MiR858b Inhibits Proanthocyanidin Accumulation by the Repression of DkMYB19 and DkMYB20 in Persimmon

Research advances of coloring mechanism regulated by MicroRNAs in plants

In plants, microRNAs (miRNAs) are a class of important small RNAs involved in their growth and development, and play a very significant role in regulating their tissue coloring. In this paper, the mechanisms on miRNA regulation of plant coloring are mainly reviewed from three aspects: macroscopic physiological and molecular foundations related to tissue coloring, miRNA biosynthesis and function, and specific analysis of miRNA regulation studies on leaf color, flower color, fruit color, and other tissue color formation in plants. Furthermore, we also systematically summarize the miRNA regulatory mechanisms identified on pigments biosynthesis and color formation in plants, and the regulatory mechanisms of these miRNAs mentioned on the existing researches can be divided into four main categories: directly targeting the related transcription factors, directly targeting the related structural genes, directly targeting the related long noncoding RNAs (LncRNAs) and miRNA-mediated production of trans-acting small interfering RNAs (ta-siRNAs). Together, these research results aim to provide a theoretical reference for the in-depth study of plant coloring mechanism and molecular breeding study of related plants in the future.

Regulatory Mechanism of Proanthocyanidins in Grape Peels Using vvi-miR828a and Its Target Gene VvMYBPA1

Anthocyanins and proanthocyanidins are considered to be essential secondary metabolites in grapes and are used to regulate metabolic processes, while miRNAs are involved in their synthesis of anthocyanins and proanthocyanidins to regulate metabolic processes. The present research work was carried out to investigate the underlying regulatory mechanism of target genes in the grape cultivars 'Italia' and 'Benitaka'. miRNA and transnscriptomic sequencing technology were employed to characterize both the profiles of miRNAs and the transcripts of grape peels at 10 and 11 weeks post flowering (10 wpf and 11 wpf). The results revealed that the expression level of vvi-miR828a in 'Italia' at 10 and 11 wpf was significantly higher than that in 'Benitaka'. miRNA-seq analysis predicted MYBPA1 to be the target gene of vvi-miR828a. In transcriptome analysis, the expression level of the VvMYBPA1 gene in 'Benitaka' was significantly higher than that in 'Italia'; in addition, the TPM values (expression levels) of VvMYBPA1 and miR828a also showed an evident negative correlation. The determination of the proanthocyanidin (PA) content in 'Italia' and 'Benitaka' peels at 11 wpf demonstrated that the PA content of 'Benitaka' was significantly higher than that of 'Italia'. The outcomes of RT-qRCR analysis exhibited that the expression levels of the VdPAL, VdCHS, VdCHI, VdDFR, VdMYB5b, VdANR, and VdMYBPA1 genes related anthocyanin and proanthocyanidin pathways were reduced, while the expression levels of all of the above genes were increased after the transient expression of the VvMYBPA1 vector into grape leaves. The results of the transient overexpression experiment of vvi-miR828a before the veraison period of strawberry fruits showed that vvi-miR828a can significantly slow down the coloration of strawberries. The vvi-miR828a negatively regulates the accumulation of proanthocyanidins in grape fruits by inhibiting the expression of VvMYBPA1.

The MpNAC72/MpERF105-MpMYB10b module regulates anthocyanin biosynthesis in Malus 'Profusion' leaves infected with Gymnosporangium yamadae

