A single nucleotide mutation of IspF gene involved in the MEP pathway for isoprenoid biosynthesis causes yellow-green leaf phenotype in rice

Advances and Future Prospects of Pigment Deposition in Pigmented Rice

Pigmented rice, particularly the black and red varieties, is popular due to its better nutritional value. Anthocyanins and proanthocyanidins are two major flavonoid subcategories with broad physiological functions and therapeutic significance. However, pigment deposition is a complex process, and the molecular mechanism involved remains unknown. This review explores the metabolites responsible for the pigmentation in various rice tissues. Moreover, the current challenges, feasible strategies, and potential future directions in pigmented rice research are reported.

Four-Dimensional Data-Independent Acquisition-Based Proteomic Profiling Combined with Transcriptomic Analysis Reveals the Involvement of the Slym1-SlFHY3-CAB3C Module in Regulating Tomato Leaf Color

In green plants, the chloroplast is responsible for light energy transition and organic assimilation. However, the molecular mechanisms underlying chloroplast development in horticultural crops remain unclear. Here, four-dimensional data-independent acquisition-based proteomic profiling identified 1,727 differentially expressed proteins between "Zhongshu 4" (ZS4) and yellowing mutant (ym) leaves, a considerable proportion of which were down-regulated chloroplast proteins. Functional analysis revealed that light harvesting and chlorophyll biosynthesis were correlated with ym leaf yellowing, validated by RNA sequencing. Quantitative PCR confirmed that chlorophyll a/b-binding protein 3C (CAB3C) related to light harvesting and NADPH:protochlorophyllide oxidoreductase 3 (POR3) involved in chlorophyll biosynthesis were repressed in ym leaves. Virus-induced gene silencing showed that suppressing CAB3C and POR3 decreased the net photosynthetic rate and chlorophyll content. Additionally, the F-box protein Slym1 negatively regulated the expression of CAB3C by depressing transcription factor SlFHY3 levels. Our findings offer insights into the regulatory mechanisms of chloroplast development in tomato.

YGL3 Encoding an IPP and DMAPP Synthase Interacts with OsPIL11 to Regulate Chloroplast Development in Rice

As the source of isoprenoid precursors, the plastidial methylerythritol phosphate (MEP) pathway plays an essential role in plant development. Here, we report a novel rice (Oryza sativa L.) mutant ygl3 (yellow-green leaf3) that exhibits yellow-green leaves and lower photosynthetic efficiency compared to the wild type due to abnormal chloroplast ultrastructure and reduced chlorophyll content. Map-based cloning showed that YGL3, one of the major genes involved in the MEP pathway, encodes 4-hydroxy-3-methylbut-2-enyl diphosphate reductase, which is localized in the thylakoid membrane. A single base substitution in ygl3 plants resulted in lower 4-hydroxy-3-methylbut-2-enyl diphosphate reductase activity and lower contents of isopentenyl diphosphate (IPP) compared to the wild type. The transcript levels of genes involved in the syntheses of chlorophyll and thylakoid membrane proteins were significantly reduced in the ygl3 mutant compared to the wild type. The phytochrome interacting factor-like gene OsPIL11 regulated chlorophyll synthesis during the de-etiolation process by directly binding to the promoter of YGL3 to activate its expression. The findings provides a theoretical basis for understanding the molecular mechanisms by which the MEP pathway regulate chloroplast development in rice.

Yellow-Green Leaf 19 Encoding a Specific and Conservative Protein for Photosynthetic Organisms Affects Tetrapyrrole Biosynthesis, Photosynthesis, and Reactive Oxygen Species Metabolism in Rice

