Current achievements and future directions in genetic engineering of European plum (Prunus domestica L.)

De novo transcriptome profiling reveals the patterns of gene expression in plum fruits with bud mutations

Bud mutation is a common technique for plant breeding and can provide a large number of breeding materials. Through traditional breeding methods, we obtained a plum plant with bud mutations (named "By") from an original plum variety (named "B"). The ripening period of "By" fruit was longer than that of "B" fruit, and its taste was better. In order to understand the characteristics of these plum varieties, we used transcriptome analysis and compared the gene expression patterns in fruits from the two cultivars. Subsequently, we identified the biological processes regulated by the differentially expressed genes (DEGs). Gene ontology (GO) analysis revealed that these DEGs were highly enriched for "single-organism cellular process" and "transferase activity". KEGG analysis demonstrated that the main pathways affected by the bud mutations were plant hormone signal transduction, starch and sucrose metabolism. The IAA, CKX, ARF, and SnRK2 genes were identified as the key regulators of plant hormone signal transduction. Meanwhile, TPP, the beta-glucosidase (EC3.2.1.21) gene, and UGT72E were identified as candidate DEGs affecting secondary metabolite synthesis. The transcriptome sequencing (RNA-seq) data were also validated using RT-qPCR experiments. The transcriptome analysis demonstrated that plant hormones play a significant role in extending the maturity period of plum fruit, with IAA, CKX, ARF, and SnRK2 serving as the key regulators of this process. Further, TPP, beta-glucosidase (EC3.2.1.21), and UGT72E appeared to mediate the synthesis of various soluble secondary metabolites, contributing to the aroma of plum fruits. The expression of BAG6 was upregulated in "B" as the fruit matured, but it was downregulated in "By". This indicated that "B" may have stronger resistance, especially fungal resistance.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-024-01472-3.

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.

CRISPR/Cas-mediated plant genome editing: outstanding challenges a decade after implementation

The discovery of the CRISPR/Cas genome-editing system has revolutionized our understanding of the plant genome. CRISPR/Cas has been used for over a decade to modify plant genomes for the study of specific genes and biosynthetic pathways as well as to speed up breeding in many plant species, including both model and non-model crops. Although the CRISPR/Cas system is very efficient for genome editing, many bottlenecks and challenges slow down further improvement and applications. In this review we discuss the challenges that can occur during tissue culture, transformation, regeneration, and mutant detection. We also review the opportunities provided by new CRISPR platforms and specific applications related to gene regulation, abiotic and biotic stress response improvement, and de novo domestication of plants.

Prunus Knotted-like Genes: Genome-Wide Analysis, Transcriptional Response to Cytokinin in Micropropagation, and Rootstock Transformation

Knotted1-like homeobox (KNOX) transcription factors are involved in plant development, playing complex roles in aerial organs. As Prunus species include important fruit tree crops of Italy, an exhaustive investigation of KNOX genes was performed using genomic and RNA-seq meta-analyses. Micropropagation is an essential technology for rootstock multiplication; hence, we investigated KNOX transcriptional behavior upon increasing 6-benzylaminopurine (BA) doses and the effects on GF677 propagules. Moreover, gene function in Prunus spp. was assessed by Gisela 6 rootstock transformation using fluorescence and peach KNOX transgenes. Based on ten Prunus spp., KNOX proteins fit into I-II-M classes named after Arabidopsis. Gene number, class member distribution, and chromosome positions were maintained, and exceptions supported the diversification of Prunus from Cerasus subgenera, and that of Armeniaca from the other sections within Prunus. Cytokinin (CK) cis-elements occurred in peach and almond KNOX promoters, suggesting a BA regulatory role in GF677 shoot multiplication as confirmed by KNOX expression variation dependent on dose, time, and interaction. The tripled BA concentration exacerbated stress, altered CK perception genes, and modified KNOX transcriptions, which are proposed to concur in in vitro anomalies. Finally, Gisela 6 transformation efficiency varied (2.6-0.6%) with the genetic construct, with 35S:GFP being more stable than 35S:KNOPE1 lines, which showed leaf modification typical of KNOX overexpression.

