Transcription factors PuPRE6/PuMYB12 and histone deacetylase PuHDAC9-like regulate sucrose levels in pear

A R2R3-MYB transcription factor LmMYB111 positively regulates chlorogenic acid and luteoloside biosynthesis in Lonicera macranthoides

Lonicera macranthoides is a vital medicinal herb frequently used in Chinese traditional medicine. Chlorogenic acid (CGA) and luteoloside are the most crucial bioactive pharmaceutical ingredients in L. macranthoides. Although CGA and luteoloside biosynthetic pathway and structural genes appeared to be extensively elucidated, the transcriptional regulation has yet to be unveiled. Here, integration of transcriptome and metabolome revealed a R2R3-MYB transcription factor LmMYB111 positively correlated with CGA concentration, which shares close homology with AtMYB111 and acts as a transcriptional activator. Overexpressing LmMYB111 in tobacco and Lonicera resulted in enhanced production of CGA and luteoloside. RNA-Seq demonstrated that overexpression of LmMYB111 dramatically upregulated CGA and luteoloside biosynthetic genes, including 10 PALs, 3 C4Hs, 7 4CLs, 4 HCT/HQTs, 3 CHSs and 5 CHIs. DNA Affinity Purification sequencing (DAP-Seq) revealed the binding motifs of LmMYB111 and 1135 downstream targets, including structural genes e.g. PAL1/PAL4s, C4H, 4CL2, CHI, and DFR as well as several transcription factors (TFs), e.g. MYB3/MYB4, bHLH62/TT8, BEL1, SCL15/SCL32 and ERF3.The electrophoretic mobility shift assay (EMSA) together with dual-luciferase reporter system (DLR) further proved that LmMYB111 bound to and activated proLmMYB4, proLmPAL1, proLm4CL2, proLmCHI and proLmDFR, therefore facilitating hyperaccumulation of CGA, luteoloside and other phenolics. These findings shed light on the participation of LmMYB111 in CGA and luteoloside biosynthetic regulatory networks in L. macranthoides mediated by controlling the expression of structural genes and TFs, which will contribute to elevate phenolics production by genetic engineering.

CsMYB1-CwINV6 Module Involves in the Promotion of Soluble Sugar Accumulation in Citrus Fruits Under Drought Stress

Drought can promote soluble sugar accumulation in fruits by increasing the fruit sink strength. Cell wall invertase (CwINV) plays a pivotal role in determining sink strength by regulating sucrose partitioning into the extracellular matrix. Research has demonstrated that drought stress significantly increases the transcript level of citrus CwINV6, but the transcriptional mechanisms governing its regulation under drought conditions remain elusive. In this study, we characterised the MYB transcription factor gene CsMYB1 from the citrus genome. CsMYB1 is localised in the cell nucleus, and CwINV6 is localised in the cell wall. Furthermore, the transcript levels of both CsMYB1 and CwINV6 significantly increased in 'Nanfeng' tangerine fruits (Citrus reticulata) in response to drought or ABA treatment. Transient overexpression of CsMYB1 or CwINV6 promoted the accumulation of glucose and fructose in 'Nanfeng' fruits. Conversely, transient VIGS of CsMYB1 or CwINV6 resulted in the opposite trend. Additionally, stable overexpression of CsMYB1 or CwINV6 significantly increased the soluble sugar content in the fruits of the 'Micro-Tom' tomato lines. Y1H and luciferase assays confirmed that CsMYB1 can bind to the CwINV6 promoter and positively regulate its expression. Taken together, our findings reveal that drought promotes soluble sugar distribution in citrus fruits by increasing sink strength via the CsMYB1-CwINV6 module.

CsbHLH122/CsMYBS3-CsSUT2 contributes to the rapid accumulation of sugar in the ripening stage of sweet orange (Citrus sinensis)

The sugar content serves as the fundamental metabolic component that determines both the flavor quality and the nutritional value of fruits. Nevertheless, the regulatory mechanism underlying the rapid accumulation of sugars during citrus fruit maturation remains elusive. In this study, we demonstrated that the expression level of sucrose transporter CsSUT2 is increased during citrus fruit ripening and sugar accumulation. Functional assays confirmed that CsSUT2 is localized in the plasma membrane and exhibits sucrose transporter activity. Homologous and heterologous overexpression of CsSUT2 in citrus juice sacs, calli, and tomato resulted in an increase in sugar content. Conversely, virus-induced gene silencing and RNAi-mediated silencing of CsSUT2 led to a decrease in sugar levels in transgenic citrus tissues. We further identified CsMYBS3 as an upstream transcription factor that positively regulates the expression of CsSUT2. Transgenic evidence supported that the induction of sugar accumulation by CsMYBS3 depends on the transcript level of CsSUT2. Additionally, we found that CsbHLH122 physically interacts with CsMYBS3 to form a transcription factor complex, enhancing promoter transcriptional activity of CsSUT2. This study expands our understanding of the function and regulatory mechanism of sugar transporter in citrus, providing valuable insights for regulating sugar accumulation and quality control in citrus fruit.