Chlorophyll is the main photosynthetic pigment and is crucial for plant photosynthesis. Leaf color mutants are widely used to identify genes involved in the synthesis or metabolism of chlorophyll. In this study, a spontaneous mutant, yellow-green leaf 19 (ygl19), was isolated from rice (Oryza sativa). This ygl19 mutant showed yellow-green leaves and decreased chlorophyll level and net photosynthetic rate. Brown necrotic spots appeared on the surface of ygl19 leaves at the tillering stage. And the agronomic traits of the ygl19 mutant, including the plant height, tiller number per plant, and total number of grains per plant, were significantly reduced. Map-based cloning revealed that the candidate YGL19 gene was LOC_Os03g21370. Complementation of the ygl19 mutant with the wild-type CDS of LOC_Os03g21370 led to the restoration of the mutant to the normal phenotype. Evolutionary analysis revealed that YGL19 protein and its homologues were unique for photoautotrophs, containing a conserved Ycf54 functional domain. A conserved amino acid substitution from proline to serine on the Ycf54 domain led to the ygl19 mutation. Sequence analysis of the YGL19 gene in 4726 rice accessions found that the YGL19 gene was conserved in natural rice variants with no resulting amino acid variation. The YGL19 gene was mainly expressed in green tissues, especially in leaf organs. And the YGL19 protein was localized in the chloroplast for function. Gene expression analysis via qRT-PCR showed that the expression levels of tetrapyrrole synthesis-related genes and photosynthesis-related genes were regulated in the ygl19 mutant. Reactive oxygen species (ROS) such as superoxide anions and hydrogen peroxide accumulated in spotted leaves of the ygl19 mutant at the tillering stage, accompanied by the regulation of ROS scavenging enzyme-encoding genes and ROS-responsive defense signaling genes. This study demonstrates that a novel yellow-green leaf gene YGL19 affects tetrapyrrole biosynthesis, photosynthesis, and ROS metabolism in rice.

Small Auxin Up RNA 56 (SAUR56) regulates heading date in rice

Heading date is a critical agronomic trait that determines crop yield. Although numerous genes associated with heading date have been identified in rice, the mechanisms involving Small Auxin Up RNA (SAUR) family have not been elucidated. In this study, the biological function of several SAUR genes was initially investigated using the CRISPR-Cas9 technology in the Japonica cultivar Zhonghua11 (ZH11) background. Further analysis revealed that the loss-of-function of OsSAUR56 affected heading date in both NLD (natural long-day) and ASD (artificial short-day). OsSAUR56 exhibited predominant expression in the anther, with its protein localized in both the cytoplasm and nucleus. OsSAUR56 regulated flowering time and heading date by modulating the expression of the clock gene OsGI, as well as two repressors Ghd7 and DTH8. Furthermore, haplotype-phenotype association analysis revealed a strong correlation between OsSAUR56 and heading date, suggesting its role in selection during the domestication of rice. In summary, these findings highlights the importance of OsSAUR56 in the regulation of heading date for further potential facilitating genetic engineering for flowering time during rice breeding.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-023-01409-w.

Exon skipping in IspE Gene is associated with abnormal chloroplast development in rice albino leaf 4 mutant

The formation of leaf color largely depends on the components of pigment accumulation in plastids, which are involved in chloroplast development and division. Here, we isolated and characterized the rice albino leaf 4 (al4) mutant, which exhibited an albino phenotype and eventually died at the three-leaf stage. The chloroplasts in al4 mutant were severely damaged and unable to form intact thylakoid structure. Further analysis revealed that the candidate gene encodes 4-diphosphocytidyl-2-C-methyl-D-erythritol kinase (IspE), which participates in the methylerythritol phosphate (MEP) pathway of isoprenoid biosynthesis. We further demonstrated that the mutation at the exon-intron junction site cause alternative splicing factors fail to distinguish the origin of the GT-AG intron, leading to exon skipping and producing a truncated OsIspE in the al4 mutant. Notably, disruption of OsIspE led to the reduced expression of chloroplast-associated genes, including chloroplast biosynthetic and translation related genes and photosynthetic associated nuclear genes (PhANGs). In summary, these findings reveal that OsIspE plays a crucial role in chloroplast biogenesis and provides novel insights into the function of CMK during chloroplast development in rice.

Cytological, genetic and transcriptomic characterization of a cucumber albino mutant

Photosynthesis, a fundamental process for plant growth and development, is dependent on chloroplast formation and chlorophyll synthesis. Severe disruption of chloroplast structure results in albinism of higher plants. In the present study, we report a cucumber albino alc mutant that presented white cotyledons under normal light conditions and was unable to produce first true leaf. Meanwhile, alc mutant could grow creamy green cotyledons under dim light conditions but died after exposure to normal light irradiation. No chlorophyll and carotenoid were detected in the alc mutant grown under normal light conditions. Using transmission electron microscopy, impaired chloroplasts were observed in this mutant. The genetic analysis indicated that the albino phenotype was recessively controlled by a single locus. Comparative transcriptomic analysis between the alc mutant and wild type revealed that genes involved in chlorophyll metabolism and the methylerythritol 4-phosphate pathway were affected in the alc mutant. In addition, three genes involved in chloroplast development, including two FtsH genes and one PPR gene, were found to have negligible expression in this mutant. The quality of RNA sequencing results was further confirmed by real-time quantitative PCR analysis. We also examined 12 homologous genes from alc mutant in other plant species, but no genetic variation in the coding sequences of these genes was found between alc mutant and wild type. Taken together, we characterized a cucumber albino mutant with albinism phenotype caused by chloroplast development deficiency and this mutant can pave way for future studies on plastid development.