Transgene-free genome editing and RNAi ectopic application in fruit trees: Potential and limitations

For the past fifteen years, significant research advances in sequencing technology have led to a substantial increase in fruit tree genomic resources and databases with a massive number of OMICS datasets (transcriptomic, proteomics, metabolomics), helping to find associations between gene(s) and performance traits. Meanwhile, new technology tools have emerged for gain- and loss-of-function studies, specifically in gene silencing and developing tractable plant models for genetic transformation. Additionally, innovative and adapted transformation protocols have optimized genetic engineering in most fruit trees. The recent explosion of new gene-editing tools allows for broadening opportunities for functional studies in fruit trees. Yet, the fruit tree research community has not fully embraced these new technologies to provide large-scale genome characterizations as in cereals and other staple food crops. Instead, recent research efforts in the fruit trees appear to focus on two primary translational tools: transgene-free gene editing via Ribonucleoprotein (RNP) delivery and the ectopic application of RNA-based products in the field for crop protection. The inherent nature of the propagation system and the long juvenile phase of most fruit trees are significant justifications for the first technology. The second approach might have the public favor regarding sustainability and an eco-friendlier environment for a crop production system that could potentially replace the use of chemicals. Regardless of their potential, both technologies still depend on the foundational knowledge of gene-to-trait relationships generated from basic genetic studies. Therefore, we will discuss the status of gene silencing and DNA-based gene editing techniques for functional studies in fruit trees followed by the potential and limitations of their translational tools (RNP delivery and RNA-based products) in the context of crop production.

Unlocking Plum Genetic Potential: Where Are We At?

Plums are a large group of closely related stone fruit species and hybrids of worldwide economic importance and diffusion. This review deals with the main aspects concerning plum agrobiodiversity and its relationship with current and potential contributions offered by breeding in enhancing plum varieties. The most recent breeding achievements are revised according to updated information proceeding from relevant scientific reports and official inventories of plum genetic resources. A special emphasis has been given to the potential sources of genetic traits of interest for breeding programs as well as to the need for efficient and coordinated efforts aimed at efficaciously preserving the rich and underexploited extant plum agrobiodiversity. The specific objective of this review was to: (i) analyze and possibly evaluate the degree of biodiversity existing in the cultivated plum germplasm, (ii) examine the set of traits of prominent agronomic and pomological interest currently targeted by the breeders, and (iii) determine how and to what extent this germplasm was appropriately exploited in breeding programs or could represent concrete prospects for the future.

Significant improvement of apple (Malus domestica Borkh.) transgenic plant production by pre-transformation with a Baby boom transcription factor

BABY BOOM (BBM) is a member of the APETALA2/ETHYLENE RESPONSE FACTOR (AP2/ERF) family and its expression has been shown to improve herbaceous plant transformation and regeneration. However, this improvement has not been shown clearly for tree species. This study demonstrated that the efficiency of transgenic apple (Malus domestica Borkh.) plant production was dramatically increased by ectopic expression of the MdBBM1 gene. "Royal Gala" apple plants were first transformed with a CaMV35S-MdBBM1 construct (MBM) under kanamycin selection. These MBM transgenic plants exhibited enhanced shoot regeneration from leaf explants on tissue culture media, with most plants displaying a close-to-normal phenotype compared with CaMV35S-GUS transgenic plants when grown under greenhouse conditions, the exception being that some plants had slightly curly leaves. Thin leaf sections revealed the MBM plants produced more cells than the GUS plants, indicating that ectopic-expression of MdBBM1 enhanced cell division. Transcriptome analysis showed that mRNA levels for cell division activators and repressors linked to hormone (auxin, cytokinin and brassinosteroid) signalling pathways were enhanced and reduced, respectively, in the MBM plants compared with the GUS plants. Plants of eight independent MBM lines were compared with the GUS plants by re-transforming them with an herbicide-resistant gene construct. The number of transgenic plants produced per 100 leaf explants was 0-3% for the GUS plants, 3-8% for five MBM lines, and 20-30% for three MBM lines. Our results provided a solution for overcoming the barriers to transgenic plant production in apple, and possibly in other trees.

Stability of Transgene Inheritance in Progeny of Field-Grown Pear Trees over a 7-Year Period

Breeding woody plants is a very time-consuming process, and genetic engineering tools have been used to shorten the juvenile phase. In addition, transgenic trees for commercial cultivation can also be used in classical breeding, but the segregation of transgenes in the progeny of perennial plants, as well as the possible appearance of unintended changes, have been poorly investigated. We studied the inheritance of the uidA gene in the progeny of field-grown transgenic pear trees for 7 years and the physical and physiological parameters of transgenic seeds. A total of 13 transgenic lines were analyzed, and the uidA gene segregated 1:1 in the progeny of 9 lines and 3:1 in the progeny of 4 lines, which is consistent with Mendelian inheritance for one and two transgene loci, respectively. Rare and random deviations from the Mendelian ratio were observed only for lines with one locus. Transgenic seeds' mass, size, and shape varied slightly, despite significant fluctuations in weather conditions during cultivation. Expression of the uidA gene in the progeny was stable. Our study showed that the transgene inheritance in the progeny of pear trees under field conditions occurs according to Mendelian ratio, does not depend on the environment, and the seed vigor of transgenic seeds does not change.