Apple rust is a serious fungal disease affecting Malus plants worldwide. Infection with the rust pathogen Gymnosporangium yamadae induces the accumulation of anthocyanins in Malus to resist rust disease. However, the mechanism of anthocyanin biosynthesis regulation in Malus against apple rust is still unclear. Here, we show that MpERF105 and MpNAC72 are key regulators of anthocyanin biosynthesis via the ethylene-dependent pathway in M. 'Profusion' leaves under rust disease stress. Exogenous ethephon treatment promoted high expression of MpERF105 and MpNAC72 and anthocyanin accumulation in G. yamadae-infected M. 'Profusion' leaves. Overexpression of MpERF105 increased the total anthocyanin content of Malus plant material and acted by positively regulating its target gene, MpMYB10b. MpNAC72 physically interacted with MpERF105 in vitro and in planta, and the two form a protein complex. Coexpression of the two leads to higher transcript levels of MpMYB10b and higher anthocyanin accumulation. In addition, overexpression of MpERF105 or MpNAC72 enhanced the resistance of M. 'Profusion' leaves to apple rust. In conclusion, our results elucidate the mechanism by which MpERF105 and MpNAC72 are induced by ethylene in G. yamadae-infected M. 'Profusion' leaves and promote anthocyanin accumulation by mediating the positive regulation of MpMYB10b expression.

The grapevine miR827a regulates the synthesis of stilbenes by targeting VqMYB14 and gives rise to susceptibility in plant immunity

Grapevine (Vitis vinifera L.) is an economically important fruit crop cultivated worldwide. In China, grapevine cultivation is very extensive, and a few Vitis grapes have excellent pathogen and stress resistance, but the molecular mechanisms underlying the grapevine response to stress remain unclear. In this study, a microRNA (miRNA; miR827a), which negatively regulates its target gene VqMYB14, a key regulatory role in the synthesis of stilbenes, was identified in Vitis quinquangularis (V. quinquangularis) using transcriptome sequencing. Using overexpression and silencing approaches, we found that miR827a regulates the synthesis of stilbenes by targeting VqMYB14. We used flagellin N-terminal 22-amino-acid peptide (flg22), the representative elicitor in plant basal immunity, as the elicitor to verify whether miR827a is involved in the basal immunity of V. quinquangularis. Furthermore, the promoter activity of miR827a was alleviated in transgenic grape protoplasts and Arabidopsis thaliana following treatment with flg22 and Pseudomonas syringae pv. Tomato DC3000 (Pst DC3000), respectively. In addition, yeast one-hybrid and dual luciferase reporter assay revealed that the ethylene transcription factor VqERF057 acted as a key regulator in the inhibition of miR827a transcription. These results will contribute to the understanding of the biological functions of miR827a in grapevine and clarify the molecular mechanism of the interaction between miR827a and VqMYB14.

The Smi-miR858a-SmMYB module regulates tanshinone and phenolic acid biosynthesis in Salvia miltiorrhiza

Tanshinones and phenolic acids are two major classes of bioactive compounds in Salvia miltiorrhiza. Revealing the regulatory mechanism of their biosynthesis is crucial for quality improvement of S. miltiorrhiza medicinal materials. Here we demonstrated that Smi-miR858a-Smi-miR858c, a miRNA family previously known to regulate flavonoid biosynthesis, also played critical regulatory roles in tanshinone and phenolic acid biosynthesis in S. miltiorrhiza. Overexpression of Smi-miR858a in S. miltiorrhiza plants caused significant growth retardation and tanshinone and phenolic acid reduction. Computational prediction and degradome and RNA-seq analyses revealed that Smi-miR858a could directly cleave the transcripts of SmMYB6, SmMYB97, SmMYB111, and SmMYB112. Yeast one-hybrid and transient transcriptional activity assays showed that Smi-miR858a-regulated SmMYBs, such as SmMYB6 and SmMYB112, could activate the expression of SmPAL1 and SmTAT1 involved in phenolic acid biosynthesis and SmCPS1 and SmKSL1 associated with tanshinone biosynthesis. In addition to directly activating the genes involved in bioactive compound biosynthesis pathways, SmMYB6, SmMYB97, and SmMYB112 could also activate SmAOC2, SmAOS4, and SmJMT2 involved in the biosynthesis of methyl jasmonate, a significant elicitor of plant secondary metabolism. The results suggest the existence of dual signaling pathways for the regulation of Smi-miR858a in bioactive compound biosynthesis in S. miltiorrhiza.

