Agrobacterium-Mediated Genetic Transformation of Grapevines: Transgenic Plants of Vitis rupestris SCHEELE and Buds of Vitis vinifera L

Establishment of an efficient callus transient transformation system for Vitis vinifera cv. 'Chardonnay'

Grape (Vitis L.), a highly valued fruit crop, poses significant challenges in genetic transformation and functional characterization of genes. Therefore, there is an urgent need for the development of a rapid and effective method for grape transformation and gene function identification. Here, we introduce a streamlined Agrobacterium-mediated transient transformation system for grape calli. Optimal conditions were established with a leaf-derived callus induction medium; chiefly B5 medium supplemented with 0.05 mg/L NAA, 0.5 mg/L 2,4-D, and 2.0 mg/L KT; and a callus proliferation medium (B5 medium supplemented with 0.5 mg/L NAA and 2.0 mg/L 6-BA), respectively. Notably, GUS enzyme activity peaked (352.96 ± 33.95 mol 4-MU/mg/min) by sonication with Agrobacterium tumefaciens EHA105 and 100 μM AS for 4 min, followed by vacuum infection for 5 min, and co-culture at 25 °C in the dark for 1 day using callus as explants at an optical density (OD600) of 0.8. VaCIPK18 gene was transiently transformed into calli, and transcripts of the gene (endogenous and exogenous) were detected at higher levels than in non-transformed calli (endogenous). Moreover, after 10 days of treatment at 4 °C or -4 °C, the callus net weight of transformed callus was significantly higher than that of the untransformed callus, indicating that the VaCIPK18-overexpressing grape callus could improve cold tolerance. Overall, we establish a simple but effective transient transformation approach for grape callus, which could serve as a useful tool for the rapid assessment of gene function in this important crop.

New Technologies and Strategies for Grapevine Breeding Through Genetic Transformation

Grapevine, as other woody perennials, has been considered a recalcitrant crop to produce transgenic plants. Since the production of transgenic and/or edited plants requires the ability to regenerate plants from transformed tissues, this step is often the biggest bottleneck in the process. The objective of this work is to review the state of the art technologies and strategies for the improvement of grapevine transformation and regeneration, focusing on three aspects: (i) problems associated with grapevine transformation; (ii) genes that promote grapevine regeneration; and (iii) vehicles for gene delivery. Concerning the first aspect, it is well documented that one of the main factors explaining the low success rate in obtaining transgenic plants is the regeneration process. After transgenic integration into receptor cells, tissue culture is required to regenerate transgenic seedlings from transformed cells. This process is time consuming and often requires the addition of environmentally damaging reagents (antibiotics and herbicides) to the culture medium to select transgenic plants. On the other hand, the expression of genes such as the so-called developmental regulators (DR), which induce specific development programs, can be used to avoid traditional tissue culture methods. The ectopic expression of specific combinations of DR in somatic cells has the potential to induce de novo meristems in diverse crops, including grapevine. Successful genome editing by de novo reprogramming of plant meristems in somatic tissues has been reported. Moreover, it has been shown that the expression of certain transcription factors can increase the regeneration efficiency in wheat, citrus, and rice. Finally, recent reports showed the use of nanoparticles, such as carbon dots (CDs), as an attractive alternative to Agrobacterium- and biolistic-mediated plant genetic transformation. In this way, the use of antibiotics in culture media is avoided, overcoming the loss of viability of plant tissues and accelerating the regeneration processes. It has been shown that CDs can act as a vehicle to transport plasmids to plant cells in transient transformation in several crops without negative impacts on photosynthesis or growth. Based on these advances, it is possible to combine these new available strategies and technologies to overcome the regeneration problems of species such as grapevine and other crops considered as recalcitrant.

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.

Grapevine (Vitis vinifera L.)