Mapping and functional verification of leaf yellowing genes in watermelon during whole growth period

Increasing light energy utilization efficiency is an effective way to increase yield and improve quality of watermelon. Leaf is the main place for photosynthesis, and the color of leaf is directly related to the change of photosynthesis. In addition, leaf yellowing can be used as a marker trait to play an important role in watermelon hybrid breeding and improve seed breeding. It can not only be used to eliminate hybrids at seedling stage, but also be used to determine seed purity. In this study, transcriptome analysis was first carried out using the whole growth period leaf yellowing watermelon mutant w-yl and inbred line ZK, and identified 2,471 differentially expressed genes (DEGs) in the comparison group w-yl-vs-ZK. Among the top 20 terms of the gene ontology (GO) enrichment pathway, 17 terms were related to photosynthesis. KEGG pathway enrichment analysis showed that the most abundant pathway was photosynthesis-antenna proteins. The F₂ population was constructed by conventional hybridization with the inbred line ZK. Genetic analysis showed that leaf yellowing of the mutant was controlled by a single recessive gene. The leaf yellowing gene of watermelon located between Ind14,179,011 and InD16,396,362 on chromosome 2 by using indel-specific PCR markers, with a region of 2.217 Mb. In the interval, it was found that five genes may have gene fragment deletion in w-yl, among which Cla97C02G036010, Cla97C02G036030, Cla97C02G036040, Cla97C02G036050 were the whole fragment loss, and Cla97C02G0360 was the C-terminal partial base loss. Gene function verification results showed that Cla97C02G036040, Cla97C02G036050 and Cla97C02G036060 may be the key factors leading to yellowing of w-yl leaves.

Comparative Analysis of the Metabolome and Transcriptome between the Green and Yellow-Green Regions of Variegated Leaves in a Mutant Variety of the Tree Species Pteroceltis tatarinowii

In nature, many different factors cause plants to develop variegated leaves. To explore the mechanism of variegated leaf formation in Pteroceltis tatarinowii, a mutant variety ('Jinyuyuan'), which was induced by ethylmethylsulfone, was selected, and its morphological structure, physiology, biochemistry, transcription and metabolism were analysed. According to differences in colour values, the colours were divided into two regions: a green region and a yellow-green region. The chlorophyll content of the two regions was significantly different. Moreover, the yellow-green regions of the leaves were significantly thinner than the green regions. The chloroplast ultrastructure in the yellow-green region revealed small chloroplasts, large vacuoles, small starch grains, obviously increased numbers of osmophilic grains, loose lamellae of the inner capsule and thin lamellae. Moreover, the yellow-green region was accompanied by oxidative stress, and the activity of the oxidative phosphorylation pathway related to oxidative activity in the transcriptome showed an upward trend. Vitamin B6 and proline contents also increased, indicating that the antioxidant activity of cells in the yellow-green region increased. Transcriptomic and metabolomic analysis showed that the differentially expressed genes (DEGs) related to chlorophyll synthesis and metabolism led to a decrease in the photosynthesis and then a decrease in the assimilation ability and contents of sucrose, starch and other assimilates. Amino acid synthesis and metabolism, lipid synthesis and the activity of metabolic pathways were obviously downregulated, and the contents of differentially accumulated metabolites associated with amino acids and lipids were also reduced. At the same time, 31 out of 32 DEGs involved in the flavonoid synthesis pathway were downregulated, which affected leaf colour. We hypothesized that the variegated leaves of P. tatarinowii 'Jinyuyuan' are caused by transcriptional and post-transcriptional regulation. Mutations in pigment and flavonoid synthesis pathway genes and transcription factor genes directly affect both pigment and flavonoid synthesis and degradation rate, which in turn affect carbon assimilation, carbon fixation, related protein synthesis and enzyme activity, lipid synthesis and degradation and the activity of other metabolic pathways, eventually leading to the formation of different colour regions.