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.

Metabolic engineering in woody plants: challenges, advances, and opportunities

Woody plant species represent an invaluable reserve of biochemical diversity to which metabolic engineering can be applied to satisfy the need for commodity and specialty chemicals, pharmaceuticals, and renewable energy. Woody plants are particularly promising for this application due to their low input needs, high biomass, and immeasurable ecosystem services. However, existing challenges have hindered their widespread adoption in metabolic engineering efforts, such as long generation times, large and highly heterozygous genomes, and difficulties in transformation and regeneration. Recent advances in omics approaches, systems biology modeling, and plant transformation and regeneration methods provide effective approaches in overcoming these outstanding challenges. Promises brought by developments in this space are steadily opening the door to widespread metabolic engineering of woody plants to meet the global need for a wide range of sustainably sourced chemicals and materials.

Stress-inducible Arabidopsis thaliana RD29A promoter constitutively drives Citrus sinensis APETALA1 and LEAFY expression and precocious flowering in transgenic Citrus spp

Transgenic 'Duncan' grapefruit (Citrus paradisi Macf.) and 'Valencia' sweet orange (Citrus sinensis [L.] Osbeck) plants ectopically expressing C. sinensis (cv. Washington navel orange) APETALA1 (CsAP1) or LEAFY (CsLFY) genes under control of the Arabidopsis thaliana stress-inducible promoter AtRD29A flowered under non-inductive (warm temperature, well-watered) greenhouse conditions, whereas their wild-type (WT) counterparts did not. The transgenic plants that flowered exhibited no altered morphological features, except the lack of thorns characteristic of juvenile WT plants. The most precocious T0 line, 'Duncan' grapefruit (Dun134-3) expressing the CsAP1 gene, flowered and fruited when it was 4.5 years old and the T1 siblings from this line flowered and fruited when they were just over 18 months old. In contrast, T1 seedlings from three lines of 'Duncan' grapefruit expressing the CsLFY gene flowered within 3 months after germination, but were unable to support fruit development. Transcript levels of corresponding transgenes in leaves were not correlated with earliness of flowering. To further study the activity of AtRD29A, leaves from three 'Carrizo' citrange (C. sinensis × Poncirus trifoliata) rootstock seedlings transformed with the green fluorescent protein (GFP) gene under regulation of the AtRD29A promoter were subjected to drought stress or well-watered conditions. Expression of GFP was not stress-dependent, consistent with the observation of flowering of CsAP1 and CsLFY transgenic plants under non-inductive conditions. Taken together, the results suggest that AtRD29A is constitutively expressed in a citrus background. Despite the loss of control over flowering time, transgenic citrus lines ectopically expressing C. sinensis AP1 or LFY genes under control of the A. thaliana RD29A promoter exhibit precocious flowering, fruit development and viable transgenic seed formation. These transformed lines can be useful tools to reduce the time between generations to accelerate breeding.

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.

Effect of Grafting on Viral Resistance of Non-transgenic Plum Scion Combined With Transgenic PPV-Resistant Rootstock

In stone fruit trees, resistance to Plum pox virus (PPV) can be achieved through the specific degradation of viral RNA by the mechanism of RNA interference (RNAi). Transgenic virus-resistant plants, however, raise serious biosafety concerns due to the insertion and expression of hairpin constructs that usually contain various selective foreign genes. Since a mature stone tree represents a combination of scion and rootstock, grafting commercial varieties onto transgenic virus-tolerant rootstocks is a possible approach to mitigate biosafety problems. The present study was aimed at answering the following question: To what extent are molecular RNAi silencing signals transmitted across graft junctions in transgrafted plum trees and how much does it affect PPV resistance in genetically modified (GM)/non-transgenic (NT) counterparts? Two combinations, NT:GM and GM:NT (scion:rootstock), were studied, with an emphasis on the first transgrafting scenario. Viral inoculation was carried out on either the scion or the rootstock. The interspecific rootstock "Elita" [(Prunus pumila L. × P. salicina Lindl.) × (P. cerasifera Ehrh.)] was combined with cv. "Startovaya" (Prunus domestica L.) as a scion. Transgenic plum lines of both cultivars were transformed with a PPV-coat protein (CP)-derived intron-separate hairpin-RNA construct and displayed substantial viral resistance. High-throughput sequence data of small RNA (sRNA) pools indicated that the accumulation of construct-specific small interfering RNA (siRNA) in transgenic plum rootstock reached over 2%. The elevated siRNA level enabled the resistance to PPV and blocked the movement of the virus through the GM tissues into the NT partner when the transgenic tissues were inoculated. At the same time, the mobile siRNA signal was not moved from the GM rootstock to the target NT tissue to a level sufficient to trigger silencing of PPV transcripts and provide reliable viral resistance. The lack of mobility of transgene-derived siRNA molecules was accompanied by the transfer of various endogenous rootstock-specific sRNAs into the NT scion, indicating the exceptional transitivity failure of the studied RNAi signal. The results presented here indicate that transgrafting in woody fruit trees remains an unpredictable practice and needs further in-depth examination to deliver molecular silencing signals.

