Regulation of BcMYB44 on Anthocyanin Synthesis and Drought Tolerance in Non-Heading Chinese Cabbage (Brassica campestris ssp. chinensis Makino)

Integration of genome-wide association studies, metabolomics, and transcriptomics reveals phenolic acid- and flavonoid-associated genes and their regulatory elements under drought stress in rapeseed flowers

Introduction

Biochemical and metabolic processes help plants tolerate the adverse effects of drought. In plants accumulating bioactive compounds, understanding the genetic control of the biosynthesis of biochemical pathways helps the discovery of candidate gene (CG)-metabolite relationships.

Methods

The metabolic profile of flowers in 119 rapeseed (Brassica napus) accessions was assessed over two irrigation treatments, one a well-watered (WW) condition and the other a drought stress (DS) regime. We integrated information gained from 52,157 single-nucleotide polymorphism (SNP) markers, metabolites, and transcriptomes to identify linked SNPs and CGs responsible for the genetic control of flower phenolic compounds and regulatory elements.

Results

In a genome-wide association study (GWAS), of the SNPs tested, 29,310 SNPs were qualified to assess the population structure and linkage disequilibrium (LD), of which several SNPs for radical scavenging activity (RSA) and total flavanol content (TFLC) were common between the two irrigation conditions and pleiotropic SNPs were found for chlorogenic and coumaric acids content. The principal component analysis (PCA) and stepwise regression showed that chlorogenic acid and epicatechin in WW and myricetin in DS conditions were the most important components for RSA. The hierarchical cluster analysis (HCA) showed that vanillic acid, myricetin, gallic acid, and catechin were closely associated in both irrigation conditions. Analysis of GWAS showed that 60 CGs were identified, of which 18 were involved in stress-induced pathways, phenylpropanoid pathway, and flavonoid modifications. Of the CGs, PAL1, CHI, UGT89B1, FLS3, CCR1, and CYP75B137 contributed to flavonoid biosynthetic pathways. The results of RNA sequencing (RNA-seq) revealed that the transcript levels of PAL, CHI, and CYP75B137 known as early flavonoid biosynthesis-related genes and FLS3, CCR1, and UGT89B1 related to the later stages were increased during drought conditions. The transcription factors (TFs) NAC035 and ERF119 related to flavonoids and phenolic acids were upregulated under drought conditions.

Discussion

These findings expand our knowledge on the response mechanisms to DS, particularly regarding the regulation of key phenolic biosynthetic genes in rapeseed. Our data also provided specific linked SNPs for marker-assisted selection (MAS) programs and CGs as resources toward realizing metabolomics-associated breeding of rapeseed.

Characterization Variation of the Differential Coloring Substances in Rapeseed Petals with Different Colors Using UPLC-HESI-MS/MS

Rapeseed's (Brassica napus L.) colorful petals have important ornamental values. However, the mechanisms of regulating petals coloration in rapeseed are still unknown. In our study, we investigated the key differential coloring substances in nine rapeseed cultivars with different petal colors, and 543 metabolites were detected and characterized through UPLC-HESI-MS/MS. Among them, the kinds and contents of flavonols, flavones, and anthocyanidins were the main contributors to petals' coloration. Tamarixetin-, quercetin-, butin-, naringenin- and luteolin-derivates were the main pigment bases in white and yellow petals. Peonidin-3,5-O-diglucoside, peonidin-3-O-(6″-O-caffeoyl)glucoside, and quercetin-derivatives were the main coloring substances in pink petals. Acylated cyanidin derivatives might lead to a series of different purple petal colors. Glycosylated anthocyanins were responsible for the coloration of rapeseed red petals, and peonidin-3-O-glucoside and kaempferol-derivatives were mainly detected from the red petals. These results provide comprehensive insights into the difference in flavonoid metabolites in rapeseed petals with different colors and supply theoretical supports for the breeding of novel colorful rapeseed cultivars.

VaMYB44 transcription factor from Chinese wild Vitis amurensis negatively regulates cold tolerance in transgenic Arabidopsis thaliana and V. vinifera

Heterologous expression of VaMYB44 gene in Arabidopsis and V. vinifera cv. 'Thompson Seedless' increases cold sensitivity, which is mediated by the interaction of VaMYC2 and VaTIFY5A with VaMYB44 MYB transcription factors play critical roles in plant stress response. However, the function of MYB44 under low temperature stress is largely unknown in grapes. Here, we isolated a VaMYB44 gene from Chinese wild Vitis amurensis acc. 'Shuangyou' (cold-resistant). The VaMYB44 is expressed in various organs and has lower expression levels in stems and young leaves. Exposure of the cold-sensitive V. vinifera cv. 'Thompson Seedless' and cold-resistant 'Shuangyou' grapevines to cold stress (-1 °C) resulted in differential expression of MYB44 in leaves with the former reaching 14 folds of the latter after 3 h of cold stress. Moreover, the expression of VaMYB44 was induced by exogenous ethylene, abscisic acid, and methyl jasmonate in the leaves of 'Shuangyou'. Notably, the subcellular localization assay identified VaMYB44 in the nucleus. Interestingly, heterologous expression of VaMYB44 in Arabidopsis and 'Thompson Seedless' grape increased freezing-induced damage compared to their wild-type counterparts. Accordingly, the transgenic lines had higher malondialdehyde content and electrolyte permeability, and lower activities of superoxide dismutase, peroxidase, and catalase. Moreover, the expression levels of some cold resistance-related genes decreased in transgenic lines. Protein interaction assays identified VaMYC2 and VaTIFY5A as VaMYB44 interacting proteins, and VaMYC2 could bind to the VaMYB44 promoter and promote its transcription. In conclusion, the study reveals VaMYB44 as the negative regulator of cold tolerance in transgenic Arabidopsis and transgenic grapes, and VaMYC2 and VaTIFY5A are involved in the cold sensitivity of plants by interacting with VaMYB44.

Transcriptome and Metabolome Profiling to Explore the Causes of Purple Leaves Formation in Non-Heading Chinese Cabbage (Brassica rapa L. ssp. chinensis Makino var. mutliceps Hort.)

Purple non-heading Chinese cabbage is one of the most popular vegetables, and is rich in various health-beneficial anthocyanins. Research related to genes associated with anthocyanin biosynthesis in non-heading Chinese cabbage is important. This study performed integrative transcriptome and metabolome analysis in the purple non-heading Chinese cabbage wild type (WT) and its green mutant to elucidate the formation of purple leaves. The anthocyanin level was higher in purple than in green plants, while the contents of chlorophyll and carotenoid were higher in the green mutant than in the purple WT. Twenty-five anthocyanins were identified in purple and green cultivars; eleven anthocyanin metabolites were identified specifically in the purple plants. RNA-seq analysis indicated that 27 anthocyanin biosynthetic genes and 83 transcription factors were significantly differentially expressed between the WT and its mutant, most of them with higher expression in the purple than green non-heading Chinese cabbage. Transcriptome and metabolome analyses showed that UGT75C1 catalyzing the formation of pelargonidin-3,5-O-diglucoside and cyanidin-3,5-O-diglucoside may play a critical role in purple leaf formation in non-heading Chinese cabbage. Therefore, these results provide crucial information for elucidating the formation of purple leaves in non-heading Chinese cabbage.