Graft-accelerated virus-induced gene silencing facilitates functional genomics in rose flowers

MADS-box protein PpDAM6 regulates chilling requirement-mediated dormancy and bud break in peach

Bud dormancy is crucial for winter survival and is characterized by the inability of the bud meristem to respond to growth-promotive signals before the chilling requirement (CR) is met. However, our understanding of the genetic mechanism regulating CR and bud dormancy remains limited. This study identified PpDAM6 (DORMANCY-ASSOCIATED MADS-box) as a key gene for CR using a genome-wide association study analysis based on structural variations in 345 peach (Prunus persica (L.) Batsch) accessions. The function of PpDAM6 in CR regulation was demonstrated by transiently silencing the gene in peach buds and stably overexpressing the gene in transgenic apple (Malus × domestica) plants. The results showed an evolutionarily conserved function of PpDAM6 in regulating bud dormancy release, followed by vegetative growth and flowering, in peach and apple. The 30-bp deletion in the PpDAM6 promoter was substantially associated with reducing PpDAM6 expression in low-CR accessions. A PCR marker based on the 30-bp indel was developed to distinguish peach plants with non-low and low CR. Modification of the H3K27me3 marker at the PpDAM6 locus showed no apparent change across the dormancy process in low- and non-low- CR cultivars. Additionally, H3K27me3 modification occurred earlier in low-CR cultivars on a genome-wide scale. PpDAM6 could mediate cell-cell communication by inducing the expression of the downstream genes PpNCED1 (9-cis-epoxycarotenoid dioxygenase 1), encoding a key enzyme for ABA biosynthesis, and CALS (CALLOSE SYNTHASE), encoding callose synthase. We shed light on a gene regulatory network formed by PpDAM6-containing complexes that mediate CR underlying dormancy and bud break in peach. A better understanding of the genetic basis for natural variations of CR can help breeders develop cultivars with different CR for growing in different geographical regions.

Genetic and Biochemical Aspects of Floral Scents in Roses

Floral scents possess high ornamental and economic values to rose production in the floricultural industry. In the past two decades, molecular bases of floral scent production have been studied in the rose as well as their genetic inheritance. Some significant achievements have been acquired, such as the comprehensive rose genome and the finding of a novel geraniol synthase in plants. In this review, we summarize the composition of floral scents in modern roses, focusing on the recent advances in the molecular mechanisms of floral scent production and emission, as well as the latest developments in molecular breeding and metabolic engineering of rose scents. It could provide useful information for both studying and improving the floral scent production in the rose.

VIGS Goes Viral: How VIGS Transforms Our Understanding of Plant Science

Virus-induced gene silencing (VIGS) has developed into an indispensable approach to gene function analysis in a wide array of species, many of which are not amenable to stable genetic transformation. VIGS utilizes the posttranscriptional gene silencing (PTGS) machinery of plants to restrain viral infections systemically and is used to downregulate the plant's endogenous genes. Here, we review the molecular mechanisms of DNA- and RNA-virus-based VIGS, its inherent connection to PTGS, and what is known about the systemic spread of silencing. Recently, VIGS-based technologies have been expanded to enable not only gene silencing but also overexpression [virus-induced overexpression (VOX)], genome editing [virus-induced genome editing (VIGE)], and host-induced gene silencing (HIGS). These techniques expand the genetic toolbox for nonmodel organisms even more. Further, we illustrate the versatility of VIGS and the methods derived from it in elucidating molecular mechanisms, using tomato fruit ripening and programmed cell death as examples. Finally, we discuss challenges of and future perspectives on the use of VIGS to advance gene function analysis in nonmodel plants in the postgenomic era.