Isolation of novel citrus and plum fruit promoters and their functional characterization for fruit biotechnology

Background

Promoters that confer expression in fruit tissues are important tools for genetic engineering of fruit quality traits, yet few fruit-specific promoters have been identified, particularly for citrus fruit development.

Results

In this study, we report five citrus fruit-specific/preferential promoters for genetic engineering. Additionally, we have characterized a novel fruit-preferential promoter from plum. Genes specifically expressed in fruit tissues were selected and their isolated promoter regions were fused with the GUSPlus reporter gene for evaluation in transgenic plants. Stable transformation in Micro-Tom tomato demonstrated that the candidate promoter regions exhibit differing levels of expression and with varying degrees of fruit specificity.

Conclusions

Among the five candidate citrus promoters characterized in this study, the CitSEP promoter showed a fruit-specific expression pattern, while the CitWAX and CitJuSac promoters exhibited high fruit-preferential expression with strong activity in the fruit, weak activity in floral tissues and low or undetectable activity in other tissues. The CitVO1, CitUNK and PamMybA promoters, while exhibiting strong fruit-preferential expression, also showed consistent weak but detectable activity in leaves and other vegetative tissues. Use of these fruit specific/preferential promoters for genetic engineering can help with precise expression of beneficial genes and help with accurate prediction of the activity of new genes in host fruit plants.

A constitutive and drought-responsive mRNA undergoes long-distance transport in pear (Pyrus betulaefolia) phloem

Pear is one of the most commercially important fruit trees worldwide and is widely cultivated in temperate zones. Drought stress can greatly limit pear fruit yield and quality. Pyrus betulaefolia Bunge, a drought-resistant pear rootstock that is commonly used in northern China, confers favourable characteristics to pear scions, allowing them to respond rapidly to drought stress via the transport of macromolecules such as phloem-mobile mRNAs. How drought-responsive mRNAs function as phloem-mobile signals remains unknown, however. Here, we used RNA sequencing (RNA-seq) combined with SNP analysis to identify mobile mRNAs in P. betulaefolia. We focused on mobile mRNAs that respond to drought stress and found that the abundance of a novel mRNA named PbDRM (P. betulaefoliaDROUGHT-RESPONSIVE MOBILE GENE) significantly increased in several different scion cultivars when they were grafted onto P. betulaefolia rootstock under drought conditions. In addition, downregulating PbDRM by virus-induced gene silencing (VIGS) increased the drought sensitivity of P. betulaefolia. CAPS RT-PCR analysis confirmed that PbDRM mRNA moves from rootstock to scion in micrografting systems. Therefore, PbDRM mRNA acts as a phloem-mobile signal in pear under drought stress.

Structure and Expression of Bud Dormancy-Associated MADS-Box Genes (DAM) in European Plum

Bud dormancy in temperate perennials ensures the survival of growing meristems under the harsh environmental conditions of autumn and winter, and facilitates an optimal growth and development resumption in the spring. Although the molecular pathways controlling the dormancy process are still unclear, DORMANCY-ASSOCIATED MADS-BOX genes (DAM) have emerged as key regulators of the dormancy cycle in different species. In the present study, we have characterized the orthologs of DAM genes in European plum (Prunus domestica L.). Their expression patterns together with sequence similarities are consistent with a role of PdoDAMs in dormancy maintenance mechanisms in European plum. Furthermore, other genes related to dormancy, flowering, and stress response have been identified in order to obtain a molecular framework of these three different processes taking place within the dormant flower bud in this species. This research provides a set of candidate genes to be genetically modified in future research, in order to better understand dormancy regulation in perennial species.

