Mapping of a Pale Green Mutant Gene and Its Functional Verification by Allelic Mutations in Chinese Cabbage (Brassica rapa L. ssp. pekinensis)

Physiological, Cytological and Transcriptome Analysis of a Yellow-Green Leaf Mutant in Magnolia sinostellata

Leaf color mutants serve as excellent models for investigating the metabolic pathways involved in chlorophyll biosynthesis, chloroplast development, and photosynthesis in plants. This study aimed to elucidate the mechanisms underlying color formation in the yellow-green leaf mutant (YL) of Magnolia sinostellata by employing physiological, cytological and transcriptomic analyses to compare the mutant with control plants (wild type Magnolia sinostellata, WT). Physiological assessments revealed a reduction in chlorophyll content, particularly chlorophyll b, alongside an increase in the flavonoid level in YL relative to WT. Cytological examinations indicated the presence of defective chloroplasts within the mesophyll cells of the mutants. Transcriptomic analysis identified 8205 differentially expressed genes, with 4159 upregulated and 4046 downregulated. Genes associated with chlorophyll metabolism, flavonoid metabolism, photosynthesis, and signaling pathways were found to play crucial roles in leaf yellowing. In conclusion, this study delineated the phenotypic, physiological, cytological, and transcriptomic differences between YL and WT leaves, offering novel insights into the mechanisms driving leaf yellowing in Magnolia sinostellata.

Effects of different light intensity on leaf color changes in a Chinese cabbage yellow cotyledon mutant

Leaf color is one of the most important phenotypic features in horticultural crops and directly related to the contents of photosynthetic pigments. Most leaf color mutants are determined by the altered chlorophyll or carotenoid, which can be affected by light quality and intensity. Our previous study obtained a Chinese cabbage yellow cotyledon mutant that exhibited obvious yellow phenotypes in the cotyledons and the new leaves. However, the underlying mechanisms in the formation of yellow cotyledons and leaves remain unclear. In this study, the Chinese cabbage yellow cotyledon mutant 19YC-2 exhibited obvious difference in leaf color and abnormal chloroplast ultrastructure compared to the normal green cotyledon line 19GC-2. Remarkably, low-intensity light treatment caused turn-green leaves and a significant decrease in carotenoid content in 19YC-2. RNA-seq analysis revealed that the pathways of photosynthesis antenna proteins and carotenoid biosynthesis were significantly enriched during the process of leaf color changes, and many differentially expressed genes related to the two pathways were identified to respond to different light intensities. Remarkably, BrPDS and BrLCYE genes related to carotenoid biosynthesis showed significantly higher expression in 19YC-2 than that in 19GC-2, which was positively related to the higher carotenoid content in 19YC-2. In addition, several differentially expressed transcription factors were also identified and highly correlated to the changes in carotenoid content, suggesting that they may participate in the regulatory pathway of carotenoid biosynthesis. These findings provide insights into the molecular mechanisms of leaf color changes in yellow cotyledon mutant 19YC-2 of Chinese cabbage.

Changes in Phytohormones and Transcriptomic Reprogramming in Strawberry Leaves under Different Light Qualities

Strawberry plants require light for growth, but the frequent occurrence of low-light weather in winter can lead to a decrease in the photosynthetic rate (Pn) of strawberry plants. Light-emitting diode (LED) systems could be used to increase Pn. However, the changes in the phytohormones and transcriptomic reprogramming in strawberry leaves under different light qualities are still unclear. In this study, we treated strawberry plants with sunlight, sunlight covered with a 50% sunshade net, no light, blue light (460 nm), red light (660 nm), and a 50% red/50% blue LED light combination for 3 days and 7 days. Our results revealed that the light quality has an effect on the contents of Chl a and Chl b, the minimal fluorescence (F0), and the Pn of strawberry plants. The light quality also affected the contents of abscisic acid (ABA), auxin (IAA), trans-zeatin-riboside (tZ), jasmonic acid (JA), and salicylic acid (SA). RNA sequencing (RNA-seq) revealed that differentially expressed genes (DEGs) are significantly enriched in photosynthesis antenna proteins, photosynthesis, carbon fixation in photosynthetic organisms, porphyrin and chlorophyll metabolisms, carotenoid biosynthesis, tryptophan metabolism, phenylalanine metabolism, zeatin biosynthesis, and linolenic acid metabolism. We then selected the key DEGs based on the results of a weighted gene co-expression network analysis (WGCNA) and drew nine metabolic heatmaps and protein-protein interaction networks to map light regulation.