Genome-Wide Association Analysis for Salt-Induced Phenotypic and Physiologic Responses in Rice at Seedling and Reproductive Stages

Salinity is one of the main adverse environmental factors severely inhibiting rice growth and decreasing grain productivity. Developing rice varieties with salt tolerance (ST) is one of the most economical approaches to cope with salinity stress. In this study, the salt tolerance of 220 rice accessions from rice diversity panel l (RDP1), representing five subpopulations, were evaluated based on 16 ST indices at both seedling and reproductive stages under salt stress. An apparent inconsistency was found for ST between the two stages. Through a gene-based/tightly linked genome-wide association study with 201,332 single nucleotide polymorphisms (SNPs) located within genes and their flanking regions were used, a total of 214 SNPs related to 251 genes, significantly associated with 16 ST-related indices, were detected at both stages. Eighty-two SNPs with low frequency favorable (LFF) alleles in the population were proposed to hold high breeding potential in improving rice ST. Fifty-four rice accessions collectively containing all these LFF alleles were identified as donors of these alleles. Through the integration of meta-quantitative trait locus (QTL) for ST and the response patterns of differential expression genes to salt stress, thirty-eight candidate genes were suggested to be involved in the regulation of rice ST. In total, the present study provides valuable information for further characterizing ST-related genes and for breeding ST varieties across whole developmental stages through marker-assisted selection (MAS).

The Genetic Components of a Natural Color Palette: A Comprehensive List of Carotenoid Pathway Mutations in Plants

Carotenoids comprise the most widely distributed natural pigments. In plants, they play indispensable roles in photosynthesis, furnish colors to flowers and fruit and serve as precursor molecules for the synthesis of apocarotenoids, including aroma and scent, phytohormones and other signaling molecules. Dietary carotenoids are vital to human health as a source of provitamin A and antioxidants. Hence, the enormous interest in carotenoids of crop plants. Over the past three decades, the carotenoid biosynthesis pathway has been mainly deciphered due to the characterization of natural and induced mutations that impair this process. Over the year, numerous mutations have been studied in dozens of plant species. Their phenotypes have significantly expanded our understanding of the biochemical and molecular processes underlying carotenoid accumulation in crops. Several of them were employed in the breeding of crops with higher nutritional value. This compendium of all known random and targeted mutants available in the carotenoid metabolic pathway in plants provides a valuable resource for future research on carotenoid biosynthesis in plant species.

Dual function of clock component OsLHY sets critical day length for photoperiodic flowering in rice

Circadian clock, an endogenous time-setting mechanism, allows plants to adapt to unstable photoperiod conditions and induces flowering with proper timing. In Arabidopsis, the central clock oscillator was formed by a series of interlocked transcriptional feedback loops, but little is known in rice so far. By MutMap technique, we identified the candidate gene OsLHY from a later flowering mutant lem1 and further confirmed it through genetic complementation, RNA interference knockdown, and CRISPR/Cas9-knockout. Global transcriptome profiling and expression analyses revealed that OsLHY might be a vital circadian rhythm component. Interestingly, oslhy flowered later under ≥12 h day length but headed earlier under ≤11 h day length. qRT-PCR results exhibited that OsLHY might function through OsGI-Hd1 pathway. Subsequent one-hybrid assays in yeast, DNA affinity purification qPCR, and electrophoretic mobility shift assays confirmed OsLHY could directly bind to the CBS element in OsGI promoter. Moreover, the critical day length (CDL) for function reversal of OsLHY in oslhy (11-12 h) was prolonged in the double mutant oslhy osgi (about 13.5 h), indicating that the CDL set by OsLHY was OsGI dependent. Additionally, the dual function of OsLHY entirely relied on Hd1, as the double mutant oslhy hd1 showed the same heading date with hd1 under about 11.5, 13.5, and 14 h day lengths. Together, OsLHY could fine-tune the CDL by directly regulating OsGI, and Hd1 acts as the final effector of CDL downstream of OsLHY. Our study illustrates a new regulatory mechanism between the circadian clock and photoperiodic flowering.

