Precocious flowering of juvenile citrus induced by a viral vector based on Citrus leaf blotch virus: a new tool for genetics and breeding

Genome-wide identification and characterization of flowering genes in Citrus sinensis (L.) Osbeck: a comparison among C. Medica L., C. Reticulata Blanco, C. Grandis (L.) Osbeck and C. Clementina

BACKGROUND

Flowering plays an important role in completing the reproductive cycle of plants and obtaining next generation of plants. In case of citrus, it may take more than a year to achieve progeny. Therefore, in order to fasten the breeding processes, the juvenility period needs to be reduced. The juvenility in plants is regulated by set of various flowering genes. The citrus fruit and leaves possess various medicinal properties and are subjected to intensive breeding programs to produce hybrids with improved quality traits. In order to break juvenility in Citrus, it is important to study the role of flowering genes. The present study involved identification of genes regulating flowering in Citrus sinensis L. Osbeck via homology based approach. The structural and functional characterization of these genes would help in targeting genome editing techniques to induce mutations in these genes for producing desirable results.

RESULTS

A total of 43 genes were identified which were located on all the 9 chromosomes of citrus. The in-silico analysis was performed to determine the genetic structure, conserved motifs, cis-regulatory elements (CREs) and phylogenetic relationship of the genes. A total of 10 CREs responsible for flowering were detected in 33 genes and 8 conserved motifs were identified in all the genes. The protein structure, protein-protein interaction network and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis was performed to study the functioning of these genes which revealed the involvement of flowering proteins in circadian rhythm pathways. The gene ontology (GO) and gene function analysis was performed to functionally annotate the genes. The structure of the genes and proteins were also compared among other Citrus species to study the evolutionary relationship among them. The expression study revealed the expression of flowering genes in floral buds and ovaries. The qRT-PCR analysis revealed that the flowering genes were highly expressed in bud stage, fully grown flower and early stage of fruit development.

CONCLUSIONS

The findings suggested that the flowering genes were highly conserved in citrus species. The qRT-PCR analysis revealed the tissue specific expression of flowering genes (CsFT, CsCO, CsSOC, CsAP, CsSEP and CsLFY) which would help in easy detection and targeting of genes through various forward and reverse genetic approaches.

Advancing tree genomics to future proof next generation orchard production

The challenges facing tree orchard production in the coming years will be largely driven by changes in the climate affecting the sustainability of farming practices in specific geographical regions. Identifying key traits that enable tree crops to modify their growth to varying environmental conditions and taking advantage of new crop improvement opportunities and technologies will ensure the tree crop industry remains viable and profitable into the future. In this review article we 1) outline climate and sustainability challenges relevant to horticultural tree crop industries, 2) describe key tree crop traits targeted for improvement in agroecosystem productivity and resilience to environmental change, and 3) discuss existing and emerging genomic technologies that provide opportunities for industries to future proof the next generation of orchards.

CRISPR/Cas as a Genome-Editing Technique in Fruit Tree Breeding

CRISPR (short for "Clustered Regularly Interspaced Short Palindromic Repeats") is a technology that research scientists use to selectively modify the DNA of living organisms. CRISPR was adapted for use in the laboratory from the naturally occurring genome-editing systems found in bacteria. In this work, we reviewed the methods used to introduce CRISPR/Cas-mediated genome editing into fruit species, as well as the impacts of the application of this technology to activate and knock out target genes in different fruit tree species, including on tree development, yield, fruit quality, and tolerance to biotic and abiotic stresses. The application of this gene-editing technology could allow the development of new generations of fruit crops with improved traits by targeting different genetic segments or even could facilitate the introduction of traits into elite cultivars without changing other traits. However, currently, the scarcity of efficient regeneration and transformation protocols in some species, the fact that many of those procedures are genotype-dependent, and the convenience of segregating the transgenic parts of the CRISPR system represent the main handicaps limiting the potential of genetic editing techniques for fruit trees. Finally, the latest news on the legislation and regulations about the use of plants modified using CRISPR/Cas systems has been also discussed.

An overview of floral regulatory genes in annual and perennial plants

The floral initiation in angiosperms is a complex process influenced by endogenous and exogenous signals. With this approach, we aim to provide a comprehensive review to integrate this complex floral regulatory process and summarize the regulatory genes and their functions in annuals and perennials. Seven primary paths leading to flowering have been discovered in Arabidopsis under several growth condition that include; photoperiod, ambient temperature, vernalization, gibberellins, autonomous, aging and carbohydrates. These pathways involve a series of interlinked signaling pathways that respond to both internal and external signals, such as light, temperature, hormones, and developmental cues, to coordinate the expression of genes that are involved in flower development. Among them, the photoperiodic pathway was the most important and conserved as some of the fundamental loci and mechanisms are shared even by closely related plant species. The activation of floral regulatory genes such as FLC, FT, LFY, and SOC1 that determine floral meristem identity and the transition to the flowering stage result from the merging of these pathways. Recent studies confirmed that alternative splicing, antisense RNA and epigenetic modification play crucial roles by regulating the expression of genes related to blooming. In this review, we documented recent progress in the floral transition time in annuals and perennials, with emphasis on the specific regulatory mechanisms along with the application of various molecular approaches including overexpression studies, RNA interference and Virus-induced flowering. Furthermore, the similarities and differences between annual and perennial flowering will aid significant contributions to the field by elucidating the mechanisms of perennial plant development and floral initiation regulation.

Generation of the transgene-free canker-resistant Citrus sinensis using Cas12a/crRNA ribonucleoprotein in the T0 generation

Citrus canker caused by Xanthomonas citri subsp. citri (Xcc) is a destructive citrus disease worldwide. Generating disease-resistant cultivars is the most effective, environmentally friendly and economic approach for disease control. However, citrus traditional breeding is lengthy and laborious. Here, we develop transgene-free canker-resistant Citrus sinensis lines in the T0 generation within 10 months through transformation of embryogenic protoplasts with Cas12a/crRNA ribonucleoprotein to edit the canker susceptibility gene CsLOB1. Among the 39 regenerated lines, 38 are biallelic/homozygous mutants, demonstrating a 97.4% biallelic/homozygous mutation rate. No off-target mutations are detected in the edited lines. Canker resistance of the cslob1-edited lines results from both abolishing canker symptoms and inhibiting Xcc growth. The transgene-free canker-resistant C. sinensis lines have received regulatory approval by USDA APHIS and are exempted from EPA regulation. This study provides a sustainable and efficient citrus canker control solution and presents an efficient transgene-free genome-editing strategy for citrus and other crops.