VvANR silencing promotes expression of VvANS and accumulation of anthocyanin in grape berries

Virus-induced gene silencing (VIGS) technology was applied to silence VvANR in cv. Zaoheibao grape berries, and the effects of VvANR silencing on berries phenotype; gene expression level of ANS, LAR1, LAR2, and UFGT; enzyme activity of ANS; and accumulations of anthocyanin and flavan-3-ol were investigated. At the third day after treatment, the VvANR silenced grape berries began to turn red slightly, which was 2 days earlier than that of the control group. And the flavan-3-ol content in VvANR-silenced grape berries had been remarkable within 1 to 5 days, the ANR enzyme activity in VvANR-silenced grapes extremely significantly decreased in 3 days, and LAR enzyme activity also decreased, but the difference was not striking. The ANS enzyme activity of the transformed berries was significantly higher than that of the control after 3 days of infection, and it was exceedingly significantly higher than that of the control after 5 to 10 days. The content of anthocyanin in transformed berries increased of a very marked difference within 3 to 15 days. pTRV2-ANR infection resulted in an extremely significant decrease in the expression of VvANR gene, and the expression of VvLAR1, VvLAR2, VvMYBPA1, VvMYBPA2, and VvDFR were also down-regulated. However, the expression of VvANS and VvUFGT was up-regulated significantly. After VvANR silencing via VIGS, VvANR expression in grape berries was extremely significantly decreased, resulting in decreased ANR enzyme activity and flavan-3-ol content; berries turned red and deeper in advance. In addition, VvANR silencing can induce up-regulation of VvANS and VvUFGT expression, significantly increase ANS enzyme activity, and increase of anthocyanin accumulation.

Virus-Induced Gene Silencing in Rose Flowers

Virus-induced gene silencing (VIGS) is a favorable method to study gene function by posttranscriptional gene silencing in plants. Here we describe a methodology of graft-accelerated VIGS in rose aimed at obtaining posttranscriptional gene silencing in the flower. The resulting phenotype can be observed within 5-6 weeks post infiltration. By using this method, we successfully silenced the expression of several genes involved in processes such as scent production, petal coloration, or flower architecture. We showed that graft-accelerated VIGS was faster, more efficient, and more convenient than conventional methods previously developed in rose such as agroinfiltration of young plantlets and in vitro cultured tissues or seeds.

Establishment of virus-induced gene silencing system and functional analysis of ScbHLH17 in Senecio cruentus

Virus-induced gene silencing (VIGS) is a technology for rapid gene functional analysis that depends on the degradation of viral RNA and is part of the natural defense mechanism in plants. Senecio cruentus is an important Compositae ornamental species that is plentiful and available in a variety of colors and has a typical blue variety that is rare in Compositae. These advantages make it a good material for studying the anthocyanin biosynthesis and blue flower formation mechanism. With the development of gene sequencing technology, the functions of many candidate genes that may be involved in anthocyanin biosynthesis in S. cruentus need to be identified. However, a stable and rapid genetic transformation system of S. cruentus is still lacking. Here, we screened two cultivars, 'Venezia' and 'Jseter', selected ScPDS and ScANS as test genes, and investigated the effect of developmental periods, bacterial cell concentrations and infection methods on gene silencing efficiency. The results showed that the silencing efficiency of S. cruentus leaves was low (13%), and it was less affected by the parameters. However, the transcription factor gene ScbHLH17 was still silenced by VIGS, which resulted in the loss of anthocyanin accumulation in leaves, and the expression levels of anthocyanin biosynthesis pathway (ABP) structural genes, including ScCHI, ScDFR3 and ScANS, were decreased significantly. The result proved that ScbHLH17 was an important transcription factor that regulated flower color formation in S. cruentus. In addition, ScANS-silencing phenotypes were observed in S. cruentus capitulum by vacuum-infiltrating S1 stage buds for 10 min after scape injection. In general, the present study provided an important technical support for the study of anthocyanin metabolism pathways in S. cruentus.

Overview and detectability of the genetic modifications in ornamental plants

The market of ornamental plants is extremely competitive, and for many species genetic engineering can be used to introduce original traits of high commercial interest. However, very few genetically modified (GM) ornamental varieties have reached the market so far. Indeed, the authorization process required for such plants has a strong impact on the profitability of the development of such products. Considering the numerous scientific studies using genetic modification on ornamental species of interest, a lot of transformed material has been produced, could be of commercial interest and could therefore be unintentionally released on the market. The unintentional use of GM petunia in breeding programs has indeed recently been observed. This review lists scientific publications using GM ornamental plants and tries to identify whether these plants could be detected by molecular biology tools commonly used by control laboratories.