The Coordinated Upregulated Expression of Genes Involved in MEP, Chlorophyll, Carotenoid and Tocopherol Pathways, Mirrored the Corresponding Metabolite Contents in Rice Leaves during De-Etiolation

Light is an essential regulator of many developmental processes in higher plants. We investigated the effect of 4-hydroxy-3-methylbut-2-enyl diphosphate reductase 1/2 genes (OsHDR1/2) and isopentenyl diphosphate isomerase 1/2 genes (OsIPPI1/2) on the biosynthesis of chlorophylls, carotenoids, and phytosterols in 14-day-old etiolated rice (Oyza sativa L.) leaves during de-etiolation. However, little is known about the effect of isoprenoid biosynthesis genes on the corresponding metabolites during the de-etiolation of etiolated rice leaves. The results showed that the levels of α-tocopherol were significantly increased in de-etiolated rice leaves. Similar to 1-deoxy-D-xylulose-5-phosphate synthase 3 gene (OsDXS3), both OsDXS1 and OsDXS2 genes encode functional 1-deoxy-D-xylulose-5-phosphate synthase (DXS) activities. Their expression patterns and the synthesis of chlorophyll, carotenoid, and tocopherol metabolites suggested that OsDXS1 is responsible for the biosynthesis of plastidial isoprenoids in de-etiolated rice leaves. The expression analysis of isoprenoid biosynthesis genes revealed that the coordinated expression of the MEP (2-C-methyl-D-erythritol 4-phosphate) pathway, chlorophyll, carotenoid, and tocopherol pathway genes mirrored the changes in the levels of the corresponding metabolites during de-etiolation. The underpinning mechanistic basis of coordinated light-upregulated gene expression was elucidated during the de-etiolation process, specifically the role of light-responsive cis-regulatory motifs in the promoter region of these genes. In silico promoter analysis showed that the light-responsive cis-regulatory elements presented in all the promoter regions of each light-upregulated gene, providing an important link between observed phenotype during de-etiolation and the molecular machinery controlling expression of these genes.

An update on the function and regulation of methylerythritol phosphate and mevalonate pathways and their evolutionary dynamics

Isoprenoids are among the largest and most chemically diverse classes of organic compounds in nature and are involved in the processes of photosynthesis, respiration, growth, development, and plant responses to stress. The basic building block units for isoprenoid synthesis-isopentenyl diphosphate and its isomer dimethylallyl diphosphate-are generated by the mevalonate (MVA) and methylerythritol phosphate (MEP) pathways. Here, we summarize recent advances on the roles of the MEP and MVA pathways in plant growth, development and stress responses, and attempt to define the underlying gene networks that orchestrate the MEP and MVA pathways in response to developmental or environmental cues. Through phylogenomic analysis, we also provide a new perspective on the evolution of the plant isoprenoid pathway. We conclude that the presence of the MVA pathway in plants may be associated with the transition from aquatic to subaerial and terrestrial environments, as lineages for its core components are absent in green algae. The emergence of the MVA pathway has acted as a key evolutionary event in plants that facilitated land colonization and subsequent embryo development, as well as adaptation to new and varied environments.

A Single Nucleotide Substitution of GSAM Gene Causes Massive Accumulation of Glutamate 1-Semialdehyde and Yellow Leaf Phenotype in Rice

BACKGROUND

Tetrapyrroles play indispensable roles in various biological processes. In higher plants, glutamate 1-semialdehyde 2,1-aminomutase (GSAM) converts glutamate 1-semialdehyde (GSA) to 5-aminolevulinic acid (ALA), which is the rate-limiting step of tetrapyrrole biosynthesis. Up to now, GSAM genes have been successively identified from many species. Besides, it was found that GSAM could form a dimeric protein with itself by x-ray crystallography. However, no mutant of GSAM has been identified in monocotyledonous plants, and no experiment on interaction of GSAM protein with itself has been reported so far.

RESULT

We isolated a yellow leaf mutant, ys53, in rice (Oryza sativa). The mutant showed decreased photosynthetic pigment contents, suppressed chloroplast development, and reduced photosynthetic capacity. In consequence, its major agronomic traits were significantly affected. Map-based cloning revealed that the candidate gene was LOC_Os08g41990 encoding GSAM protein. In ys53 mutant, a single nucleotide substitution in this gene caused an amino acid change in the encoded protein, so its ALA-synthesis ability was significantly reduced and GSA was massively accumulated. Complementation assays suggested the mutant phenotype of ys53 could be rescued by introducing wild-type OsGSAM gene, confirming that the point mutation in OsGSAM is the cause of the mutant phenotype. OsGSAM is mainly expressed in green tissues, and its encoded protein is localized to chloroplast. qRT-PCR analysis indicated that the mutation of OsGSAM not only affected the expressions of tetrapyrrole biosynthetic genes, but also influenced those of photosynthetic genes in rice. In addition, the yeast two-hybrid experiment showed that OsGSAM protein could interact with itself, which could largely depend on the two specific regions containing the 81th-160th and the 321th-400th amino acid residues at its N- and C-terminals, respectively.