Identification and Characterization of a Novel Quanzhou Mulberry Virus from Mulberry (Morus alba)

In this study, we discovered a new virus named Quanzhou mulberry virus (QMV), which was identified from the leaves of an ancient mulberry tree. This tree is over 1300 years old and is located at Fujian Kaiyuan Temple, a renowned cultural heritage site in China. We obtained the complete genome sequence of QMV using RNA sequencing followed by rapid amplification of complementary DNA ends (RACE). The QMV genome is 9256 nucleotides (nt) long and encodes five open reading frames (ORFs). Its virion was made of icosahedral particles. Phylogenetic analysis suggests that it belongs to the unclassified Riboviria. An infectious clone for QMV was generated and agroinfiltrated into Nicotiana benthamiana and mulberry, resulting in no visible disease symptoms. However, systemic movement of the virus was only observed in mulberry seedlings, suggesting that it has a host-specific pattern of movement. Our findings provide a valuable reference for further studies on QMV and related viruses, contributing to the understanding of viral evolution and biodiversity in mulberry.

Genotyping by Sequencing for SNP-Based Linkage Analysis and the Development of KASPar Markers for Male Sterility and Polyembryony in Citrus

Polyembryony and male sterility (MS) are essential characters for citrus breeding. MS, coupled with parthenocarpy, allows for addressing the diversification of diploid seedless mandarin varieties, and nucleocytoplasmic MS is the most prevalent system. Polyembryony limits the use of seed parents in scion breeding programs, and the recovery of monoembryonic hybrids to be used as female parents is a crucial pre-breeding component. The objectives of this work were the identification of SNPs closely linked with the genes implied in these traits for marker-assisted selection. Genotyping by sequencing was used to genotype 61 diploid hybrids from an F1 progeny recovered from crossing 'Kiyomi' and 'Murcott' tangors. A total of 6444 segregating markers were identified and used to establish the two parental genetic maps. They consisted of 1374 and 697 markers encompassing 1416.287 and 1339.735 cM for 'Kiyomi' and 'Murcott', respectively. Phenotyping for MS and polyembryony was performed. The genotype-trait association study identified a genomic region on LG8 which was significantly associated with MS, and a genomic region on LG1 which was significantly associated with polyembryony. Annotation of the identified region for MS revealed 19 candidate genes. One SNP KASPar marker was developed and fully validated for each trait.

Identification of Three Viruses Infecting Mulberry Varieties

Viruses-mediated genome editing in plants is a powerful strategy to develop plant cultivars with important and novel agricultural traits. Mulberry alba is an important economic tree species that has been cultivated in China for more than 5000 years. So far, only a few viruses have been identified from mulberry trees, and their application potential is largely unknown. Therefore, mining more virus resources from the mulberry tree can pave the way for the establishment of useful engineering tools. In this study, eight old mulberry plants were gathered in seven geographic areas for virome analysis. Based on transcriptome analysis, we discovered three viruses associated with mulberries: Citrus leaf blotch virus isolate mulberry alba 2 (CLBV-ML2), Mulberry-associated virga-like virus (MaVLV), and Mulberry-associated narna-like virus (MaNLV). The genome of CLBV-ML2 was completely sequenced and exhibited high homology with Citriviruses, considered to be members of the genus Citrivirus, while the genomes of MaVLV and MaNLV were nearly completed lacking the 5' and 3' termini sequences. We tentatively consider MaVLV to be members of the family Virgaviridae and MaNLV to be members of the genus Narnavirus based on the results of phylogenetic trees. The infection experiments showed that CLBV-ML2 could be detected in the inoculated seedlings of both N. benthamiana and Morus alba, while MaVLV could only be detected in N. benthamiana. All of the infected seedlings did not show obvious symptoms.

Highly efficient hairy root genetic transformation and applications in citrus

Highly efficient genetic transformation technology is greatly beneficial for crop gene function analysis and precision breeding. However, the most commonly used genetic transformation technology for woody plants, mediated by Agrobacterium tumefaciens, is time-consuming and inefficient, which limits its utility for gene function analysis. In this study, a simple, universal, and highly efficient genetic transformation technology mediated by A. rhizogenes K599 is described. This technology can be applied to multiple citrus genotypes, and only 2-8 weeks were required for the entire workflow. Genome-editing experiments were simultaneously conducted using 11 plasmids targeting different genomic positions and all corresponding transformants with the target knocked out were obtained, indicating that A. rhizogenes-mediated genome editing was highly efficient. In addition, the technology is advantageous for investigation of specific genes (such as ACD2) for obtaining "hard-to-get" transgenic root tissue. Furthermore, A. rhizogenes can be used for direct viral vector inoculation on citrus bypassing the requirement for virion enrichment in tobacco, which facilitates virus-induced gene silencing and virus-mediated gene expression. In summary, we established a highly efficient genetic transformation technology bypassing tissue culture in citrus that can be used for genome editing, gene overexpression, and virus-mediated gene function analysis. We anticipate that by reducing the cost, required workload, experimental period, and other technical obstacles, this genetic transformation technology will be a valuable tool for routine investigation of endogenous and exogenous genes in citrus.

From Floral Induction to Blooming: The Molecular Mysteries of Flowering in Woody Plants

Flowering is a pivotal developmental process in response to the environment and determines the start of a new life cycle in plants. Woody plants usually possess a long juvenile nonflowering phase followed by an adult phase with repeated flowering cycles. The molecular mechanism underlying flowering regulation in woody plants is believed to be much more complex than that in annual herbs. In this review, we briefly describe the successive but distinct flowering processes in perennial trees, namely the vegetative phase change, the floral transition, floral organogenesis, and final blooming, and summarize in detail the most recent advances in understanding how woody plants regulate flowering through dynamic gene expression. Notably, the florigen gene FLOWERING LOCUS T(FT) and its antagonistic gene TERMINAL FLOWER 1 (TFL1) seem to play a central role in various flowering transition events. Flower development in different taxa requires interactions between floral homeotic genes together with AGL6 conferring floral organ identity. Finally, we illustrate the issues and corresponding measures of flowering regulation investigation. It is of great benefit to the future study of flowering in perennial trees.