BrKCS6 mutation conferred a bright glossy phenotype to Chinese cabbage

KEY MESSAGE

BrKCS6 encoding 3-ketoacyl-CoA synthases was cloned through MutMap and KASP analysis, and its function was verified via allelic mutants in Chinese cabbage. Bright and glossy green appearance is an attractive commodity character for leafy vegetables and is mainly caused by the absence of epicuticular wax crystals. In this study, two allelic epicuticular wax crystal deficiency mutants, wdm9 and wdm10, were obtained from an EMS mutagenesis population of Chinese cabbage (Brassica rapa L. ssp. pekinensis). Genetic analysis revealed that the mutant phenotype was controlled by a recessive nuclear gene. BrKCS6 encoding 3-ketoacyl-CoA synthases was identified as the candidate gene by MutMap and KASP analysis. A SNP (G to A) on BrKCS6 in wdm9 led to the amino acid substitution from serine (S) to phenylalanine (F), and another SNP (G to A) in wdm10 resulted in the amino acid substitution from serine (S) to leucine (L). Both SNPs are located in the ACP_syn_III_C conserved domain, corresponding to two highly conserved sites among BrKCS6 and its homologs. These two amino acid substitutions changed the secondary structure and the three-dimensional structure of BrKCS6 protein. qRT-PCR results showed that the relative expression of BrKCS6 significantly decreased in the flower, stem, and leaves in mutant, and the relative expressions of the downstream key genes of BrKCS6 were down-regulated in mutant. We were the first to clone the precious glossy bright gene BrKCS6 which has a great potential for commodity quality breeding in Chinese cabbage.

Research progress and applications of colorful Brassica crops

MAIN CONCLUSION

We review the application and the molecular regulation of anthocyanins in colorful Brassica crops, the creation of new germplasm resources, and the development and utilization of colorful Brassica crops. Brassica crops are widely cultivated: these include oilseed crops, such as rapeseed, mustards, and root, leaf, and stem vegetable types, such as turnips, cabbages, broccoli, and cauliflowers. Colorful variants exist of these crop species, and asides from increased aesthetic appeal, these may also offer advantages in terms of nutritional content and improved stress resistances. This review provides a comprehensive overview of pigmentation in Brassica as a reference for the selection and breeding of new colorful Brassica varieties for multiple end uses. We summarize the function and molecular regulation of anthocyanins in Brassica crops, the creation of new colorful germplasm resources via different breeding methods, and the development and multifunctional utilization of colorful Brassica crop types.

Role of Brassica orphan gene BrLFM on leafy head formation in Chinese cabbage (Brassica rapa)

Brassica orphan gene BrFLM, identified by two allelic mutants, was involved in leafy head formation in Chinese cabbage. Leafy head formation is a unique agronomic trait of Chinese cabbage that determines its yield and quality. In our previous study, an EMS mutagenesis Chinese cabbage mutant library was constructed using the heading Chinese cabbage double haploid (DH) line FT as the wild-type. Here, we screened two extremely similar leafy head deficiency mutants lfm-1 and lfm-2 with geotropic growth leaves from the library to investigate the gene(s) related to leafy head formation. Reciprocal crossing results showed that these two mutants were allelic. We utilized lfm-1 to identify the mutant gene(s). Genetic analysis showed that the mutated trait was controlled by a single nuclear gene Brlfm. Mutmap analysis showed that Brlfm was located on chromosome A05, and BraA05g012440.3C or BraA05g021450.3C were the candidate gene. Kompetitive allele-specific PCR analysis eliminated BraA05g012440.3C from the candidates. Sanger sequencing identified an SNP from G to A at the 271st nucleotide on BraA05g021450.3C. The sequencing of lfm-2 detected another non-synonymous SNP (G to A) located at the 266st nucleotide on BraA05g021450.3C, which verified its function on leafy head formation. We blasted BraA05g021450.3C on database and found that it belongs to a Brassica orphan gene encoding an unknown 13.74 kDa protein, named BrLFM. Subcellular localization showed that BrLFM was located in the nucleus. These findings reveal that BrLFM is involved in leafy head formation in Chinese cabbage.

Identification of an epicuticular wax crystal deficiency gene Brwdm1 in Chinese cabbage (Brassica campestris L. ssp. pekinensis)

Introduction

The cuticle wax covering the plant surface is a whitish hydrophobic protective barrier in Chinese cabbage, and the epicuticular wax crystal deficiency normally has higher commodity value for a tender texture and glossy appearance. Herein, two allelic epicuticular wax crystal deficiency mutants, wdm1 and wdm7, were obtained from the EMS mutagenesis population of a Chinese cabbage DH line 'FT'.

