Plant glutathione S-transferases (GSTs) are ubiquitous and multifunctional enzymes encoded by large gene families. A characteristic feature of GST genes is their high inducibility by a wide range of stress conditions including biotic stress. Early studies on the role of GSTs in plant biotic stress showed that certain GST genes are specifically up-regulated by microbial infections. Later numerous transcriptome-wide investigations proved that distinct groups of GSTs are markedly induced in the early phase of bacterial, fungal and viral infections. Proteomic investigations also confirmed the accumulation of multiple GST proteins in infected plants. Furthermore, functional studies revealed that overexpression or silencing of specific GSTs can markedly modify disease symptoms and also pathogen multiplication rates. However, very limited information is available about the exact metabolic functions of disease-induced GST isoenzymes and about their endogenous substrates. The already recognized roles of GSTs are the detoxification of toxic substances by their conjugation with glutathione, the attenuation of oxidative stress and the participation in hormone transport. Some GSTs display glutathione peroxidase activity and these GSTs can detoxify toxic lipid hydroperoxides that accumulate during infections. GSTs can also possess ligandin functions and participate in the intracellular transport of auxins. Notably, the expression of multiple GSTs is massively activated by salicylic acid and some GST enzymes were demonstrated to be receptor proteins of salicylic acid. Furthermore, induction of GST genes or elevated GST activities have often been observed in plants treated with beneficial microbes (bacteria and fungi) that induce a systemic resistance response (ISR) to subsequent pathogen infections. Further research is needed to reveal the exact metabolic functions of GST isoenzymes in infected plants and to understand their contribution to disease resistance.

Glutathione S-Transferase Enzymes in Plant-Pathogen Interactions.

With the global climate anomalies and the destruction of ecological balance, the water shortage has become a serious ecological problem facing all mankind, and drought has become a key factor restricting the development of agricultural production. Therefore, it is essential to study the drought tolerance of crops. Based on previous studies, we reviewed the effects of drought stress on plant morphology and physiology, including the changes of external morphology and internal structure of root, stem, and leaf, the effects of drought stress on osmotic regulation substances, drought-induced proteins, and active oxygen metabolism of plants. In this paper, the main drought stress signals and signal transduction pathways in plants are described, and the functional genes and regulatory genes related to drought stress are listed, respectively. We summarize the above aspects to provide valuable background knowledge and theoretical basis for future agriculture, forestry breeding, and cultivation.

Response Mechanism of Plants to Drought Stress

Rapeseed ( Brassica napus L.) is the largest oilseed crop in China and accounts for about 20% of world production. For the last 10 years, the production, planting area, and yield of rapeseed have been stable, with improvement of seed quality and especially seed oil content. China is among the leading countries in rapeseed genomic research internationally, having jointly with other countries accomplished the whole genome sequencing of rapeseed and its two parental species, Brassica oleracea and Brassica rapa . Progress on functional genomics including the identification of QTL governing important agronomic traits such as yield, seed oil content, fertility regulation, disease and insect resistance, abiotic stress, nutrition use efficiency, and pod shattering resistance has been achieved. As a consequence, molecular markers have been developed and used in breeding programs. During 2005–2014, 215 rapeseed varieties were registered nationally, including 210 winter- and 5 spring-type varieties. Mechanization across the whole process of rapeseed production was investigated and operating instructions for all relevant techniques were published. Modern techniques for rapeseed field management such as high-density planting, controlled-release fertilizer, and biocontrol of disease and pests combined with precision tools such as drones have been developed and are being adopted in China. With the application of advanced breeding and production technologies, in the near future, the oil yield and quality of rapeseed varieties will be greatly increased, and more varieties with desirable traits, especially early maturation, high yield, high resistance to biotic and abiotic stress, and suitability for mechanized harvesting will be developed. Application of modern technologies on the mechanized management of rapeseed will greatly increase grower profit.

Rapeseed research and production in China

Plants serve as host to numerous microorganisms. The members of these microbial communities interact among each other and with the plant, and there is increasing evidence to suggest that the microbial community may promote plant growth, improve drought tolerance, facilitate pathogen defense and even assist in environmental remediation. Therefore, it is important to better understand the mechanisms that influence the composition and structure of microbial communities, and what role the host may play in the recruitment and control of its microbiome. In particular, there is a growing body of research to suggest that plant defense systems not only provide a layer of protection against pathogens but may also actively manage the composition of the overall microbiome. In this review, we provide an overview of the current research into mechanisms employed by the plant host to select for and control its microbiome. We specifically review recent research that expands upon the role of keystone microbial species, phytohormones, and abiotic stress; in how they relate to plant driven dynamic microbial structuring.