Different Cell Types Affect the Transition from Juvenile to Mature Phase in Citrus Plants Regenerated through Somatic Embryogenesis

Robust protocols for the regeneration of somatic embryos in vitro are essential for the efficient use of the most modern biotechnologies. Unfortunately, in perennial trees such as Citrus, plants regenerated from juvenile tissues usually exhibit strong, undesirable juvenile characters such as thorny habit and delayed flowering and fruit production. In this work, we tested whether the cell types (nucellar and stigma/style) used to regenerate Citrus plants through somatic embryogenesis affected the transition from the juvenile to mature phase. The results show that regenerants from nucellar cells presented persistent juvenile characters, whereas plants originating from stigma/style explants transited to the mature phase more rapidly. Our observations support the hypothesis that the totipotent cells originated from different cell types are not equivalent, possibly by maintaining memory of their previously differentiated state.

Simplifying plant gene silencing and genome editing logistics by a one-Agrobacterium system for simultaneous delivery of multipartite virus vectors

Viral vectors provide a quick and effective way to express exogenous sequences in eukaryotic cells and to engineer eukaryotic genomes through the delivery of CRISPR/Cas components. Here, we present JoinTRV, an improved vector system based on tobacco rattle virus (TRV) that simplifies gene silencing and genome editing logistics. Our system consists of two mini T-DNA vectors from which TRV RNA1 (pLX-TRV1) and an engineered version of TRV RNA2 (pLX-TRV2) are expressed. The two vectors have compatible origins that allow their cotransformation and maintenance into a single Agrobacterium cell, as well as their simultaneous delivery to plants by a one-Agrobacterium/two-vector approach. The JoinTRV vectors are substantially smaller than those of any known TRV vector system, and pLX-TRV2 can be easily customized to express desired sequences by one-step digestion-ligation and homology-based cloning. The system was successfully used in Nicotiana benthamiana for launching TRV infection, for recombinant protein production, as well as for robust virus-induced gene silencing (VIGS) of endogenous transcripts using bacterial suspensions at low optical densities. JoinTRV-mediated delivery of single-guide RNAs in a Cas9 transgenic host allowed somatic cell editing efficiencies of ≈90%; editing events were heritable and >50% of the progeny seedlings showed mutations at the targeted loci.

Promotion of natural flowers by JcFT depends on JcLFY in the perennial woody species Jatropha curcas

Production of normal gametes is necessary for flowering plant reproduction, which involves the transition from vegetative to reproductive stage and floral organ development. Such transitions and floral development are modulated by various environmental and endogenous stimuli and controlled by sophisticated regulatory networks. FLOWERING LOCUS T (FT) and LEAFY (LFY) are two key genes that integrate signals from multiple genetic pathways in Arabidopsis. However, the comprehensive functions and relationship between these two genes in trees are poorly understood. In this study, we found that JcFT played a vital role in regulating the flowering transition in the perennial woody species Jatropha curcas. JcLFY also involved in regulating this transition and controlled floral organ development. The non-flowering phenotype of JcFT-RNAi was rescued successfully by overexpression of JcLFY, while the abnormal flowers produced by JcLFY silencing were not recovered by JcFT overexpression via hybridization. These results indicate that JcFT, in which a mutation leads to a nonflowering phenotype, is the central gene of the floral meristem transition and that JcLFY, in which a mutation leads to striking changes in flowering and often sterility, is the central floral and inflorescence development gene. Moreover, our hybridization results suggest that JcLFY acts downstream of JcFT in Jatropha.

Mobility of FLOWERING LOCUS T protein as a systemic signal in trifoliate orange and its low accumulation in grafted juvenile scions

The long juvenile period of perennial woody plants is a major constraint in breeding programs. FLOWERING LOCUS T (FT) protein is an important mobile florigen signal that induces plant flowering. However, whether FT can be transported in woody plants to shorten the juvenile period is unknown, and its transport mechanism remains unclear. In this study, trifoliate orange FT (ToFT) and Arabidopsis FT (AtFT, which has been confirmed to be transportable in Arabidopsis) as a control were transformed into tomato and trifoliate orange, and early flowering was induced in the transgenic plants. Long-distance and two-way (upward and downward) transmission of ToFT and AtFT proteins was confirmed in both tomato and trifoliate orange using grafting and western blot analysis. However, rootstocks of transgenic trifoliate orange could not induce flowering in grafted wild-type juvenile scions because of the low accumulation of total FT protein in the grafted scions. It was further confirmed that endogenous ToFT protein was reduced in trifoliate orange, and the accumulation of the transported ToFT and AtFT proteins was lower than that in grafted juvenile tomato scions. Furthermore, the trifoliate orange FT-INTERACTING PROTEIN1 homolog (ToFTIP1) was isolated by yeast two-hybrid analysis. The FTIP1 homolog may regulate FT transport by interacting with FT in tomato and trifoliate orange. Our findings suggest that FT transport may be conserved between the tomato model and woody plants. However, in woody plants, the transported FT protein did not accumulate in significant amounts in the grafted wild-type juvenile scions and induce the scions to flower.

Horticultural innovation by viral-induced gene regulation of carotenogenesis

Multipartite viral vectors provide a simple, inexpensive and effective biotechnological tool to transiently manipulate (i.e. reduce or increase) gene expression in planta and characterise the function of genetic traits. The development of virus-induced gene regulation (VIGR) systems usually involve the targeted silencing or overexpression of genes involved in pigment biosynthesis or degradation in plastids, thereby providing rapid visual assessment of success in establishing RNA- or DNA-based VIGR systems in planta. Carotenoids pigments provide plant tissues with an array of yellow, orange, and pinkish-red colours. VIGR-induced transient manipulation of carotenoid-related gene expression has advanced our understanding of carotenoid biosynthesis, regulation, accumulation and degradation, as well as plastid signalling processes. In this review, we describe mechanisms of VIGR, the importance of carotenoids as visual markers of technology development, and knowledge gained through manipulating carotenogenesis in model plants as well as horticultural crops not always amenable to transgenic approaches. We outline how VIGR can be utilised in plants to fast-track the characterisation of gene function(s), accelerate fruit tree breeding programs, edit genomes, and biofortify plant products enriched in carotenoid micronutrients for horticultural innovation.