Virus-Induced Gene Silencing in Olive Tree (Oleaceae)

Research on gene functions in non-model tree species is hampered by a number of difficulties such as time-consuming genetic transformation protocols and extended period for the production of healthy transformed offspring, among others. Virus-induced gene silencing (VIGS) is an alternative approach to transiently knock out an endogenous gene of interest (GOI) by the introduction of viral sequences encompassing a fragment of the GOI and to exploit the posttranscriptional gene silencing (PTGS) mechanism of the plant, thus triggering silencing of the GOI. Here we describe the successful application of Tobacco rattle virus (TRV)-mediated VIGS through agroinoculation of olive plantlets. This methodology is expected to serve as a fast tracking and powerful tool enabling researchers from diversified fields to perform functional genomic analyses in the olive tree.

A detached petal disc assay and virus-induced gene silencing facilitate the study of Botrytis cinerea resistance in rose flowers

Fresh-cut roses (Rosa hybrida) are one of the most important ornamental crops worldwide, with annual trade in the billions of dollars. Gray mold disease caused by the pathogen Botrytis cinerea is the most serious fungal threat to cut roses, causing extensive postharvest losses. In this study, we optimized a detached petal disc assay (DPDA) for artificial B. cinerea inoculation and quantification of disease symptoms in rose petals. Furthermore, as the identification of rose genes involved in B. cinerea resistance could provide useful genetic and genomic resources, we devised a virus-induced gene silencing (VIGS) procedure for the functional analysis of B. cinerea resistance genes in rose petals. We used RhPR10.1 as a reporter of silencing efficiency and found that the rose cultivar 'Samantha' showed the greatest decrease in RhPR10.1 expression among the cultivars tested. To determine whether jasmonic acid and ethylene are required for B. cinerea resistance in rose petals, we used VIGS to silence the expression of RhLOX5 and RhEIN3 (encoding a jasmonic acid biosynthesis pathway protein and an ethylene regulatory protein, respectively) and found that petal susceptibility to B. cinerea was affected. Finally, a VIGS screen of B. cinerea-induced rose transcription factors demonstrated the potential benefits of this method for the high-throughput identification of gene function in B. cinerea resistance. Collectively, our data show that the combination of the DPDA and VIGS is a reliable and high-throughput method for studying B. cinerea resistance in rose.

Chalcone Synthase-Encoding AeCHS is Involved in Normal Petal Coloration in Actinidia eriantha

Studies on anthocyanin biosynthesis have been mainly concentrated on the fruit, whereas few have focused the mechanism of flower coloration in kiwifruit. Here, we report that the structural gene, AeCHS, is involved in anthocyanin accumulation and indispensable for normal petal coloration in Actinidia eriantha. Petals from three different species including Actinidia eriantha (red petals), Actinidia hemsleyana (light pink petals) and Actinidia arguta (white petals) were selected for anthocyanin determination and gene expression analysis. The anthocyanin components in A. eriantha were significantly higher than in A. hemsleyana or A. arguta. Consistently, gene expression profiles suggested that AeCHS expression in A. eriantha was higher than in A. hemsleyana or A. arguta. Cluster analysis showed that AeCHS was clustered into a single group and distinctly separated from other genes, indicating the expression pattern of AeCHS gene was different from any other. Additionally, correlation analysis revealed AeCHS expression significantly correlated with anthocyanin content. The complete coding sequence of AeCHS was cloned from petals of A. eriantha 'Zaoxu', showing the length of AeCHS was 1170 bp encoding a protein of 389 amino acids. AeCHS was located in the cytoplasm, indicating it is indeed a structural gene involved in anthocyanin biosynthesis. AeCHS silencing performed by infiltration grafting-mediated virus-induced gene silencing (VIGS) reduced petal anthocyanin content and bleached red petals in A. eriantha. Our results confirm a crucial role of AeCHS in anthocyanin biosynthesis and accumulation in A. eriantha petals; furthermore, they offer important basic information and constitute a reference point for further research.

