ripening, which suggests that these genes are involved in grape ripening and softening. Furthermore, overexpression of VvPL15 affects the contents of water-soluble pectin (WSP) and acid-soluble pectin (ASP) in the leaves of Arabidopsis and significantly changes the growth of Arabidopsis plants. The relationship between VvPL15 and pectin content was further determined by antisense expression of VvPL15. In addition, we also studied the effect of VvPL15 on fruit in transgenic tomato plants, which showed that VvPL15 accelerated fruit ripening and softening. Our results indicate that VvPL15 plays an important role in grape berry softening during ripening by depolymerizing pectin.

Identification and functional characterization of AcMYB113 in anthocyanin metabolism of Aesculus chinensis Bunge var. chinensis leaves

Anthocyanins can be induced by environmental factors such as low-temperature and play essential roles in plant color formation. In this study, leaves of Aesculus chinensis Bunge var. chinensis with different colors under natural low-temperature in autumn were collected and grouped into green leaf (GL) and red leaf (RL). To reveal the underlying mechanism of color formation in RL, a combined analysis of the metabolome and transcriptome was conducted with GL and RL. Metabolic analyses revealed that total anthocyanin content and primary anthocyanin components were increased RL relative to GL and cyanidin was the main anthocyanin compound in RL. Transcriptome analysis provided a total of 18720 differentially expressed genes (DEGs), of which 9150 DEGs were upregulated and 9570 DEGs were downregulated in RL relative to GL. KEGG analysis showed that DEGs were mainly enriched in flavonoid biosynthesis, phenylalanine metabolism, and phenylpropanoid biosynthesis. Furthermore, co-expression network analysis indicated that 56 AcMYB transcription factors were highly expressed in RL compared with GL, among which AcMYB113 (an R2R3-MYB TF) had a strong correlation with anthocyanins. Overexpression of AcMYB113 in apple resulted in dark-purple transgenic calluses. In addition, the transient expression experiment showed that AcMYB113 enhanced anthocyanin synthesis by activating pathways of anthocyanin biosynthesis in leaves of Aesculus chinensis Bunge var. chinensis. Taken together, our findings reveal new insights into the molecular mechanism of anthocyanin accumulation in RL and provide candidate genes for the breeding of anthocyanin-rich cultivars.

Melon yellow-green plant (Cmygp) encodes a Golden2-like transcription factor regulating chlorophyll synthesis and chloroplast development

A SNP mutation in CmYGP gene encoding Golden2-like transcription factor is responsible for melon yellow-green plant trait. Chlorophylls are essential and beneficial substances for both plant and human health. Identifying the regulatory network of chlorophyll is necessary to improve the nutritional quality of fruits. At least six etiolation genes have been identified in different melon varieties, but none of them have been cloned, and the molecular mechanisms underlying chlorophyll synthesis and chloroplast development in melon remain unclear. Here, the NSL73046, a yellow-green plant (Cmygp) mutant, enabled the map-based cloning of the first etiolation gene in melon. CmYGP encodes a Golden2-like transcription factor. Spatiotemporal expression analyses confirmed the high CmYGP expression in all green tissues, particularly in young leaves and fruit peels. Virus-induced gene silencing and the development of near-isogenic line by marker-assisted selection further confirmed that downregulation of CmYGP can reduce chloroplast number and chlorophyll content, thereby resulting in yellow-green leaves and fruits in melon, and overexpression of CmYGP in tomatoes also led to dark-green leaves and fruits. RNA-seq analysis revealed that CmYGP greatly affected the expression of key genes associated with chloroplast development. Taken together, these findings demonstrated that CmYGP regulate chlorophyll synthesis and chloroplast development thus affect fruit development in melon. This study also offers a new strategy to enhance fruit quality in melon.

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.

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.