Efficiency of graft-transmitted JcFT for floral induction in woody perennial species of the Jatropha genus depends on transport distance

FLOWERING LOCUS T (FT) promotes flowering by integrating six genetic pathways. In Arabidopsis, the FT protein is transported from leaves to shoot apices and induces flowering. However, contradictory conclusions about floral induction via graft-transmitted FT in trees were reported in previous studies. We obtained extremely early-flowering transgenic woody Jatropha curcas L. by overexpression of J. curcas FT using Arabidopsis thaliana SUCROSE TRANSPORTER 2 (SUC2) promoter (SUC2:JcFT) and non-flowering transgenic J. curcas by RNA interference (RNAi), which were used to investigate the function of graft-transmitted JcFT in floral induction in woody perennials. Scions from five wild-type species of the Jatropha genus and from JcFT-RNAi transgenic J. curcas were grafted onto SUC2:JcFT rootstocks. Most grafted plants produced flowers in 1-2 months, and the flowering percentage and frequency of various grafted plants decreased with increasing scion length. Consistently, FT protein abundance in scions also decreased with increasing distance from graft junctions to the buds. These findings suggest that FT proteins can be transmitted by grafting and can induce the floral transition in woody perennials, and the efficiency of graft-transmitted JcFT for floral induction depends on the scion length, which may help explain previous seemingly contradictory observations regarding floral induction via graft-transmitted FT in trees.

Citrus Genetic Transformation: An Overview of the Current Strategies and Insights on the New Emerging Technologies

Citrus are among the most prevailing fruit crops produced worldwide. The implementation of effective and reliable breeding programs is essential for coping with the increasing demands of satisfactory yield and quality of the fruit as well as to deal with the negative impact of fast-spreading diseases. Conventional methods are time-consuming and of difficult application because of inherent factors of citrus biology, such as their prolonged juvenile period and a complex reproductive stage, sometimes presenting infertility, self-incompatibility, parthenocarpy, or polyembryony. Moreover, certain desirable traits are absent from cultivated or wild citrus genotypes. All these features are challenging for the incorporation of the desirable traits. In this regard, genetic engineering technologies offer a series of alternative approaches that allow overcoming the difficulties of conventional breeding programs. This review gives a detailed overview of the currently used strategies for the development of genetically modified citrus. We describe different aspects regarding genotype varieties used, including elite cultivars or extensively used scions and rootstocks. Furthermore, we discuss technical aspects of citrus genetic transformation procedures via Agrobacterium, regular physical methods, and magnetofection. Finally, we describe the selection of explants considering young and mature tissues, protoplast isolation, etc. We also address current protocols and novel approaches for improving the in vitro regeneration process, which is an important bottleneck for citrus genetic transformation. This review also explores alternative emerging transformation strategies applied to citrus species such as transient and tissue localized transformation. New breeding technologies, including cisgenesis, intragenesis, and genome editing by clustered regularly interspaced short palindromic repeats (CRISPR), are also discussed. Other relevant aspects comprising new promoters and reporter genes, marker-free systems, and strategies for induction of early flowering, are also addressed. We provided a future perspective on the use of current and new technologies in citrus and its potential impact on regulatory processes.

Advances in genomics and genome editing for breeding next generation of fruit and nut crops

Fruit tree crops are an essential part of the food production systems and are key to achieve food and nutrition security. Genetic improvement of fruit trees by conventional breeding has been slow due to the long juvenile phase. Advancements in genomics and molecular biology have paved the way for devising novel genetic improvement tools like genome editing, which can accelerate the breeding of these perennial crops to a great extent. In this article, advancements in genomics of fruit trees covering genome sequencing, transcriptome sequencing, genome editing technologies (GET), CRISPR-Cas system based genome editing, potential applications of CRISPR-Cas9 in fruit tree crops improvement, the factors influencing the CRISPR-Cas editing efficiency and the challenges for CRISPR-Cas9 applications in fruit tree crops improvement are reviewed. Besides, base editing, a recently emerging more precise editing system, and the future perspectives of genome editing in the improvement of fruit and nut crops are covered.

Selection and Evaluation of a Thornless and HLB-Tolerant Bud-Sport of Pummelo Citrus With an Emphasis on Molecular Mechanisms

The selection of elite bud-sports is an important breeding approach in horticulture. We discovered and evaluated a thornless pummelo bud-sport (TL) that grew more vigorously and was more tolerant to Huanglongbing (HLB) than the thorny wild type (W). To reveal the underlying molecular mechanisms, we carried out whole-genome sequencing of W, and transcriptome comparisons of W, TL, and partially recovered thorny "mutants" (T). The results showed W, TL, and T varied in gene expression, allelic expression, and alternative splicing. Most genes/pathways with significantly altered expression in TL compared to W remained similarly altered in T. Pathway and gene ontology enrichment analysis revealed that the expression of multiple pathways, including photosynthesis and cell wall biosynthesis, was altered among the three genotypes. Remarkably, two polar auxin transporter genes, PIN7 and LAX3, were expressed at a significantly lower level in TL than in both W and T, implying alternation of polar auxin transport in TL may be responsible for the vigorous growth and thornless phenotype. Furthermore, 131 and 68 plant defense-related genes were significantly upregulated and downregulated, respectively, in TL and T compared with W. These genes may be involved in enhanced salicylic acid (SA) dependent defense and repression of defense inducing callose deposition and programmed cell death. Overall, these results indicated that the phenotype changes of the TL bud-sport were associated with tremendous transcriptome alterations, providing new clues and targets for breeding and gene editing for citrus improvement.

IDA (INFLORESCENCE DEFICIENT IN ABSCISSION)-like peptides and HAE (HAESA)-like receptors regulate corolla abscission in Nicotiana benthamiana flowers

BACKGROUND

Abscission is an active, organized, and highly coordinated cell separation process enabling the detachment of aerial organs through the modification of cell-to-cell adhesion and breakdown of cell walls at specific sites on the plant body known as abscission zones. In Arabidopsis thaliana, abscission of floral organs and cauline leaves is regulated by the interaction of the hormonal peptide INFLORESCENCE DEFICIENT IN ABSCISSION (IDA), a pair of redundant receptor-like protein kinases, HAESA (HAE) and HAESA-LIKE2 (HSL2), and SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) co-receptors. However, the functionality of this abscission signaling module has not yet been demonstrated in other plant species.

RESULTS

The expression of the pair of NbenIDA1 homeologs and the receptor NbenHAE.1 was supressed at the base of the corolla tube by the inoculation of two virus-induced gene silencing (VIGS) constructs in Nicotiana benthamiana. These gene suppression events arrested corolla abscission but did not produce any obvious effect on plant growth. VIGS plants retained a higher number of corollas attached to the flowers than control plants, an observation related to a greater corolla breakstrength. The arrest of corolla abscission was associated with the preservation of the parenchyma tissue at the base of the corolla tube that, in contrast, was virtually collapsed in normal corollas. In contrast, the inoculation of a viral vector construct that increased the expression of NbenIDA1A at the base of the corolla tube negatively affected the growth of the inoculated plants accelerating the timing of both corolla senescence and abscission. However, the heterologous ectopic overexpression of citrus CitIDA3 and Arabidopsis AtIDA in N. benthamiana did not alter the standard plant phenotype suggesting that the proteolytic processing machinery was unable to yield active peptides.