The bHLH transcription factor DlbHLH68 positively regulates DlSPS1 expression to promote sucrose biosynthesis in longan

Sucrose is an important factor affecting plant growth and fruit quality, but the molecular regulatory mechanism of sucrose biosynthesis in longan is not yet understood. Here, we characterized a transcription factor, DlbHLH68, positively regulates sucrose accumulation in longan. Subcellular localization and transcriptional activity analysis indicated that DlbHLH68 is a nuclear transcriptional activator. Overexpressing DlbHLH68 in Arabidopsis enhanced sucrose content, plant height, and the relative expression level of sucrose phosphate synthase genes (AtSPS1 and AtSPS2). Yeast one-hybrid and dual-luciferase reporter assays indicated that DlbHLH68 was able to activate the expression of DlSPS1, the homology gene of AtSPS1. As expected, overexpression of DlSPS1 significantly increased the sucrose content in transgenic Arabidopsis and longan fruits. Collectively, this study reveals that DlbHLH68 is a positive regulator in sucrose accumulation by activating DlSPS1 expression to mediate sucrose biosynthesis, which is helpful for understanding the molecular basis of sucrose biosynthesis and accumulation in longan fruit and provides candidate genes for further breeding.

Cloning the promoter of the sucrose transporter gene PsSUT2 and screening its upstream transcription factors in tree peony

Sucrose is an essential energy substance for tree peony (Paeonia Suffruticosa) floral organ development. However, little is known about the sucrose regulatory network in tree peony. In this study, the promoter sequence of the tree peony sucrose transporter gene PsSUT2 was cloned. Through cis-acting elements analysis and weighted gene co-expression network analysis (WGCNA), 6 transcription factors potentially regulating PsSUT2 were screened. Expression analysis revealed that the 6 transcription factors had similar expression trends with the PsSUT2 in all parts of peony at the full bloom stage. Furthermore, a yeast one-hybrid assay revealed that PsMYB20 and PsMADS9 bind to the PsSUT2 promoter. Dual-luciferase reporter assay demonstrated that PsMYB20 and PsMADS9 could activate PsSUT2 expression. Taken together, our findings suggest that PsMYB20 and PsMADS9 positively regulate PsSUT2, laying the foundation for the construction of a gene network for sucrose regulation in tree peony.

Transcription factors PuNAC37/PuWRKY74 and E3 ubiquitin ligase PuRDUF2 inhibit volatile ester synthesis in 'Nanguo' pear

As major contributors to flavor in many fruit species, volatile esters are useful for investigating the regulation of the biosynthesis pathways of volatile aroma compounds in plants. Ethylene promotes the biosynthesis of volatile esters during fruit storage while accelerating fruit ripening; thus, the ethylene perception inhibitor 1-methylcyclopropene (1-MCP) is employed to prolong the shelf life of fruits. However, the mechanisms by which 1-MCP regulates volatiles synthesis remain unclear. In this study, we analyzed the pathway of 1-MCP-mediated volatile ester synthesis in 'Nanguo' pear (Pyrus ussuriensis). 1-MCP significantly decreased volatile ester synthesis during storage. Comparative transcriptome analysis showed that the genes encoding two transcription factors (PuNAC37 and PuWRKY74) and a RING-type E3 ubiquitin ligase that have a domain of unknown function (PuRDUF2) were expressed at high levels, whereas ALCOHOL ACYLTRANSFERASE 1 (PuAAT1), encoding an enzyme responsible for volatile ester synthesis, was expressed at low levels in 1-MCP-treated fruit. Moreover, PuNAC37 inhibited the expression of PuWRKY74 via transcriptional regulation, whereas PuNAC37 and PuWRKY74, after directly binding to the promoter of PuAAT1, synergistically inhibited its expression in 1-MCP-treated fruit. In addition, in vitro and in vivo ubiquitination experiments revealed that PuRDUF2 functions as an E3 ubiquitin ligase that ubiquitinates PuAAT1, causing its degradation via the 26S proteasome pathway following 1-MCP treatment. Subsequent PuAAT1 degradation resulted in a reduction of volatile esters during fruit storage. Our findings provide insights into the complex transcriptional regulation of volatile ester formation in 'Nanguo' pears and contribute to future research on AAT protein ubiquitination in other species.

Comparative transcriptome analysis identifies candidate genes related to sucrose accumulation in longan (Dimocarpus longan Lour.) pulp

Sucrose content is one of the important factors to determine longan fruit flavor quality. To gain deep insight of molecular mechanism on sucrose accumulation in longan, we conducted comparative transcriptomic analysis between low sucrose content longan cultivar 'Qingkebaoyuan' and high sucrose content cultivar 'Songfengben'. A total of 12,350 unique differentially expressed genes (DEGs) were detected across various development stages and different varieties, including hexokinase (HK) and sucrose-phosphate synthase (SPS), which are intricately linked to soluble sugar accumulation and metabolism. Weighted gene co-expression network analysis (WGCNA) identified magenta module, including DlSPS gene, was significantly positively correlated with sucrose content. Furthermore, transient expression unveiled DlSPS gene play crucial role in sucrose accumulation. Moreover, 5 transcription factors (MYB, ERF, bHLH, C2H2, and NAC) were potentially involved in DlSPS regulation. Our findings provide clues for sucrose metabolism, and lay the foundation for longan breeding in the future.