Methods

The cuticle wax morphology was observed by Cryo-scanning electron microscopy (Cryo-SEM) and the composition of wax was determined by GC-MS. The candidate mutant gene was found by MutMap and validated by KASP. The function of candidate gene was verified by allelic variation.

Results

The mutants had fewer wax crystals and lower leaf primary alcohol and ester content. Genetic analysis revealed that the epicuticular wax crystal deficiency phenotype was controlled by a recessive nuclear gene, named Brwdm1. MutMap and KASP analyses indicated that BraA01g004350.3C, encoding an alcohol-forming fatty acyl-CoA reductase, was the candidate gene for Brwdm1. A SNP 2,113,772 (C to T) variation in the 6^th exon of Brwdm1 in wdm1 led to the 262^nd amino acid substitution from threonine (T) to isoleucine (I), which existed in a rather conserved site among the amino acid sequences from Brwdm1 and its homologs. Meanwhile, the substitution changed the three-dimensional structure of Brwdm1. The SNP 2,114,994 (G to A) in the 10^th exon of Brwdm1 in wdm7 resulted in the change of the 434^th amino acid from valine (V) to isoleucine (I), which occurred in the STERILE domain. KASP genotyping showed that SNP 2,114,994 was co-segregated with glossy phenotype. Compared with the wild type, the relative expression of Brwdm1 was significantly decreased in the leaves, flowers, buds and siliques of wdm1.

Discussion

These results indicated that Brwdm1 was indispensable for the wax crystals formation and its mutation resulted in glossy appearance in Chinese cabbage.

BrCWM Mutation Disrupted Leaf Flattening in Chinese Cabbage (Brassica rapa L. ssp. pekinensis)

Leaf flattening plays a vital role in the establishment of plant architecture, which is closely related to plant photosynthesis and, thus, influences the product yield and quality of Chinese cabbage. In this study, we used the doubled haploid line ‘FT’ of Chinese cabbage as the wild type for ethyl methanesulfonate (EMS) mutagenesis and obtained a mutant cwm with stably inherited compact and wrinkled leaves. Genetic analysis revealed that the mutated trait was controlled by a single recessive nuclear gene, Brcwm. Brcwm was preliminarily mapped to chromosome A07 based on bulked segregant RNA sequencing (BSR-seq) and fine-mapped to a 205.66 kb region containing 39 genes between Indel12 and Indel21 using SSR and Indel analysis. According to the whole-genome re-sequencing results, we found that there was only one nonsynonymous single nucleotide polymorphism (SNP) (C to T) within the target interval on exon 4 of BraA07g021970.3C, which resulted in a proline to serine amino acid substitution. The mutated trait co-segregated with the SNP. Quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) revealed that BraA07g021970.3C expression was dramatically higher in ‘FT’ leaves than that in cwm leaves. BraA07g021970.3C is homologous to AT3G55000 encoding a protein related to cortical microtubule organization. A similar phenotype of dwarfism and wrinkled leaves was observed in the recessive homozygous mutant cwm-f1 of AT3G55000, and its T3 transgenic lines were restored to the Arabidopsis wild-type phenotype through ectopic overexpression of BraA07g021970.3C. These results verified that BraA07g021970.3C was the target gene essential for leaf flattening in Chinese cabbage.

Identification of a biomass unaffected pale green mutant gene in Chinese cabbage (Brassica rapa L. ssp. pekinensis)

Chlorophyll (Chl) is an essential component of the photosynthetic apparatus and pigments in plant greening. Leaf color is an important agronomic and commercial trait of Chinese cabbage. In this study, we identified a pale green mutant pgm created by ethyl methane sulfonate (EMS) mutagenesis in Chinese cabbage. Compared with wild-type (FT), pgm had a lower Chl content with a higher Chl a/b ratio, imperfect chloroplast structure, and lower non-photochemical quenching. However, its net photosynthetic rate and biomass showed no significant differences. Genetic analysis revealed that the pale green phenotype of pgm was controlled by a recessive nuclear gene, designated as Brpgm. We applied BSR-Seq, linkage analysis, and whole-genome resequencing to map Brpgm and predicted that the target gene was BraA10g007770.3C (BrCAO), which encodes chlorophyllide a oxygenase (CAO). Brcao sequencing results showed that the last nucleotide of its first intron changed from G to A, causing the deletion of the first nucleotide in its second CDS and termination of the protein translation. The expression of BrCAO in pgm was upregulated, and the enzyme activity of CAO in pgm was significantly decreased. These results provide an approach to explore the function of BrCAO and create a pale green variation in Chinese cabbage.