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Plant Host-Associated Mechanisms for Microbial Selection.

Yellow seed is a desirable trait with great potential for improving seed quality in Brassica crops. Unfortunately, no natural or induced yellow seed germplasms have been found in Brassica napus, an important oil crop, which likely reflects its genome complexity and the difficulty of the simultaneous random mutagenesis of multiple gene copies with functional redundancy. Here, we demonstrate the first application of CRISPR/Cas9 for creating yellow-seeded mutants in rapeseed. The targeted mutations of the BnTT8 gene were stably transmitted to successive generations, and a range of homozygous mutants with loss-of-function alleles of the target genes were obtained for phenotyping. The yellow-seeded phenotype could be recovered only in targeted mutants of both BnTT8 functional copies, indicating that the redundant roles of BnA09.TT8 and BnC09.TT8b are vital for seed colour. The BnTT8 double mutants produced seeds with elevated seed oil and protein content and altered fatty acid (FA) composition without any serious defects in the yield-related traits, making it a valuable resource for rapeseed breeding programmes. Chemical staining and histological analysis showed that the targeted mutations of BnTT8 completely blocked the proanthocyanidin (PA)-specific deposition in the seed coat. Further, transcriptomic profiling revealed that the targeted mutations of BnTT8 resulted in the broad suppression of phenylpropanoid/flavonoid biosynthesis genes, which indicated a much more complex molecular mechanism underlying seed colour formation in rapeseed than in Arabidopsis and other Brassica species. In addition, gene expression analysis revealed the possible mechanism through which BnTT8 altered the oil content and fatty acid composition in seeds.

https://onlinelibrary.wiley.com/doi/epdf/10.1111/pbi.13281

Targeted mutagenesis of BnTT8 homologs controls yellow seed coat development for effective oil production in Brassica napus L.

Brassica napus is one of the most important oil crops in the world, and stem rot caused by the fungus Sclerotinia sclerotiorum results in major losses in yield and quality. To elucidate resistance genes and pathogenesis-related genes, genome-wide association analysis of 347 accessions was performed using the Illumina 60K Brassica SNP (single nucleotide polymorphism) array. In addition, the detached stem inoculation assay was used to select five highly resistant (R) and susceptible (S) B. napus lines, 48 h postinoculation with S. sclerotiorum for transcriptome sequencing. We identified 17 significant associations for stem resistance on chromosomes A8 and C6, five of which were on A8 and 12 on C6. The SNPs identified on A8 were located in a 409-kb haplotype block, and those on C6 were consistent with previous QTL mapping efforts. Transcriptome analysis suggested that S. sclerotiorum infection activates the immune system, sulphur metabolism, especially glutathione (GSH) and glucosinolates in both R and S genotypes. Genes found to be specific to the R genotype related to the jasmonic acid pathway, lignin biosynthesis, defence response, signal transduction and encoding transcription factors. Twenty-four genes were identified in both the SNP-trait association and transcriptome sequencing analyses, including a tau class glutathione S-transferase (GSTU) gene cluster. This study provides useful insight into the molecular mechanisms underlying the plant's response to S. sclerotiorum.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/pbi.12501

Genome-wide association analysis and differential expression analysis of resistance to Sclerotinia stem rot in Brassica napus.

Oilseed rape (Brassica napus L.) is the second highest yielding oil crop worldwide. In addition to being used as an edible oil and a feed for livestock, rapeseed has high ornamental value. In this study, we identified and characterized the main floral major constituents, including phenolic acids and flavonoids components, in rapeseed accessions with different-colored petals. A total of 144 constituents were identified using ultrahigh-performance liquid chromatography-HESI-mass spectrometry (UPLC-HESI-MS/MS), 57 of which were confirmed and quantified using known standards and mainly contained phenolic acids, flavonoids, and glucosinolates compounds. Most of the epicatechin, quercetin, and isorhamnetin derivates were found in red and pink petals of B. napus, while kaempferol derivates were in yellow and pale white petals. Moreover, petal-specific compounds, including a putative hydroxycinnamic acid derivative, sinapoyl malate, 1-O-sinapoyl-β-d-glucose, feruloyl glucose, naringenin-7-O-glucoside, cyanidin-3-glucoside, cyanidin-3,5-di-O-glucoside, petunidin-3-O-β-glucopyranoside, isorhamnetin-3-O-glucoside, kaempferol-3-O-glucoside-7-O-glucoside, quercetin-3,4'-O-di-β-glucopyranoside, quercetin-3-O-glucoside, and delphinidin-3-O-glucoside, might contribute to a variety of petal colors in B. napus. In addition, bound phenolics were tentatively identified and contained three abundant compounds (p-coumaric acid, ferulic acid, and 8-O-4'-diferulic acid). These results provide insight into the molecular mechanisms underlying petal color and suggest strategies for breeding rapeseed with a specific petal color in the future.