CONCLUSIONS

We successfully characterized rice GSAM gene by a yellow leaf mutant and map-based cloning approach. Meanwhile, we verified that OsGSAM protein could interact with itself mainly by means of the two specific regions of amino acid residues at its N- and C-terminals, respectively.

Integrated transcriptomics and metabolomics provide novel insight into changes in specialized metabolites in an albino tea cultivar (Camellia sinensis (L.) O. Kuntz)

Tea varieties with specific colours have often been studied by researchers. However, previous studies on the albinism of tea plants have mostly been based on plants with different genetic backgrounds or focused on common components in albino tea leaves, such as amino acids, flavones, and carotenoids. In this study, we conducted widely targeted metabolic and transcriptomic analyses between a wildtype tea genotype (Shuixian, LS) and its albino mutant (Huangjinshuixian, HS). At the molecular level, alteration of gene expression levels in the MEP pathway may have reduced the production of chlorophyll and carotenoids in HS, which could be the main cause of the phenotypic changes in HS. At the metabolite level, a large number of metabolites related to light protection that significantly accumulated in HS, including flavones, anthocyanins, flavonols, flavanones, vitamins and their derivatives, polyphenols, phenolamides. This result, combined with an enzyme activity experiment, suggested that the absence of photosynthetic pigments made the albino tea leaves of HS more vulnerable to UV stress, even under normal light conditions. In addition, except for the common amino acids, we also identified numerous nitrogen-containing compounds, including nucleotides and their derivates, amino acid derivatives, glycerophospholipids, and phenolamides, which implied that significant accumulation of NH₄⁺ in albino tea leaves could not only promote amino acid synthesis but could also activate other specialized metabolic pathways related to nitrogen metabolism. In conclusion, our results provide new information to guide further studies of the extensive metabolic reprogramming events caused by albinism in tea plants.

A study of leaf-senescence genes in rice based on a combination of genomics, proteomics and bioinformatics

Leaf senescence is a highly complex, genetically regulated and well-ordered process with multiple layers and pathways. Delaying leaf senescence would help increase grain yields in rice. Over the past 15 years, more than 100 rice leaf-senescence genes have been cloned, greatly improving the understanding of leaf senescence in rice. Systematically elucidating the molecular mechanisms underlying leaf senescence will provide breeders with new tools/options for improving many important agronomic traits. In this study, we summarized recent reports on 125 rice leaf-senescence genes, providing an overview of the research progress in this field by analyzing the subcellular localizations, molecular functions and the relationship of them. These data showed that chlorophyll synthesis and degradation, chloroplast development, abscisic acid pathway, jasmonic acid pathway, nitrogen assimilation and ROS play an important role in regulating the leaf senescence in rice. Furthermore, we predicted and analyzed the proteins that interact with leaf-senescence proteins and achieved a more profound understanding of the molecular principles underlying the regulatory mechanisms by which leaf senescence occurs, thus providing new insights for future investigations of leaf senescence in rice.

A mutation in CsHD encoding a histidine and aspartic acid domain-containing protein leads to yellow young leaf-1 (yyl-1) in cucumber (Cucumis sativus L.)

Leaf color mutants are an ideal tool to study chlorophyll biosynthesis, chloroplast development and photosynthesis. In this study, we identified an EMS-induced yellow young leaf mutant C777. The mutant exhibited yellow cotyledons and emerging true leaves with stay-green dots that turn green gradually with leaf growth. Segregation analysis in several populations indicated that the mutant C777 was controlled by a recessive gene yyl-1. Fine mapping delimited the yyl-1 locus to a 45.3 kb region harboring 8 putative genes, but only one SNP (G to A) was identified between C777 and its wild-type parental line in this region which occurred in the 13th exon of CsHD that encodes a histidine and aspartic acid (HD) domain containing protein. This nonsense mutation introduced a stop codon and thus a premature protein. Uniqueness of this mutant allele was verified in 515 cucumber lines. Quantitative real-time PCR revealed significantly reduced expression of CsHD gene in the mutant. Further, silencing the NbHD gene by VIGS in tobacco resulted in virescent young leaves and significantly down-regulated expression of HD gene. These results strongly supported the association of the CsHD gene with the virescent young leaf phenotype in C777. This is the first report to clone and characterize the CsHD gene in the horticultural crops. The results may help understand the functions of the HD gene in chloroplast development and chlorophyll biosynthesis in plants.

