TRIPTYCHON-LIKE regulates aspects of both fruit flavor and color in citrus

WRKY75 regulates anthocyanin accumulation in juvenile citrus tissues

The anthocyanin accumulation in juvenile tissues can enhance the ornamental value, attract pollinators, and help improve abiotic stress. Although transcriptional regulation studies of anthocyanin have been relatively extensive, there are few reports on the mechanism of anthocyanin accumulation in young tissues. This study reveals that many juvenile citrus tissues (flowers, leaves, and pericarp) undergo transient accumulation of anthocyanins, exhibiting a red coloration. Using weighted gene co-expression network analysis (WGCNA) identified CitWRKY75 as a candidate gene. After detecting the expression levels of CitWRKY75 in various citrus juvenile tissues, the expression trend of CitWRKY75 was highly consistent with the red exhibiting and fading. Overexpression of CitWRKY75 in tobacco significantly increased the anthocyanin content. LUC and yeast one-hybrid assay demonstrated that CitWRKY75 could bind to the promoter of CitRuby1(encoding the key transcription factor promoting anthocyanin accumulation) and promote its expression. Finally, comparing the expression levels of CitWRKY75 and CitRuby1 in the late development stage of blood orange found that CitWRKY75 was not the main regulatory factor for anthocyanin accumulation in the later stage. This study used reverse genetics to identify a transcription factor, CitWRKY75, upstream of CitRuby1, which promotes anthocyanin accumulation in citrus juvenile tissues.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-024-01490-9.

Seasonal drought promotes citrate accumulation in citrus fruit through the CsABF3-activated CsAN1-CsPH8 pathway

Plenty of rainfall but unevenly seasonal distribution happens regularly in southern China. Seasonal drought from summer to early autumn leads to citrus fruit acidification, but how seasonal drought regulates citrate accumulation remains unknown. Herein, we employed a set of physiological, biochemical, and molecular approaches to reveal that CsABF3 responds to seasonal drought stress and modulates citrate accumulation in citrus fruits by directly regulating CsAN1 and CsPH8. Here, we demonstrated that irreversible acidification of citrus fruits is caused by drought lasting for > 30 d during the fruit enlargement stage. We investigated the transcriptome characteristics of fruits affected by drought and corroborated the pivotal roles of a bHLH transcription factor (CsAN1) and a P3A-ATPase gene (CsPH8) in regulating citrate accumulation in response to drought. Abscisic acid (ABA)-responsive element binding factor 3 (CsABF3) was upregulated by drought in an ABA-dependent manner. CsABF3 activated CsAN1 and CsPH8 expression by directly and specifically binding to the ABA-responsive elements (ABREs) in the promoters and positively regulated citrate accumulation. Taken together, this study sheds new light on the regulatory module ABA-CsABF3-CsAN1-CsPH8 responsible for citrate accumulation under drought stress, which advances our understanding of quality formation of citrus fruit.

Multi-omics analysis provides new insights into the changes of important nutrients and fructose metabolism in loquat bud sport mutant

Bud sport is a common and stable somatic variation in perennial fruit trees, and often leads to significant modification of fruit traits and affects the breeding value. To investigate the impact of bud sport on the main metabolites in the fruit of white-fleshed loquat, we conducted a multi-omics analysis of loquat fruits at different developmental stages of a white-fleshed bud sport mutant of Dongting loquat (TBW) and its wild type (TBY). The findings from the detection of main fruit quality indices and metabolites suggested that bud sport resulted in a reduction in the accumulation of carotenoids, fructose, titratable acid and terpenoids at the mature stage of TBW, while leading to the accumulation of flavonoids, phenolic acids, amino acids and lipids. The comparably low content of titratable acid further enhances the balanced and pleasent taste profile of TBW. Expression patterns of differentially expressed genes involved in fructose metabolism exhibited a significant increase in the expression level of S6PDH (EVM0006243, EVM0044405) prior to fruit maturation. The comparison of protein sequences and promoter region of S6PDH between TBY and TBW revealed no structural variations that would impact gene function or expression, indicating that transcription factors may be responsible for the rapid up-regulation of S6PDH before maturation. Furthermore, correlation analysis helped to construct a comprehensive regulatory network of fructose metabolism in loquat, including 23 transcription factors, six structural genes, and nine saccharides. Based on the regulatory network and existing studies, it could be inferred that transcription factors such as ERF, NAC, MYB, GRAS, and bZIP may promote fructose accumulation in loquat flesh by positively regulating S6PDH. These findings improve our understanding of the nutritional value and breeding potential of white-fleshed loquat bud sport mutant, as well as serve as a foundation for exploring the genes and transcription factors that regulate fructose metabolism in loquat.