Comparative transcriptomics analysis of developing peanut (Arachis hypogaea L.) pods reveals candidate genes affecting peanut seed size

Pod size is one of the most important agronomic features of peanuts, which directly affects peanut yield. Studies on the regulation mechanism underpinning pod size in cultivated peanuts remain hitherto limited compared to model plant systems. To better understand the molecular elements that underpin peanut pod development, we conducted a comprehensive analysis of chronological transcriptomics during pod development in four peanut accessions with similar genetic backgrounds, but varying pod sizes. Several plant transcription factors, phytohormones, and the mitogen-activated protein kinase (MAPK) signaling pathways were significantly enriched among differentially expressed genes (DEGs) at five consecutive developmental stages, revealing an eclectic range of candidate genes, including PNC, YUC, and IAA that regulate auxin synthesis and metabolism, CYCD and CYCU that regulate cell differentiation and proliferation, and GASA that regulates seed size and pod elongation via gibberellin pathway. It is plausible that MPK3 promotes integument cell division and regulates mitotic activity through phosphorylation, and the interactions between these genes form a network of molecular pathways that affect peanut pod size. Furthermore, two variant sites, GCP4 and RPPL1, were identified which are stable at the QTL interval for seed size attributes and function in plant cell tissue microtubule nucleation. These findings may facilitate the identification of candidate genes that regulate pod size and impart yield improvement in cultivated peanuts.

Post-translational regulation of metabolic checkpoints in plant tetrapyrrole biosynthesis

Tetrapyrrole biosynthesis produces metabolites that are essential for critical reactions in photosynthetic organisms, including chlorophylls, heme, siroheme, phytochromobilins, and their derivatives. Due to the paramount importance of tetrapyrroles, a better understanding of the complex regulation of tetrapyrrole biosynthesis promises to improve plant productivity in the context of global climate change. Tetrapyrrole biosynthesis is known to be controlled at multiple levels-transcriptional, translational and post-translational. This review addresses recent advances in our knowledge of the post-translational regulation of tetrapyrrole biosynthesis and summarizes the regulatory functions of the various auxiliary factors involved. Intriguingly, the post-translational network features three prominent metabolic checkpoints, located at the steps of (i) 5-aminolevulinic acid synthesis (the rate-limiting step in the pathway), (ii) the branchpoint between chlorophyll and heme synthesis, and (iii) the light-dependent enzyme protochlorophyllide oxidoreductase. The regulation of protein stability, enzymatic activity, and the spatial organization of the committed enzymes in these three steps ensures the appropriate flow of metabolites through the tetrapyrrole biosynthesis pathway during photoperiodic growth. In addition, we offer perspectives on currently open questions for future research on tetrapyrrole biosynthesis.

Comparative Transcriptome Analysis Identified Key Pathways and Genes Regulating Differentiated Stigma Color in Melon (Cucumis melo L.)

Stigma color is an important morphological trait in many flowering plants. Visual observations in different field experiments have shown that a green stigma in melons is more attractive to natural pollinators than a yellow one. In the current study, we evaluated the characterization of two contrasted melon lines (MR-1 with a green stigma and M4-7 with a yellow stigma). Endogenous quantification showed that the chlorophyll and carotenoid content in the MR-1 stigmas was higher compared to the M4-7 stigmas. The primary differences in the chloroplast ultrastructure at different developmental stages depicted that the stigmas of both melon lines were mainly enriched with granum, plastoglobulus, and starch grains. Further, comparative transcriptomic analysis was performed to identify the candidate pathways and genes regulating melon stigma color during key developmental stages (S1–S3). The obtained results indicated similar biological processes involved in the three stages, but major differences were observed in light reactions and chloroplast pathways. The weighted gene co-expression network analysis (WGCNA) of differentially expressed genes (DEGs) uncovered a “black” network module (655 out of 5302 genes), mainly corresponding to light reactions, light harvesting, the chlorophyll metabolic process, and the chlorophyll biosynthetic process, and exhibited a significant contribution to stigma color. Overall, the expression of five key genes of the chlorophyll synthesis pathway—CAO (MELO03C010624), CHLH (MELO03C007233), CRD (MELO03C026802), HEMA (MELO03C011113), POR (MELO03C016714)—were checked at different stages of stigma development in both melon lines using quantitative real time polymerase chain reaction (qRT-PCR). The results exhibited that the expression of these genes gradually increased during the stigma development of the MR-1 line but decreased in the M4-7 line at S2. In addition, the expression trends in different stages were the same as RNA-seq, indicating data accuracy. To sum up, our research reveals an in-depth molecular mechanism of stigma coloration and suggests that chlorophyll and related biological activity play an important role in differentiating melon stigma color.