CONCLUSION

Here, we demonstrate that the pair of NbenIDA1 homeologs encoding small peptides of the IDA-like family and the receptor NbenHAE.1 control cellular breakdown at the base of the corolla tube awhere an adventitious AZ should be formed and, therefore, corolla abscission in N. benthamiana flowers. Altogether, our results provide the first evidence supporting the notion that the IDA-HAE/HSL2 signaling module is conserved in angiosperms.

Transient reprogramming of crop plants for agronomic performance

The development of a new crop variety is a time-consuming and costly process due to the reliance of plant breeding on gene shuffling to introduce desired genes into elite germplasm, followed by backcrossing. Here, we propose alternative technology that transiently targets various regulatory circuits within a plant, leading to operator-specified alterations of agronomic traits, such as time of flowering, vernalization requirement, plant height or drought tolerance. We redesigned techniques of gene delivery, amplification and expression around RNA viral transfection methods that can be implemented on an industrial scale and with many crop plants. The process does not involve genetic modification of the plant genome and is thus limited to a single plant generation, is broadly applicable, fast, tunable and versatile, and can be used throughout much of the crop cultivation cycle. The RNA-based reprogramming may be especially useful in plant pathogen pandemics but also for commercial seed production and for rapid adaptation of orphan crops.

SuperSour: A New Strategy for Breeding Superior Citrus Rootstocks

Citrus crops have a long history of cultivation as grafted trees on selected rootstock cultivars, but all current rootstocks have significant limitations and traditional methods of rootstock breeding take at least 2-3 decades to develop and field test new rootstocks. Citrus production in the United States, and other parts of the world, is impaired by a wide range of biotic and abiotic problems, with especially severe damage caused by the disease huanglongbing (HLB) associated with Candidatus Liberibacter asiaticus. All major commercial citrus scion cultivars are damaged by HLB, but tree tolerance is significantly improved by some rootstocks. To overcome these challenges, the USDA citrus breeding program has implemented a multi-pronged strategy for rootstock breeding that expands the diversity of germplasm utilized in rootstock breeding, significantly increases the number of new hybrids evaluated concurrently, and greatly reduces the time from cross to potential cultivar release. We describe the key components and methodologies of this new strategy, termed "SuperSour," along with reference to the historical favorite rootstock sour orange (Citrus aurantium), and previous methods employed in citrus rootstock breeding. Rootstock propagation by cuttings and tissue culture is one key to the new strategy, and by avoiding the need for nucellar seeds, eliminates the 6- to 15-year delay in testing while waiting for new hybrids to fruit. In addition, avoiding selection of parents and progeny based on nucellar polyembryony vastly expands the potential genepool for use in rootstock improvement. Fifteen new field trials with more than 350 new hybrid rootstocks have been established under the SuperSour strategy in the last 8 years. Detailed multi-year performance data from the trials will be used to identify superior rootstocks for commercial release, and to map important traits and develop molecular markers for the next generation of rootstock development. Results from two of these multi-year replicated field trials with sweet orange scion are presented to illustrate performance of 97 new hybrid rootstocks relative to four commercial rootstocks. Through the first 7 years in the field with endemic HLB, many of the new SuperSour hybrid rootstocks exhibit greatly superior fruit yield, yield efficiency, canopy health, and fruit quality, as compared with the standard rootstocks included in the trials.

The vascular targeted citrus FLOWERING LOCUS T3 gene promotes non-inductive early flowering in transgenic Carrizo rootstocks and grafted juvenile scions

Shortening the juvenile stage in citrus and inducing early flowering has been the focus of several citrus genetic improvement programs. FLOWERING LOCUS T (FT) is a small phloem-translocated protein that regulates precocious flowering. In this study, two populations of transgenic Carrizo citrange rootstocks expressing either Citrus clementina FT1 or FT3 genes under the control of the Arabidopsis thaliana phloem specific SUCROSE SYNTHASE 2 (AtSUC2) promoter were developed. The transgenic plants were morphologically similar to the non-transgenic controls (non-transgenic Carrizo citrange), however, only AtSUC2-CcFT3 was capable of inducing precocious flowers. The transgenic lines produced flowers 16 months after transformation and flower buds appeared 30-40 days on juvenile immature scions grafted onto transgenic rootstock. Gene expression analysis revealed that the expression of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and APETALA1 (AP1) were enhanced in the transgenics. Transcriptome profiling of a selected transgenic line showed the induction of genes in different groups including: genes from the flowering induction pathway, APETALA2/ETHYLENE RESPONSE FACTOR (AP2/ERF) family genes, and jasmonic acid (JA) pathway genes. Altogether, our results suggested that ectopic expression of CcFT3 in phloem tissues of Carrizo citrange triggered the expression of several genes to mediate early flowering.

Plant Virology Delivers Diverse Toolsets for Biotechnology

Over a hundred years of research on plant viruses has led to a detailed understanding of viral replication, movement, and host-virus interactions. The functions of vast viral genes have also been annotated. With an increased understanding of plant viruses and plant-virus interactions, various viruses have been developed as vectors to modulate gene expressions for functional studies as well as for fulfilling the needs in biotechnology. These approaches are invaluable not only for molecular breeding and functional genomics studies related to pivotal agronomic traits, but also for the production of vaccines and health-promoting carotenoids. This review summarizes the latest progress in these forefronts as well as the available viral vectors for economically important crops and beyond.

Recent Advances of In Vitro Culture for the Application of New Breeding Techniques in Citrus

Citrus is one of the most important fruit crops in the world. This review will discuss the recent findings related to citrus transformation and regeneration protocols of juvenile and adult explants. Despite the many advances that have been made in the last years (including the use of inducible promoters and site-specific recombination systems), transformation efficiency, and regeneration potential still represent a bottleneck in the application of the new breeding techniques in commercial citrus varieties. The influence of genotype, explant type, and other factors affecting the regeneration and transformation of the most used citrus varieties will be described, as well as some examples of how these processes can be applied to improve fruit quality and resistance to various pathogens and pests, including the potential of using genome editing in citrus. The availability of efficient regeneration and transformation protocols, together with the availability of the source of resistance, is made even more important in light of the fast diffusion of emerging diseases, such as Huanglongbing (HLB), which is seriously challenging citriculture worldwide.