Cytological, genetic, and proteomic analysis of a sesame (Sesamum indicum L.) mutant Siyl-1 with yellow-green leaf color

BACKGROUND

Both photosynthetic pigments and chloroplasts in plant leaf cells play an important role in deciding on the photosynthetic capacity and efficiency in plants. Systematical investigating the regulatory mechanism of chloroplast development and chlorophyll (Chl) content variation is necessary for clarifying the photosynthesis mechanism for crops.

OBJECTIVE

This study aims to explore the critical regulatory mechanism of leaf color mutation in a yellow-green leaf sesame mutant Siyl-1.

METHODS

We performed the genetic analysis of the yellow-green leaf color mutation using the F2 population of the mutant Siyl-1. We compared the morphological structure of the chloroplasts, chlorophyll content of the three genotypes of the mutant F2 progeny. We performed the two-dimensional gel electrophoresis (2-DE) and compared the protein expression variation between the mutant progeny and the wild type.

RESULTS

Genetic analysis indicated that there were 3 phenotypes of the F2 population of the mutant Siyl-1, i.e., YY type with light-yellow leaf color (lethal); Yy type with yellow-green leaf color, and yy type with normal green leaf color. The yellow-green mutation was controlled by an incompletely dominant nuclear gene, Siyl-1. Compared with the wild genotype, the chloroplast number and the morphological structure in YY and Yy mutant lines varied evidently. The chlorophyll content also significantly decreased (P < 0.05). The 2-DE comparison showed that there were 98 differentially expressed proteins (DEPs) among YY, Yy, and yy lines. All the 98 DEPs were classified into 5 functional groups. Of which 82.7% DEPs proteins belonged to the photosynthesis and energy metabolism group.

CONCLUSION

The results revealed the genetic character of yellow-green leaf color mutant Siyl-1. 98 DEPs were found in YY and Yy mutant compared with the wild genotype. The regulation pathway related with the yellow leaf trait mutation in sesame was analyzed for the first time. The findings supplied the basic theoretical and gene basis for leaf color and chloroplast development mechanism in sesame.

Tandem 13-Lipoxygenase Genes in a Cluster Confers Yellow-Green Leaf in Cucumber

Some lipoxygenase (LOX) isoenzymes can co-oxidize carotenoids. Carotenoids are collectors of light energy for photosynthesis and can protect plants from reactive oxygen species and coloration. This study isolated the cucumber (Cucumis sativus L.) yellow-green leaf mutant (ygl1), which had yellow-green leaves with decreased chlorophyll synthesis, increased relative carotenoid content, and delayed chloroplast development. Genetic analysis demonstrated that the phenotype of ygl1 was caused by a recessive mutation in a nuclear gene. The bulked segregants were resequenced, and the candidate ygl1 locus identified was mapped to the 9.2 kb region of the chromosome 4. Sequence analysis revealed that ygl1 encodes the tandem 13-LOX genes in a cluster. Four missense mutations were found in four tandem 13-LOX genes (Csa4M286960, Csa4M287550, Csa4M288070, and Csa4M288080) in the ygl1 mutant, and the four 13-LOX genes showed high similarity with one another. The transient RNA interference and virus-induced gene silencing of these genes simultaneously resulted in yellow-green leaves with a reduced amount of chloroplasts and increased relative carotenoid content, which were observed in the ygl1 mutant. This evidence supported the non-synonymous SNPs (Single Nucleotide Polymorphism) in the four tandem 13-LOX genes as being the causative mutation for the yellow-green leaves. Furthermore, this study provides a new allele for breeding cucumbers with yellow-green leaves and serves as an additional resource for studying carotenoid biosynthesis.