Regulation of tomato fruit elongation by transcription factor BZR1.7 through promotion of SUN gene expression

Fruit shape is an important biological trait that is also of special commercial value in tomato. The SUN gene has been known as a key regulator of tomato fruit elongation for years, but the molecular mechanisms underlying its transcriptional regulation remain little understood. Here, a unique BZR1-like transcription factor, BZR1.7, was identified as a trans-acting factor of the SUN gene promoter that bound to the conserved E-box of the promoter to promote SUN gene expression. Overexpression of BZR1.7 in tomato led to elevated SUN gene expression and formation of elongated fruits. Plants of the BZR1.7 knockout mutant created by gene editing did not exhibit an observable fruit shape phenotype, suggesting possible functional redundancy of BZR1-like genes in tomato. There were seven BZR1-like genes in the tomato genome and overexpression of BZR1.5 and BZR1.6 led to elongated fruit phenotypes similar to those observed in the BZR1.7 overexpression lines, further supporting the notion of functional redundancy of BZR1-like genes in tomato fruit shape specification. Microscopic analysis revealed that there was a decreased number of cell layers in the fruit pericarp in the BZR1.7 overexpression lines. These findings offer new insights into the regulatory mechanism by which BZR1.7 promotes SUN gene expression and regulates fruit elongation in tomato.

Genetic analysis of chlorophyll synthesis and degradation regulated by BALANCE of CHLOROPHYLL METABOLISM

Chlorophyll (Chl) serves a number of essential functions, capturing and converting light energy as a component of photosystem supercomplexes. Chl degradation during leaf senescence is also required for adequate degeneration of chloroplasts and salvaging of nutrients from senescent leaves. In this study, we performed genetic analysis to determine the functions of BALANCE of CHLOROPHYLL METABOLISM1 (BCM1) and BCM2, which control Chl levels by regulating synthesis and degradation, and STAY-GREEN (SGR)1 (also known as NON-YELLOWING1 [NYE1]) and SGR2, which encode Mg-dechelatase and catalyze Chl a degradation in Arabidopsis (Arabidopsis thaliana). Analysis of bcm1 bcm2 revealed that both BCM1 and BCM2 are involved in the regulation of Chl levels in presenescent leaves and Chl degradation in senescing leaves. Analysis of bcm1 bcm2 nye1 nye2 suggested that BCMs repress Chl-degrading activity in both presenescent and senescing leaves by regulating SGR activity. Furthermore, transactivation analysis and chromatin immunoprecipitation (ChIP) assay revealed that GOLDEN2-LIKE1 (GLK1), a central transcription factor regulating the expression of genes encoding photosystem-related proteins, such as light-harvesting Chl a/b-binding proteins (LHCPs), directly regulates the transcription of BCM1. LHCPs are stabilized by Chl binding, suggesting that GLKs control the amount of LHCP through transcriptional and post-translational regulation via BCM-mediated Chl-level regulation. Meanwhile, we generated a mutant of the BCM ortholog in lettuce (Lactuca sativa) by genome editing and found that it showed an early yellowing phenotype, but only a slight reduction in Chl in presenescent leaves. Thus, this study revealed a conserved but slightly diversified regulation of Chl and LHCP levels via the GLK-BCM pathway in eudicots.

Genetic Mapping and QTL Analysis of Stigma Color in Melon (Cucumis melo L.)

Melon is an important Cucurbitaceae crop. Field observations had shown that the green stigmas of melon are more attractive to pollinators than yellow stigmas. In this study, F₂ and F_2:3 populations obtained by crossing MR-1 (green stigma) and M4-7 (yellow stigma) were used for genetic analysis and mapping. A genetic map of 1,802.49 cm was constructed with 116 cleaved amplified polymorphism sequence (CAPS) markers. Two stable quantitative trait loci (QTLs) linked to the trait of stigma color were identified on chromosomes 2 (SC2.1) and 8 (SC8.1), respectively. An expanded F₂ population was used to narrow down the confidence regions of SC2.1 and SC8.1. As a result, SC2.1 was further mapped to a 3.6 cm region between CAPS markers S2M3 and S2B1-3, explaining 9.40% phenotypic variation. SC8.1 was mapped to a 3.7-cm region between CAPS markers S8E7 and S8H-1, explaining 25.92% phenotypic variation. This study broadens our understanding of the mechanisms of stigma color regulation and will be of benefit to the breeding of melon.

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.