Virus-induced flowering-a tool for cereals

This article comments on:

Yuan C, Li H, Qin C, Zhang X, Chen Q, Zhang P, Xu X, He M, Zhang X, Tör M, Xue Dawei, Wang H, Jackson S, He Y, Liu Y, Shi N, Hong Y. 2020. Foxtail mosaic virus-induced flowering assays in monocot crops. Journal of Experimental Botany 71, 3012–3023.

Foxtail mosaic virus-induced flowering assays in monocot crops

Virus-induced flowering (VIF) exploits RNA or DNA viruses to express flowering time genes to induce flowering in plants. Such plant virus-based tools have recently attracted widespread attention for their fundamental and applied uses in flowering physiology and in accelerating breeding in dicotyledonous crops and woody fruit-trees. We now extend this technology to a monocot grass and a cereal crop. Using a Foxtail mosaic virus (FoMV)-based VIF system, dubbed FoMViF, we showed that expression of florigenic Flowering Locus T (FT) genes can promote early flowering and spikelet development in proso millet, a C4 grass species with potential as a nutritional food and biofuel resource, and in non-vernalized C3 wheat, a major food crop worldwide. Floral and spikelet/grain induction in the two monocot plants was caused by the virally expressed untagged or FLAG-tagged FT orthologs, and the florigenic activity of rice Hd3a was more pronounced than its dicotyledonous counterparts in proso millet. The FoMViF system is easy to use and its efficacy to induce flowering and early spikelet/grain production is high. In addition to proso millet and wheat, we envisage that FoMViF will be also applicable to many economically important monocotyledonous food and biofuel crops.

Fast-track breeding system to introduce CTV resistance of trifoliate orange into citrus germplasm, by integrating early flowering transgenic plants with marker-assisted selection

BACKGROUND

Global warming will expand the range of new and invasive pathogens in orchards, and subsequently increase the risk of disease epidemics and economic losses. The development of new resistant plant varieties can help to reduce the impact of pathogens, however, the breeding speed can be extremely slow, due to the growth rates of the plants, and the availability of resistance genes. Citrus trees are suffering immense damage from serious diseases such as citrus canker (XCC), huanglongbing (HLB), and citrus tristeza virus (CTV). A fast-track breeding system, that aimed at shortening the duration for disease resistance breeding by incorporating the resistance genes from related species to commercial varieties, has been developed using the integration of precocious transgenic trifoliate orange with the overexpression of CiFT and MAS. It was applied here to incorporate CTV resistance of trifoliate orange into citrus germplasm.

RESULTS

One generation of backcrossed breeding, that would normally take at least 5 years, was achieved in a single year by fast-track breeding system. Linkage analysis using the corresponding DNA markers revealed that CTV resistance and T-DNA integrated regions were found in different linkage groups, and they were independently segregated in the BC progenies. The CTV resistant null segregants, in which the T-DNA integrated region was removed from their genome, were feasibly obtained by MAS in each generation of the BC progenies, and their CTV resistance was confirmed by immunological analysis. Several BC₃ null segregants, whose genetic backgrounds had been substituted into citrus germplasm, except for the haplotype block of CTV resistance, were successfully obtained. CGH and NGS analyses revealed that the T-DNA integrated region was safely segregated out in null segregants.

CONCLUSION

Fast-track breeding systems are expected to shorten the required breeding time by more than one-fifth in comparison with conventional cross breeding techniques. Using this system, we obtained BC₃-8, whose genetic background was successfully substituted except for the CTV resistance locus, and could be a novel mandarin breeding material. The fast-track breeding system will be useful to introduce important traits from related species to citrus germplasm while also drastically reducing the time required for breeding.

Improvement of alfalfa forage quality and management through the down-regulation of MsFTa1

Alfalfa (Medicago sativa L.) is one of the most important forage crops worldwide. As a perennial, alfalfa is cut several times each year. Farmers face a dilemma: if cut earlier, forage nutritive value is much higher but regrowth is affected and the longevity of the stand is severely compromised. On the other hand, if alfalfa is cut later at full flower, stands persist longer and more biomass may be harvested, but the nutritive value diminishes. Alfalfa is a strict long-day plant. We reasoned that by manipulating the response to photoperiod, we could delay flowering to improve forage quality and widen each harvesting window, facilitating management. With this aim, we functionally characterized the FLOWERING LOCUS T family of genes, represented by five members: MsFTa1, MsFTa2, MsFTb1, MsFTb2 and MsFTc. The expression of MsFTa1 correlated with photoperiodic flowering and its down-regulation led to severe delayed flowering. Altogether, with late flowering, low expression of MsFTa1 led to changes in plant architecture resulting in increased leaf to stem biomass ratios and forage digestibility. By manipulating photoperiodic flowering, we were able to improve the quality of alfalfa forage and management, which may allow farmers to cut alfalfa of high nutritive value without compromising stand persistence.

New Plant Breeding Techniques in Citrus for the Improvement of Important Agronomic Traits. A Review

New plant breeding techniques (NPBTs) aim to overcome traditional breeding limits for fruit tree species, in order to obtain new varieties with improved organoleptic traits and resistance to biotic and abiotic stress, and to maintain fruit quality achieved over centuries by (clonal) selection. Knowledge on the gene(s) controlling a specific trait is essential for the use of NPBTs, such as genome editing and cisgenesis. In the framework of the international scientific community working on fruit tree species, including citrus, NPBTs have mainly been applied to address pathogen threats. Citrus could take advantage of NPBTs because of its complex species biology (seedlessness, apomixis, high heterozygosity, and long juvenility phase) and aptitude for in vitro manipulation. To our knowledge, genome editing in citrus via transgenesis has successful for induced resistance to Citrus bacterial canker in sweet orange and grapefruit using the resistance gene CsLOB1. In the future, NPBTs will also be used to improve fruit traits, making them healthier. The regeneration of plants following the application of NPBTs is a bottleneck, making it necessary to optimize the efficiency of current protocols. The strengths and weaknesses of using explants from young in vitro plantlets, and from mature plants, will be discussed. Other major issues addressed in this review are related to the requirement for marker-free systems and shortening the long juvenility phase. This review aims to summarize methods and approaches available in the literature that are suitable to citrus, focusing on the principles observed before the use of NPBTs.