GRA78 encoding a putative S-sulfocysteine synthase is involved in chloroplast development at the early seedling stage of rice

Cysteine functions not only as an amino acid in proteins but also as a precursor for a large number of essential biomolecules. Cysteine is synthesized via the incorporation of sulfide to O-acetylserine under the catalysis of O-acetylserine(thiol)lyase (OASTL). In dicotyledonous Arabidopsis, nine OASTL genes have been reported. However, in their null mutants, only the mutant of CS26 encoding S-sulfocysteine synthase showed the visible phenotypic changes, displaying significantly small plants and pale-green leaves under long-day condition but not short-day condition. Up to now, no OASTL gene or mutant has been identified in monocotyledon. In this study, we isolated a green-revertible albino mutant gra78 in rice (Oryza sativa). Its albino phenotype at the early seedling stage was sensitive to temperature but independent of photoperiod. Map-based cloning revealed that candidate gene LOC_Os01g59920 of GRA78 encodes a putative S-sulfocysteine synthase showing significant similarity with Arabidopsis CS26. Complementation experiment confirmed that mutation in LOC_Os01g59920 accounted for the mutant phenotype of gra78. GRA78 is constitutively expressed in all tissues and its encoded protein is targeted to the chloroplast. In addition, qRT-PCR suggested that expression levels of four OASTL homolog genes and five photosynthetic genes were remarkably down-regulated.

A Single Nucleotide Mutation of the IspE Gene Participating in the MEP Pathway for Isoprenoid Biosynthesis Causes a Green-Revertible Yellow Leaf Phenotype in Rice

Plant isoprenoids are dependent on two independent pathways, the cytosolic mevalonate (MVA) pathway and the plastidic methylerythritol phosphate (MEP) pathway. IspE is one of seven known enzymes in the MEP pathway. Currently, no IspE gene has been identified in rice. In addition, no virescent mutants have been reported to result from a gene mutation affecting the MEP pathway. In this study, we isolated a green-revertible yellow leaf mutant gry340 in rice. The mutant exhibited a reduced level of photosynthetic pigments, and an arrested development of chloroplasts and mitochondria in its yellow leaves. Map-based cloning revealed a missense mutation in OsIspE (LOC_Os01g58790) in gry340 mutant plants. OsIspE is constitutively expressed in all tissues, and its encoded protein is targeted to the chloroplast. Further, the mutant phenotype of gry340 was rescued by introduction of the wild-type gene. Therefore, we have successfully identified an IspE gene in monocotyledons via map-based cloning, and confirmed that the green-revertible yellow leaf phenotype of gry340 does result from a single nucleotide mutation in the IspE gene. In addition, the ispE ispF double mutant displayed an etiolation lethal phenotype, indicating that the isoprenoid precursors from the cytosol cannot efficiently compensate for the deficiency of the MEP pathway in rice chloroplasts. Furthermore, real-time quantitative reverse transcription-PCR suggested that this functional defect in OsIspE affected the expression of not only other MEP pathway genes but also that of MVA pathway genes, photosynthetic genes and mitochondrial genes.

Genetics and Physiology of the Nuclearly Inherited Yellow Foliar Mutants in Soybean

Plant photosynthetic pigments are important in harvesting the light energy and transfer of energy during photosynthesis. There are several yellow foliar mutants discovered in soybean and chromosomal locations for about half of them have been deduced. Viable-yellow mutants are capable of surviving with decreased photosynthesis, while lethal-yellow mutants die shortly after germination. In addition to the decreased chlorophyll content, other features associated with yellow mutants include altered Chl a and Chl b ratio, reduction in chloroplast size and number, lower levels of other photosynthetic pigments, inability of thylakoids to stack into granum, lack of lamellae to interconnect granum and reduced size of the light harvesting complex. For some yellow mutants, temperature and/or light play a critical role in the manifestation of phenotype. Although yellow foliar mutants are viewed as undesirable for crop production, there is the possibility of these mutants to create a positive impact by reducing the total amount of chlorophyll and diverting resources toward increased biochemical photosynthetic capacity leading to increased yield. Recent advances in model plants led to the isolation and characterization of various genes associated with yellow foliar phenotype. Knowledge gained from the model plants can be applied using homology based cloning approach to isolate genes in soybean and understanding the modes of actions of the involved proteins. Identifying and characterizing yellow foliar mutants will not only aid in understanding the biosynthetic pathways involved in the photosynthetic machinery, but may also provide ways to increase soybean productivity.