Revisiting decade-old questions in proanthocyanidin biosynthesis: current understanding and new challenges

Proanthocyanidins (PAs), one of the most abundant natural polymers found in plants, are gaining increasing attention because of their beneficial effects for agriculture and human health. The study of PA biosynthesis has been active for decades, and progress has been drastically accelerated since the discovery of key enzymes such as Anthocyanidin Reductase (ANR), Leucoanthocyanidin Reductase (LAR), and key transcription factors such as Transparent Testa 2 (TT2) and Transparent Testa 8 (TT8) in the early 2000s. Scientists raised some compelling questions regarding PA biosynthesis about two decades ago in the hope that addressing these questions would lead to an enhanced understanding of PA biosynthesis in plants. These questions focus on the nature of starter and extension units for PA biosynthesis, the stereochemistry of PA monomers and intermediates, and how and where the polymerization or condensation steps work subcellularly. Here, I revisit these long-standing questions and provide an update on progress made toward answering them. Because of advanced technologies in genomics, bioinformatics and metabolomics, we now have a much-improved understanding of functionalities of key enzymes and identities of key intermediates in the PA biosynthesis and polymerization pathway. Still, several questions, particularly the ones related to intracellular PA transportation and deposition, as well as enzyme subcellular localization, largely remain to be explored. Our increasing understanding of PA biosynthesis in various plant species has led to a new set of compelling open questions, suggesting future research directions to gain a more comprehensive understanding of PA biosynthesis.

GhmiR858 Inhibits the Accumulation of Proanthocyanidins by Targeting GhTT2L in Cotton (Gossypium hirsutum)

Proanthocyanidins (PAs) are predominantly regulated at the transcriptional level by sophisticated regulatory networks. In cotton, the role of miRNAs as key regulatory factors at the post-transcriptional level is still unclear. Here, we demonstrated that GhmiR858 negatively regulates PA accumulation in cotton leaves and calli by targeting GhTT2L. Excessive expression of GhmiR858 restrained the expression of GhTT2L, resulting in a significant decrease in PA abundance. Conversely, a reduction in GhmiR858 activity upregulated GhTT2L, which increased PA accumulation. Additionally, GhTT2L was found to positively regulate PA accumulation in both cotton and Arabidopsis. Further analyses showed that GhTT2L interacted with transcription factor GhTTG1, which directly binds to the GhANR promoter, to facilitate its transcription. This study provides new information to guide future studies of the PA regulatory mechanisms affected by miRNAs as well as the breeding of novel varieties of colored cotton with rich PAs.

The eTM-miR858-MYB62-like module regulates anthocyanin biosynthesis under low-nitrogen conditions in Malus spectabilis

The anthocyanin content increases in Malus spectabilis leaves under low-nitrogen conditions. Noncoding RNAs are indicated to play key regulatory roles in anthocyanin biosynthesis. However, the functional roles of noncoding RNAs in anthocyanin biosynthesis under low-nitrogen conditions remain elusive. In this study, miR858 was screened as a key regulator of anthocyanin biosynthesis under low-nitrogen conditions through whole-transcriptome sequencing. Then, we used miR858 as an entry point to explore the regulatory network of lncRNA-miRNA-mRNA by dual-luciferase reporter assays and GUS histochemical staining assays, as well as to identify the mechanism of this regulatory network in anthocyanin biosynthesis by both transient and stable transformation experiments in Malus. MiR858 overexpression increased total anthocyanin content. MiR858 acted by negatively regulating its target gene, MsMYB62-like, under the low-nitrogen condition. MsMYB62-like inhibited the expression of MsF3'H, thereby negatively regulating anthocyanin biosynthesis. In addition, eTM858-1 and eTM858-2 were identified as endogenous target mimics of miR858 that bind to miR858 to prevent cleavage of MsMYB62-like and thereby negatively regulate anthocyanin biosynthesis. The results clarify the mechanism through which the eTM-miR858-MYB62-like module regulates anthocyanin biosynthesis in Malus under low-nitrogen conditions.