Identification and Characterization of Major Constituents in Different-Colored Rapeseed Petals by UPLC–HESI-MS/MS

Candidate genes associated with lignin and lodging traits were identified by combining phenotypic, genotypic, and gene expression data in B. napus.
 Brassica napus is one of the world’s most important oilseed crops, but its yield can be dramatically reduced by lodging, bending, and falling of its vertical stems. Lignin has been shown to contribute to stem mechanical strength. In this study, we found that the syringyl/guaiacyl (S/G) monolignol ratio exhibits a significant negative correlation with disease and lodging resistance. A total of 92 and 50 SNP and SSR loci, respectively, were found to be significantly associated with five traits, breaking force, breaking strength, lodging coefficient, acid detergent lignin content, and the S/G monolignol ratio using GWAS. To identify novel genes involved in lignin biosynthesis, transcriptome sequencing of high- (H) and low (L)-ADL content accessions was performed. The up-regulated genes were mainly involved in glycoside catabolic processes (especially glucosinolate catabolism) and cell wall biogenesis, while down-regulated genes were involved in glucosinolate biosynthesis, indicating that crosstalk exists between glucosinolate metabolic processes and lignin biosynthesis. Integrating this differential expression with the GWAS analysis, we identified four candidate genes regulating lignin, including glycosyl hydrolase (BnaA01g00480D), CYT1 (BnaA04g22820D), and two encoding transcription factors, SHINE1 (ERF family) and DAR6 (LIM family). This study provides insight into the genetic control of lodging and lignin in B. napus.

Genetic and transcriptomic analyses of lignin- and lodging-related traits in Brassica napus

Silencing of BnTT1 family genes affects seed flavonoid biosynthesis and alters seed fatty acid composition in Brassica napus

FATTY ACID DESATURASE 2 (FAD2, EC 1.3.1.35), also known as delta-12 oleate desaturase, is a key enzyme for linoleic acid and α-linolenic acid biosynthesis. Chia (Salvia hispanica) seeds contain the highest known proportion of α-linolenic acid in any plant sources. In this study, two full-length FAD2 genes, named as ShFAD2-1 and ShFAD2-2, were isolated from S. hispanica based on RACE method. Both ShFAD2-1 and ShFAD2-2 proteins possess strong transmembrane helices, three histidine motifs and a C-terminal ER-located signal (YNNKL). Phylogenetic analysis showed that both ShFAD2-1 and ShFAD2-2 are grouped with constitutive plant FAD2s. Heterologous expression in Saccharomyces cerevisiae indicated that ShFAD2-1 and ShFAD2-2 genes both encode a bio-functional delta-12 oleate desaturase. ShFAD2-2 was mainly expressed in flowers and early-stage seeds while ShFAD2-1 expression was almost constitutive in different organs. qRT-PCR results demonstrated that ShFAD2-1 and ShFAD2-2 show a cold-induced and heat-repressed expression pattern, whereas they also were differentially up-regulated or repressed by other abiotic stresses. This is the first cloning and function characterization of FAD2 from S. hispanica, which can provide insights into molecular mechanism of high ALA traits of S. hispanica and enrich our understanding of the roles of FAD2 genes in various abiotic stresses.

Nengwen Yin

Papers

Genome-wide association analysis and differential expression analysis of resistance to Sclerotinia stem rot in Brassica napus.

Identification and Characterization of Major Constituents in Different-Colored Rapeseed Petals by UPLC–HESI-MS/MS

Genetic and transcriptomic analyses of lignin- and lodging-related traits in Brassica napus

Silencing of BnTT1 family genes affects seed flavonoid biosynthesis and alters seed fatty acid composition in Brassica napus

Molecular cloning and expression analysis of two FAD2 genes from chia (Salvia hispanica)