15-cis-Phytoene Desaturase and 15-cis-Phytoene Synthase Can Catalyze the Synthesis of β-Carotene and Influence the Color of Apricot Pulp

Fruit color affects its commercial value. β-carotene is the pigment that provides color for many fruits and vegetables. However, the molecular mechanism of β-carotene metabolism during apricot ripening is largely unknown. Here, we investigated whether β-carotene content affects apricot fruit color. First, the differences in β-carotene content between orange apricot 'JTY' and white apricot 'X15' during nine developmental stages (S1-S9) were compared. β-carotene contents highly significantly differed between 'JTY' and 'X15' from S5 (color transition stage) onwards. Whole-transcriptome analysis showed that the β-carotene synthesis genes 15-cis-phytoene desaturase (PaPDS) and 15-cis-phytoene synthase (PaPSY) significantly differed between the two cultivars during the color transition stage. There was a 5 bp deletion in exon 11 of PaPDS in 'X15', which led to early termination of amino acid translation. Gene overexpression and virus-induced silencing analysis showed that truncated PaPDS disrupted the β-carotene biosynthesis pathway in apricot pulp, resulting in decreased β-carotene content and a white phenotype. Furthermore, virus-induced silencing analysis showed that PaPSY was also a key gene in β-carotene biosynthesis. These findings provide new insights into the molecular regulation of apricot carotenoids and provide a theoretical reference for breeding new cultivars of apricot.

Comprehensive analyses of the citrus WRKY gene family involved in the metabolism of fruit sugars and organic acids

Sugars and organic acids are the main factors determining the flavor of citrus fruit. The WRKY transcription factor family plays a vital role in plant growth and development. However, there are still few studies about the regulation of citrus WRKY transcription factors (CsWRKYs) on sugars and organic acids in citrus fruit. In this work, a genome-wide analysis of CsWRKYs was carried out in the citrus genome, and a total of 81 CsWRKYs were identified, which contained conserved WRKY motifs. Cis-regulatory element analysis revealed that most of the CsWRKY promoters contained several kinds of hormone-responsive and abiotic-responsive cis-elements. Furthermore, gene expression analysis and fruit quality determination showed that multiple CsWRKYs were closely linked to fruit sugars and organic acids with the development of citrus fruit. Notably, transcriptome co-expression network analysis further indicated that three CsWRKYs, namely, CsWRKY3, CsWRKY47, and CsWRKY46, co-expressed with multiple genes involved in various pathways, such as Pyruvate metabolism and Citrate cycle. These CsWRKYs may participate in the metabolism of fruit sugars and organic acids by regulating carbohydrate metabolism genes in citrus fruit. These findings provide comprehensive knowledge of the CsWRKY family on the regulation of fruit quality.

Evolutionary assessment of SQUAMOSA PROMOTER BINDING PROTEIN-LIKE genes in citrus relatives with a specific focus on flowering

Phase transition and floral induction in citrus requires several years of juvenility after germination. Such a long period of juvenility has been a major hindrance to its genetic improvement program. Studies have shown that miR156 along with its downstream genes SQUAMOSA PROMOTER BINDING PROTEINS (SBP) and SBP-LIKE (SPL) mediate the phase transition and floral induction process in plants. Our current study has systematically analyzed SPLs in 15 different citrus-related species, systematically annotated them based on their close homology to their respective Arabidopsis orthologs, and confirmed the functional attributes of the selected members in floral precocity. The majority of the species harbored 15 SPLs. Their cis-element assessment suggested the involvement of the SPLs in diverse developmental and physiological processes in response to different biotic and abiotic cues. Among all, SPL5, SPL9, and SPL11 stood out as consistently differentially expressed SPLs in the adult and young tissues of different citrus-related species. Independent overexpression of their F. hindsii orthologs (FhSPL5, FhSPL9, and FhSPL11) brought an enhanced expression of endogenous FLOWERING LOCUS T leading to the significantly precocious flowering in transgenic Arabidopsis lines. Future study of the genes in the citrus plant itself is expected to conclude the assessments made in the current study.

Weighted gene coexpression correlation network analysis reveals the potential molecular regulatory mechanism of citrate and anthocyanin accumulation between postharvest 'Bingtangcheng' and 'Tarocco' blood orange fruit

BACKGROUND

Organic acids and anthocyanins are the most important compounds for the flavor and nutritional quality of citrus fruit. However, there are few reports on the involvement of co-regulation of citrate and anthocyanin metabolism. Here, we performed a comparative transcriptome analysis to elucidate the genes and pathways involved in both citrate and anthocyanin accumulation in postharvest citrus fruit with 'Tarocco' blood orange (TBO; high accumulation) and 'Bingtangcheng' sweet orange (BTSO; low accumulation).

RESULTS

A robust core set of 825 DEGs were found to be temporally associated with citrate and anthocyanin accumulation throughout the storage period through transcriptome analysis. Further according to the results of weighted gene coexpression correlation network analysis (WGCNA), the turquoise and brown module was highly positively correlated with both of the content of citrate and anthocyanin, and p-type ATPase (PH8), phosphoenolpyruvate carboxylase kinase (PEPCK), chalcone isomerase (CHI), flavanone 3-hydroxylase (F3H), flavonoid 3'-hydroxylase (F3'H) and glutathione S transferase (GST) were considered key structural genes. Moreover, MYB family transcription factor (PH4), Zinc finger PHD-type transcription factor (CHR4, HAC12), Zinc finger SWIM-type transcription factor (FAR1) and Zinc finger C3H1-type transcription factor (ATC3H64) were considered hub genes related to these structural genes. Further qRT-PCR analysis verified that these transcription factors were highly expressed in TBO fruit and their expression profiles were significantly positively correlated with the structural genes of citrate and anthocyanin metabolism as well as the content of citrate and anthocyanin content.