UV-C Promotes the Accumulation of Flavane-3-ols in Juvenile Fruit of Grape through Positive Regulating VvMYBPA1

Flavane-3-ol monomers are the precursors of proanthocyanidins (PAs), which play a crucial role in grape resistance. Previous studies showed that UV-C positively regulated leucoanthocyanidin reductase (LAR) enzyme activity to promote the accumulation of total flavane-3-ols in juvenile grape fruit, but its molecular mechanism was still unclear. In this paper, we found that the contents of flavane-3-ol monomers increased dramatically at the early development stage grape fruit after UV-C treatment, and the expression of its related transcription factor VvMYBPA1 was also enhanced significantly. The contents of (-)-epicatechin and (+)-catechin, the expression level of VvLAR1 and VvANR, and the activities of LAR and anthocyanidin reductase (ANR) were improved significantly in the VvMYBPA1 overexpressed grape leaves compared to the empty vector. Both VvMYBPA1 and VvMYC2 could interact with VvWDR1 using bimolecular fluorescence complementation (BiFC) and yeast two hybrid (Y2H). Finally, VvMYBPA1 was proven to bind with the promoters of VvLAR1 and VvANR by yeast one hybrid (Y1H). To sum up, we found that the expression of VvMYBPA1 increased in the young stage of grape fruit after UV-C treatment. VvMYBPA1 formed a trimer complex with VvMYC2 and VvWDR1 to regulate the expression of VvLAR1 and VvANR, thus positively promoting the activities of LAR and ANR enzyme, and eventually improved the accumulation of flavane-3-ols in grape fruit.

Tomato SlMYB15 transcription factor targeted by sly-miR156e-3p positively regulates ABA-mediated cold tolerance

Cold is a common abiotic stress that seriously affects plant growth and development. MYB transcription factors are regulatory molecules that play important roles in various biological processes. We have previously demonstrated that SlMYB15 positively regulates cold tolerance in tomato. However, the underlying mechanism of SlMYB15-induced cold tolerance remains largely unexplored. Here, cold-induced SlMYB15 was found to be targeted by Solanum lycopersicum (sly)-miR156e-3p, which was decreased by cold stimulus in tomato. Tomato plants overexpressing sly-MIR156e-3p displayed significant enhancement in susceptibility to cold stress, while silencing of sly-miR156e-3p by an artificial microRNA interference strategy caused tomato plants to be more tolerant to cold. Moreover, both overexpression of SlMYB15 and silencing of sly-miR156e-3p increased the accumulation of ABA. SlMYB15 directly binds to the promoter regions of ABA biosynthesis and signalling genes, SlNCED1 and SlABF4, resulting in enhanced cold tolerance. Further experiments showed that SlMYB15 and sly-miR156e-3p also coordinated the cold tolerance of tomato via the reactive oxygen species (ROS) signalling pathway, as reflected by the increased expression of SlRBOH1, enhanced H2O2 and O2•-accumulation, and amplified activity of antioxidant enzymes in SlMYB15-overexpressing and sly-miR156e-3p-silenced plants. Taken together, our results demonstrate that SlMYB15 targeted by sly-miR156e-3p confers higher survivability to cold stress via ABA and ROS signals. This study provides valuable information for breeding improved crop cultivars better equipped with cold tolerance.

MicroRNAs: emerging regulators in horticultural crops

Horticulture is one of the oldest agricultural practices with great popularity throughout the world. Horticultural crops include fruits, vegetables, ornamental plants, as well as medicinal and beverage plants. They are cultivated for food, specific nutrition, and medical use, or for aesthetic pleasure. MicroRNAs (miRNAs), which constitute a major class of endogenous small RNAs in plants, affect a multitude of developmental and physiological processes by imparting sequence specificity to gene regulation. Over the past decade, tens of thousands of miRNAs have been identified in more than 100 horticultural crops and their critical roles in regulating quality development of diverse horticultural crops have been demonstrated. Here, we review how miRNAs have emerged as important regulators and promising tools for horticultural crop improvement.