Citrus Taste Modification Potentials by Genetic Engineering

Citrus fruits are mainly consumed as fresh fruit and processed juice products. They serve as nutritional and a tasty diet in our daily life. However, the formidable bitterness and delayed bitterness significantly impact the citrus industry attributable to the two major bitter compounds naringin and limonin. The extremely sour and acidic also negatively affects the sensory quality of citrus products. Citrus breeding programs have developed different strategies to improve citrus quality and a wealth of studies have aimed to uncover the genetic and biochemical basis of citrus flavor. In this minireview, we outline the major genes characterized to be involved in pathways shaping the sweet, bitter, or sour taste in citrus, and discuss briefly about the possible approaches to modify citrus taste by genetic engineering.

Plant Virus Vectors 3.0: Transitioning into Synthetic Genomics

Plant viruses were first implemented as heterologous gene expression vectors more than three decades ago. Since then, the methodology for their use has varied, but we propose it was the merging of technologies with virology tools, which occurred in three defined steps discussed here, that has driven viral vector applications to date. The first was the advent of molecular biology and reverse genetics, which enabled the cloning and manipulation of viral genomes to express genes of interest (vectors 1.0). The second stems from the discovery of RNA silencing and the development of high-throughput sequencing technologies that allowed the convenient and widespread use of virus-induced gene silencing (vectors 2.0). Here, we briefly review the events that led to these applications, but this treatise mainly concentrates on the emerging versatility of gene-editing tools, which has enabled the emergence of virus-delivered genetic queries for functional genomics and virology (vectors 3.0).

Harnessed viruses in the age of metagenomics and synthetic biology: an update on infectious clone assembly and biotechnologies of plant viruses

Recent metagenomic studies have provided an unprecedented wealth of data, which are revolutionizing our understanding of virus diversity. A redrawn landscape highlights viruses as active players in the phytobiome, and surveys have uncovered their positive roles in environmental stress tolerance of plants. Viral infectious clones are key tools for functional characterization of known and newly identified viruses. Knowledge of viruses and their components has been instrumental for the development of modern plant molecular biology and biotechnology. In this review, we provide extensive guidelines built on current synthetic biology advances that streamline infectious clone assembly, thus lessening a major technical constraint of plant virology. The focus is on generation of infectious clones in binary T-DNA vectors, which are delivered efficiently to plants by Agrobacterium. We then summarize recent applications of plant viruses and explore emerging trends in microbiology, bacterial and human virology that, once translated to plant virology, could lead to the development of virus-based gene therapies for ad hoc engineering of plant traits. The systematic characterization of plant virus roles in the phytobiome and next-generation virus-based tools will be indispensable landmarks in the synthetic biology roadmap to better crops.

Agrobacterium-Mediated Transformation of Tree Fruit Crops: Methods, Progress, and Challenges

Genetic engineering based on Agrobacterium-mediated transformation has been a desirable tool to manipulate single or multiple genes of existing genotypes of woody fruit crops, for which conventional breeding is a difficult and lengthy process due to heterozygosity, sexual incompatibility, juvenility, or a lack of natural sources. To date, successful transformation has been reported for many fruit crops. We review the major progress in genetic transformation of these fruit crops made in the past 5 years, emphasizing reproducible transformation protocols as well as the strategies that have been tested in fruit crops. While direct transformation of scion cultivars was mostly used for fruit quality improvement, biotic and abiotic tolerance, and functional gene analysis, transgrafting on genetically modified (GM) rootstocks showed a potential to produce non-GM fruit products. More recently, genome editing technology has demonstrated a potential for gene(s) manipulation of several fruit crops. However, substantial efforts are still needed to produce plants from gene-edited cells, for which tremendous challenge remains in the context of either cell's recalcitrance to regeneration or inefficient gene-editing due to their polyploidy. We propose that effective transient transformation and efficient regeneration are the key for future utilization of genome editing technologies for improvement of fruit crops.

Virus-induced gene silencing and virus-induced flowering in strawberry (Fragaria × ananassa) using apple latent spherical virus vectors

Apple latent spherical virus (ALSV) vector is a convenient alternative to genetic transformation in horticultural plants, especially in species recalcitrant to genetic transformation. ALSV, an RNA virus, can infect a wide variety of plant species including major horticultural plants without inducing symptoms. Here, methodologies were developed for infection of ALSV vectors to strawberry seedlings and plantlets cultured in vitro. A seed-propagated F₁ hybrid strawberry cultivar 'Yotsuboshi' was aseptically grown on half-strength Murashige-Skoog medium for 1 month and true leaves were inoculated with an ALSV RNA preparation by particle bombardment. ALSV vector infection rates varied from 58 to 100% according to the insertion sequences, in 'Yotsuboshi' seedlings. Plantlets ('Dover') propagated in vitro could also be infected with ALSV vector at a similar infection rate. For virus-induced gene silencing (VIGS), we prepared an ALSV vector carrying a 201 nucleotide segment of the strawberry phytoene desaturase gene. 'Yotsuboshi' and 'Dover' plants infected by this vector generated completely white leaves at fifth or sixth true leaves and above. For virus-induced flowering (VIF), we used an ALSV vector expressing the Arabidopsis thaliana flowering locus T gene. Strawberry seedlings infected by this vector started to flower from about 2 months post inoculation and bore fruits with viable seeds. The ALSV vector was no longer detected in any of the seedlings from early-flowered strawberries. Thus, the ALSV vector may be beneficial for examination of gene functions by VIGS in strawberry, and VIF using ALSV vector constitutes an effective new plant breeding technique for the promotion of cross-breeding in strawberry.

Precocious genotypes and homozygous tendency generated by self-pollination in walnut

BACKGROUND

Observations of precocious (early bearing) genotypes of walnut (Juglans regia L.) under natural conditions encouraged us to study the origin and genetic control of these fascinating traits.

RESULTS

In this study, the self-fertility, progeny performance, and simple sequence repeat (SSR) locus variation of iron walnut (Juglans sigillata Dode), an ecotype of J. regia, were investigated. The average self-pollinated fruit set rate of J. sigillata cv. 'Dapao' (DP) was 7.0% annually from 1979 to 1982. The average germination rate of self-pollinated seeds was 45.2% during the 4-year period. Most progeny had inbreeding depression. Nine representative self-pollinated progeny (SP₁-SP₉), with special or typical traits of DP, were selected. SP₁-SP₄ were precocious because they initiated flowers as early as 2 years after germination, compared to the 7-10-yr period that is typical of DP. SP₉ had not flowered since 1980. Twelve SSR markers were used to analyze the SP and DP. The genome of SP had a tendency toward high levels of homozygosity. The high levels of homozygosity reported in 18 additional precocious walnut genotypes complemented the results of this study.