CONCLUSIONS

The findings suggest that the CHR4, FAR1, ATC3H64 and HAC12 may be the new transcription regulators participate in controlling the level of citrate and anthocyanin in postharvest TBO fruit in addition to PH4. These results may providing new insight into the regulation mechanism of citrate and anthocyanin accumulation in citrus fruit.

The transcription factors SbMYB45 and SbMYB86.1 regulate flavone biosynthesis in Scutellaria baicalensis

Scutellaria baicalensis Georgi is an important Chinese medicinal plant that is rich in the flavones baicalin, wogonoside, and wogonin, providing it with anti-cancer, anti-inflammatory, and antibacterial properties. However, although the biosynthetic pathways of baicalin and its derivates have been elucidated, the regulation of flavone biosynthesis in S. baicalensis is poorly understood. Here, we found that the contents of baicalin and its derivates increased and that baicalin biosynthetic pathway genes were induced in response to light, and baicalin and baicalein are not exclusively produced in the roots of S. baicalensis. Based on the fact that MYB transcription factors are known to play important roles in flavone biosynthesis, we identified SbMYB45 and SbMYB86.1 in S. baicalensis and determined that they bind to the promoter of the flavone biosynthesis gene SbCHI to enhance its transcription. Moreover, overexpressing SbMYB45 and SbMYB86.1 enhanced the accumulation of baicalin in S. baicalensis leaves. We demonstrate that SbMYB45 and SbMYB86.1 bind to the cis-acting element MBSII in the promoter of CHI to redundantly induce its expression upon light exposure. These findings indicate that SbMYB45 and SbMYB86.1 transcriptionally activate SbCHI in response to light and enhance flavone contents in S. baicalensis.

The high-quality genome of pummelo provides insights into the tissue-specific regulation of citric acid and anthocyanin during domestication

Citric acid and anthocyanin contents were co-selected during Citrus domestication. Pummelo is a founding species in the Citrus genus, but the domestication of pummelo has not been well studied. Here, we compared the citric acid and anthocyanin contents of a low citric acid pummelo (Citrus maxima LCA) and its high citric acid variety (HCA) from the same cultivation area in China. Our study revealed that, unlike the LCA type, the HCA variety accumulated anthocyanin in the pericarp early in fruit development. To investigate the genetic basis of acid and anthocyanin enrichment in HCA pulp and pericarp, respectively, we generated a chromosome-scale HCA genome using long-read sequence reads and Hi-C sequencing data. Transcriptome analysis and transient overexpression assays showed that the accumulation of citric acid and anthocyanin was associated with high expression of CgANTHOCYANIN1 (CgAN1), and two different MYBs transcription factors (CgPH4 and CgRuby1), respectively. Moreover, the CgAN1 promoter was more methylated in the LCA pulp than in the HCA pulp. Treatment with a DNA methylation inhibitor, 5-azacytidine, alleviated the CgAN1 promoter hypermethylation in the LCA pulp, leading to increased CgAN1 expression and citric acid content. This study provides a new high-quality pummelo genome and insight into the molecular mechanism behind the change in tissue-specific citric acid and anthocyanin accumulation during pummelo domestication and provides a conceptual basis for precise genetic manipulation in fruit flavor breeding.

Overexpression of leucoanthocyanidin reductase or anthocyanidin reductase elevates tannins content and confers cassava resistance to two-spotted spider mite

The two-spotted spider mite (TSSM) is a destructive cassava pest. Intensive demonstration of resistance mechanism greatly facilitates the creation of TSSM-resistant cassava germplasm. Gene to metabolite network plays a crucial role in modulating plant resistance, but little is known about the genes and related metabolites which are responsible for cassava resistance to TSSM. Here, a highly resistant (HR) and a highly susceptible (HS) cassava cultivar were used, integrative and comparative transcriptomic and metabolomic analyses between these two cultivars after TSSM infestation revealed that several genes and metabolites were closely related and significantly different in abundance. In particular, the expression of leucoanthocyanidin reductase (LAR) and anthocyanidin reductase (ANR) genes showed a high positive correlation with most of the metabolites in the tannin biosynthesis pathway. Furthermore, transgenic cassava lines overexpressing either of the genes elevated tannin concentrations and conferred cassava resistance to TSSM. Additionally, different forms of tannins possessed distinct bioactivity on TSSM, of which total condensed tannins (LC₅₀ = 375.68 mg/l) showed maximum lethal effects followed by procyanidin B1 (LC₅₀ = 3537.10 mg/l). This study accurately targets LAR, ANR and specific tannin compounds as critical genes and metabolites in shaping cassava resistance to TSSM, which could be considered as biomarkers for evaluation and creation of pest-resistant cassava germplasm.