A TCP Transcription Factor in Malus halliana, MhTCP4, Positively Regulates Anthocyanins Biosynthesis

Anthocyanins belong to a group of flavonoids, which are the most important flower pigments. Clarifying the potential anthocyanins biosynthesis molecular mechanisms could facilitate artificial manipulation of flower pigmentation in plants. In this paper, we screened a differentially expressed gene, MhTCP4, from the transcriptome data of Malus halliana petals at different development stages and explored its role in anthocyanins biosynthesis. The transcriptome data and qRT-PCR analysis showed that the expression level of MhTCP4 gradually decreased from the flower color fades. Tissue specific expression analysis showed MhTCP4 was expressed in the petal, leaf, and fruit of M. halliana, and was highly expressed in the scarlet petal. Overexpression of MhTCP4 promoted anthocyanins accumulation and increased pigments in infected parts of M. 'Snowdrift' and M. 'Fuji' fruit peels. In contrast, when endogenous MhTCP4 was silenced, the anthocyanins accumulation was inhibited and pigments decreased in the infected peels. The qRT-PCR analysis revealed that overexpression or silence of MhTCP4 caused expression changes of a series of structural genes included in anthocyanins biosynthesis pathway. The yeast two-hybrid assays indicated that MhTCP4 did not interact with MhMYB10. Furthermore, the yeast one-hybrid assays indicated that MhTCP4 did not directly bind to the promoter of MhMYB10, but that of the anthocyanins biosynthesis genes, MhCHI and MhF3'H. Dual luciferase assays further confirmed that MhTCP4 can strongly activate the promoters of MhCHI and MhF3'H in tobacco. Overall, the results suggest that MhTCP4 positively regulates anthocyanins biosynthesis by directly activated MhCHI and MhF3'H in M. halliana flowers.

An Efficient Agrobacterium-Mediated Genetic Transformation System for Persimmon (Diospyros kaki Thunb.)

Persimmon (Diospyros kaki Thunb.) is an important fruit crop in East Asia and has great commercial potential due to the high demand for fresh edible fruits or processed dried persimmon, as well as its being a source of raw materials for the chemical industry. Genetic engineering has shown great potential in persimmon fruit improvement and compensated for the limitations of conventional breeding. In this study, we developed an efficient transformation system and in vivo regeneration protocol by using the leaf disc culture of ‘Gongcheng Shuishi’ persimmon through the optimization of different basic media and hormone combinations. Based on the secondary induction system on 1/2DKW (DKW medium) + ZT (2.0 mg/L) + TDZ (0.5 mg/L) and MS (1/2 N) + ZT (2.0 mg/L) + IAA (0.1 mg/L), the callus induction rate and adventitious bud induction rate were 96.0% and 94.0%, respectively, and RNA interference of DkANR (ANR, anthocyanidin reductase) was achieved after transformation, thus resulting in a reduction in proanthocyanidin accumulation. The GFP (green fluorescent protein) fluorescence signal was visualized in the transgenic seedlings with GFP gene cotransformation. A total of 17.9% of the positive regenerated transgenic seedlings with empty vector were confirmed by both hyg gene and 35S promoter PCR analysis. Southern blotting assays showed that 20 positive transgenic persimmon seedlings were obtained from 135 regenerated transformants derived from DkANRi. Altogether, a reliable and highly efficient transformation system was achieved in persimmon, which may contribute to gene function identification and genetic improvement in the future.

Reinforcing the bulwark: unravelling the efficient applications of plant phenolics and tannins against environmental stresses

Phenolic compounds are plant secondary metabolites that play a vital role in plant resistance. They are mainly synthetized from the amino acid L-phenylalanine, which is converted to trans-cinnamic acid in a series of biochemical reactions. These compounds take part in the regulation of seed germination and cooperate in regulating the growth of plants, also taking part in defense responses during infection, UV exposure, injuries, and heavy metal stress. The aim of this review is to discuss the role of phenolic compounds in the interactions of plants with various stress factors, both biotic and abiotic with special attention to their antioxidant properties. Therefore, understanding the biochemical potential of the phenylpropanoid derivatives would be beneficial in sustaining the metabolic processes used by plants to thrive and endure under adverse conditions.