CONCLUSIONS

These results provide evidence of precocious phenotypes and genomes with high levels of homozygosity that might be generated from self-pollinating walnut. This suggests that self-pollination might facilitate the generation of unique homozygous parents for subsequent use in walnut-breeding programs. The results also indicate that more attention should be focused on adequate management of precocious walnut to avoid early depression in the production of nuts.

Strategies for Engineering Reproductive Sterility in Plantation Forests

A considerable body of research exists concerning the development of technologies to engineer sterility in forest trees. The primary driver for this work has been to mitigate concerns arising from gene flow from commercial plantings of genetically engineered (GE) trees to non-GE plantations, or to wild or feral relatives. More recently, there has been interest in the use of sterility technologies as a means to mitigate the global environmental and socio-economic damage caused by the escape of non-native invasive tree species from planted forests. The current sophisticated understanding of the molecular processes underpinning sexual reproduction in angiosperms has facilitated the successful demonstration of a number of control strategies in hardwood tree species, particularly in the model hardwood tree Poplar. Despite gymnosperm softwood trees, such as pines, making up the majority of the global planted forest estate, only pollen sterility, via cell ablation, has been demonstrated in softwoods. Progress has been limited by the lack of an endogenous model system, long timescales required for testing, and key differences between softwood reproductive pathways and those of well characterized angiosperm model systems. The availability of comprehensive genome and transcriptome resources has allowed unprecedented insights into the reproductive processes of both hardwood and softwood tree species. This increased fundamental knowledge together with the implementation of new breeding technologies, such as gene editing, which potentially face a less oppressive regulatory regime, is making the implementation of engineered sterility into commercial forestry a realistic possibility.

The resistance of sour orange to Citrus tristeza virus is mediated by both the salicylic acid and RNA silencing defence pathways

Citrus tristeza virus (CTV) induces in the field the decline and death of citrus varieties grafted on sour orange (SO) rootstock, which has forced the use of alternative decline-tolerant rootstocks in affected countries, despite the highly desirable agronomic features of the SO rootstock. Declining citrus plants display phloem necrosis below the bud union. In addition, SO is minimally susceptible to CTV compared with other citrus varieties, suggesting partial resistance of SO to CTV. Here, by silencing different citrus genes with a Citrus leaf blotch virus-based vector, we have examined the implication of the RNA silencing and salicylic acid (SA) defence pathways in the resistance of SO to CTV. Silencing of the genes RDR1, NPR1 and DCL2/DCL4, associated with these defence pathways, enhanced virus spread and accumulation in SO plants in comparison with non-silenced controls, whereas silencing of the genes NPR3/NPR4, associated with the hypersensitive response, produced a slight decrease in CTV accumulation and reduced stunting of SO grafted on CTV-infected rough lemon plants. We also found that the CTV RNA silencing suppressors p20 and p23 also suppress the SA signalling defence, with the suppressor activity being higher in the most virulent isolates.

Expression patterns of flowering genes in leaves of 'Pineapple' sweet orange [Citrus sinensis (L.) Osbeck] and pummelo (Citrus grandis Osbeck)

BACKGROUND

In citrus the transition from juvenility to mature phase is marked by the capability of a tree to flower and fruit consistently. The long period of juvenility in citrus severely impedes the use of genetic based strategies to improve fruit quality, disease resistance, and responses to abiotic environmental factors. One of the genes whose expression signals flower development in many plant species is FLOWERING LOCUS T (FT).

RESULTS

In this study, gene expression levels of flowering genes CiFT1, CiFT2 and CiFT3 were determined using reverse-transcription quantitative real-time PCR in citrus trees over a 1 year period in Florida. Distinct genotypes of citrus trees of different ages were used. In mature trees of pummelo (Citrus grandis Osbeck) and 'Pineapple' sweet orange (Citrus sinensis (L.) Osbeck) the expression of all three CiFT genes was coordinated and significantly higher in April, after flowering was over, regardless of whether they were in the greenhouse or in the field. Interestingly, immature 'Pineapple' seedlings showed significantly high levels of CiFT3 expression in April and June, while CiFT1 and CiFT2 were highest in June, and hence their expression induction was not simultaneous as in mature plants.

CONCLUSIONS

In mature citrus trees the induction of CiFTs expression in leaves occurs at the end of spring and after flowering has taken place suggesting it is not associated with dormancy interruption and further flower bud development but is probably involved with shoot apex differentiation and flower bud determination. CiFTs were also seasonally induced in immature seedlings, indicating that additional factors must be suppressing flowering induction and their expression has other functions.

A Virus-Induced Assay for Functional Dissection and Analysis of Monocot and Dicot Flowering Time Genes

Virus-induced flowering (VIF) uses virus vectors to express Flowering Locus T (FT) to induce flowering in plants. This approach has recently attracted wide interest for its practical applications in accelerating breeding in crops and woody fruit trees. However, the insight into VIF and its potential as a powerful tool for dissecting florigenic proteins remained to be elucidated. Here, we describe the mechanism and further applications of Potato virus X (PVX)-based VIF in the short-day Nicotiana tabacum cultivar Maryland Mammoth. Ectopic delivery of Arabidopsis (Arabidopsis thaliana) AtFT by PVX/AtFT did not induce the expression of the endogenous FT ortholog NtFT4; however, it was sufficient to trigger flowering in Maryland Mammoth plants grown under noninductive long-day conditions. Infected tobacco plants developed no systemic symptoms, and the PVX-based VIF did not cause transgenerational flowering. We showed that the PVX-based VIF is a much more rapid method to examine the impacts of single amino acid mutations on AtFT for floral induction than making individual transgenic Arabidopsis lines for each mutation. We also used the PVX-based VIF to demonstrate that adding a His- or FLAG-tag to the N or C terminus of AtFT could affect its florigenic activity and that this system can be applied to assay the function of FT genes from heterologous species, including tomato (Solanum lycopersicum) SFT and rice (Oryza sativa) Hd3a Thus, the PVX-based VIF represents a simple and efficient system to identify individual amino acids that are essential for FT-mediated floral induction and to test the ability of mono- and dicotyledonous FT genes and FT fusion proteins to induce flowering.

Virus-Induced Flowering: An Application of Reproductive Biology to Benefit Plant Research and Breeding

Virus-induced flowering combines fundamental research in reproductive biology with efficient tools for manipulating gene expression in nonmodel systems to accelerate discovery and breeding.