Do Grapholita funebrana Infestation Rely on Specific Plum Fruit Features?

The effective control of the plum fruit moth, Grapholita funebrana (Lepidoptera: Tortricidae) still represents a difficult challenge for organic plum farming. Little information is available on the susceptibility of plum cultivars to this moth pest. We investigated the roles of several fruit parameters (i.e., shape, volume, hardness, fruit colour, and physiochemical properties) on the susceptibility of four different plum cultivars (Angeleno, Friar, President and Stanley) to G. funebrana attack. Field data demonstrated the importance of some fruit parameters (i.e., elongation index, sugar degree, titratable acidity, cover colour percentage) on susceptibility to G. funebrana infestation. Under laboratory conditions, colour and shape had a significant role in determining the time spent on false fruits, i.e., female moths preferred yellow and rounded fruits over elongated red or green fruits. Angeleno (yellow and rounded fruits) and Stanley (green and elongated fruits) were the most and least susceptible cultivars, respectively. Overall, this study adds useful knowledge about plum cultivar susceptibility to G. funebrana. Information reported here may be useful to improve integrated pest management strategies both in conventional and organic orchards because the use of less susceptible cultivars may reduce insecticidal treatments, limiting the development of resistance in target insects and the harmful side effects on beneficial species.

Agrobacterium-Mediated Transformation of Russian Commercial Plum cv. "Startovaya" (Prunus domestica L.) With Virus-Derived Hairpin RNA Construct Confers Durable Resistance to PPV Infection in Mature Plants

In modern horticulture Plum pox virus (PPV) imposes serious threats to commercial plantations of a wide range of fruit species belonging to genera Prunus. Given the lack of natural genetic resources, which display reliable resistance to PPV infection, there has been considerable interest in using genetic engineering methods for targeted genome modification of stone fruit trees to control Sharka disease caused by PPV. Among the many virus defense mechanisms, RNA interference is shown to be the most promising transgenic disease-control strategy in plant biotechnology. The present study describes the production of transgenic PPV resistant European plum "Startovaya" (P. domestica L.) through the Agrobacterium-mediated transformation of in vitro leaf explants. Due to organogenesis from leaves, the established protocol allows the genetic engineering of the plum genome without losing clonal fidelity of original cultivar. Seven independent transgenic plum lines containing the self-complementary fragments of PPV-CP gene sequence separated by a PDK intron were generated using hpt as a selective gene and uidA as a reporter gene. The transformation was verified through the histochemical staining for β-glucuronidase activity, PCR amplification of appropriate vector products from isolated genomic DNA and Southern blot analysis of hairpin PPV-CP gene fragments. To clarify the virus resistance, plum buds infected by PPV-M strain were grafted onto 1-year-old transgenic plants, which further were grown into mature trees in the greenhouse. As evaluated by RT-PCR, DAS-ELISA, Western blot, ImmunoStrip test, and visual observations, GM plum trees remained uninfected over 9 years. Infected branches that developed from grafted buds displayed obvious symptoms of Sharka disease over the years and maintained the high level of virus accumulation, whereby host transgenic trees had been constantly challenged with the pathogen. Since the virus was unable to spread to transgenic tissues, the stable expression of PPV-derived gene construct encoding intron-spliced hairpin RNAs provided a highly effective protection of plum trees against permanent viral infection. At the same time, this observation indicates the lack of the systemic spread of resistance from GM tissues to an infected plum graft even after years of joint growth.

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.

Plant Cellular and Molecular Biotechnology: Following Mariotti's Steps

This review is dedicated to the memory of Prof. Domenico Mariotti, who significantly contributed to establishing the Italian research community in Agricultural Genetics and carried out the first experiments of Agrobacterium-mediated plant genetic transformation and regeneration in Italy during the 1980s. Following his scientific interests as guiding principles, this review summarizes the recent advances obtained in plant biotechnology and fundamental research aiming to: (i) Exploit in vitro plant cell and tissue cultures to induce genetic variability and to produce useful metabolites; (ii) gain new insights into the biochemical function of Agrobacterium rhizogenes rol genes and their application to metabolite production, fruit tree transformation, and reverse genetics; (iii) improve genetic transformation in legume species, most of them recalcitrant to regeneration; (iv) untangle the potential of KNOTTED1-like homeobox (KNOX) transcription factors in plant morphogenesis as key regulators of hormonal homeostasis; and (v) elucidate the molecular mechanisms of the transition from juvenility to the adult phase in Prunus tree species.