RcAP1, a Homolog of APETALA1, is Associated with Flower Bud Differentiation and Floral Organ Morphogenesis in Rosa chinensis

Rosa chinensis is one of the most popular flower plants worldwide. The recurrent flowering trait greatly enhances the ornamental value of roses, and is the result of the constant formation of new flower buds. Flower bud differentiation has always been a major topic of interest among researchers. The APETALA1 (AP1) MADS-box (Mcm1, Agamous, Deficiens and SRF) transcription factor-encoding gene is important for the formation of the floral meristem and floral organs. However, research on the rose AP1 gene has been limited. Thus, we isolated AP1 from Rosa chinensis ‘Old Blush’. An expression analysis revealed that RcAP1 was not expressed before the floral primordia formation stage in flower buds. The overexpression of RcAP1 in Arabidopsis thaliana resulted in an early-flowering phenotype. Additionally, the virus-induced down-regulation of RcAP1 expression delayed flowering in ‘Old Blush’. Moreover, RcAP1 was specifically expressed in the sepals of floral organs, while its expression was down-regulated in abnormal sepals and leaf-like organs. These observations suggest that RcAP1 may contribute to rose bud differentiation as well as floral organ morphogenesis, especially the sepals. These results may help for further characterization of the regulatory mechanisms of the recurrent flowering trait in rose.

Functional characterization of the eugenol synthase gene (RcEGS1) in rose

The floral volatile compound eugenol is an important constituent in many aromatic plants, being a floral attractant for pollinators as well as having antimicrobial activity. Rose flowers emit eugenol and its derivatives. We recently reported a eugenol synthase gene (RcEGS1) (JQ522949) that was present in petals of R. chinensis cv. Old Blush. RcEGS1 has its highest expression levels in the petals compared to other tissues; it has higher transcript levels at the developmental blooming stage and lower levels at budding and senescence stages. Here, we overexpressed the RcEGS1 protein in Escherichia coli, and showed by Western-blot analysis that its expression was mainly detected in stamens and petals at the flower opening stage. RcEGS1 was principally localized in the upper and lower epidermal layers, which are the major sites of scent emission in roses. Furthermore, we demonstrated that down-regulation of RcEGS1 expression in flowers by virus-induced gene silencing led to a reduction of the relative content of eugenol. We suggested that RcEGS1 was responsible for eugenol biosynthesis in roses.

An APETALA2 Homolog, RcAP2, Regulates the Number of Rose Petals Derived From Stamens and Response to Temperature Fluctuations

Rosa chinensis, which is a famous traditional flower in China, is a major ornamental plant worldwide. Long-term cultivation and breeding have resulted in considerable changes in the number of rose petals, while most wild Rosaceae plants have only one whorl consisting of five petals. The petals of double flowers reportedly originate from stamens, but the underlying molecular mechanism has not been fully characterized. In this study, we observed that the number of petals of R. chinensis 'Old Blush' flowers increased and decreased in response to low- and high-temperature treatments, respectively, similar to previous reports. We characterized these variations in further detail and found that the number of stamens exhibited the opposite trend. We cloned an APETALA2 homolog, RcAP2. A detailed analysis of gene structure and promoter cis-acting elements as well as RcAP2 temporospatial expression patterns and responses to temperature changes suggested that RcAP2 expression may be related to the number of petals from stamen origin. The overexpression of RcAP2 in Arabidopsis thaliana transgenic plants may induce the transformation of stamens to petals, thereby increasing the number of petals. Moreover, silencing RcAP2 in 'Old Blush' plants decreased the number of petals. Our results may be useful for clarifying the temperature-responsive mechanism involved in petaloid stamen production, which may be relevant for the breeding of new rose varieties with enhanced flower traits.