Grapevine (Vitis) is considered to be one of the major fruit crops in the world based on hectares cultivated and economic value. Grapes are used not only for wine but also for fresh fruit, dried fruit, and juice production. Wine is by far the major product of grapes, and the focus of this chapter is on wine grape cultivars. Grapevine cultivars of Vitis vinifera L. have a reputation for producing premium quality wines. These premium quality wines are produced from a small number of cultivars that enjoy a high level of consumer acceptance and are firmly entrenched in the market place because of varietal name branding and the association of certain wine styles and regions with specific cultivars. In light of this situation, grapevine improvement by a transgenic approach is attractive when compared to a classical breeding approach. The transfer of individual traits as single genes with a minimum disruption to the original genome would leave the traditional characteristics of the cultivar intact. However, a reliable transformation system is required for a successful transgenic approach to grapevine improvement. There are three criteria for achieving an efficient Agrobacterium-mediated transformation system: (1) the production of highly regenerative transformable tissue, (2) optimal cocultivation conditions for both grapevine tissue and Agrobacterium, and (3) an efficient selection regime for transgenic plant regeneration. In this chapter, we describe a grapevine transformation system that meets these criteria. We also describe a protocol for the production of transformed roots suitable for functional gene studies and for the production of semi-transgenic grafted plants.

Control of viruses infecting grapevine

Grapevine is a high value vegetatively propagated fruit crop that suffers from numerous viruses, including some that seriously affect the profitability of vineyards. Nowadays, 64 viruses belonging to different genera and families have been reported in grapevines and new virus species will likely be described in the future. Three viral diseases namely leafroll, rugose wood, and infectious degeneration are of major economic importance worldwide. The viruses associated with these diseases are transmitted by mealybugs, scale and soft scale insects, or dagger nematodes. Here, we review control measures of the major grapevine viral diseases. More specifically, emphasis is laid on (i) approaches for the production of clean stocks and propagative material through effective sanitation, robust diagnosis, as well as local and regional certification efforts, (ii) the management of vectors of viruses using cultural, biological, and chemical methods, and (iii) the production of resistant grapevines mainly through the application of genetic engineering. The benefits and limitations of the different control measures are discussed with regard to accomplishments and future research directions.

Actin marker lines in grapevine reveal a gatekeeper function of guard cells

Resistance to abiotic and biotic stress is a central topic for sustainable agriculture, especially in grapevine, one of the field crops with the highest economic output per acreage. As early cellular factors for plant defense, actin microfilaments (AF) are of high relevance. We therefore generated a transgenic actin marker line for grapevine by expressing a fusion protein between green fluorescent protein and the second actin-binding domain of Arabidopsis (Arabidopsis thaliana) fimbrin, AtFIM1. Based on this first cytoskeletal-marker line in grapevine, the response of AFs to phytopathogenic microorganisms could be followed in vivo. Upon inoculation with fluorescently labeled strains of phytopathogenic bacteria, actin responses were confined to the guard cells. In contrast, upon contact with zoospores of Plasmopara viticola, not only the guard cells, but also epidermal pavement cells, where no zoospores had attached responded with the formation of a perinuclear actin basket. Our data support the hypothesis that guard cells act as pacemakers of defense, dominating the responses of the remaining epidermal cells.

Citrus biotechnology: Achievements, limitations and future directions

Citrus is one of the most important commercial and nutritional fruit crops in the world, hence it needs to be improved to cater to the diverse needs of consumers and crop breeders. Genetic manipulation through conventional techniques in this genus is invariably a difficult task for plant breeders as it poses various biological limitations comprising long juvenile period, high heterozygosity, sexual incompatibility, nucellar polyembryony and large plant size that greatly hinder cultivar improvement. Hence, several attempts were made to improve Citrus sps. by using various in vitro techniques. Citrus sps are widely known for their recalcitrance to transformation and subsequent rooting, but constant research has led to the establishment of improved protocols to ensure the production of uniformly transformed plants, albeit with relatively low efficiency, depending upon the genotype. Genetic modification through Agrobacterium-mediated transformation has emerged as an important tool for introducing agronomically important genes into Citrus sps. Somatic hybridization has been applied to overcome self and cross-incompatibility barriers and generated inter-specific and inter-generic hybrids. Encouraging results have been achieved through transgenics for resistance against viruses and bacteria, thereby augmenting the yield and quality of the fruit. Now, when major transformation and regeneration protocols have sufficiently been standardized for important cultivars, ongoing citrus research focuses mainly on incorporating such genes in citrus genotypes that can combat different biotic and abiotic stresses. This review summarizes the advances made so far in Citrus biotechnology, and suggests some future directions of research in this fruit crop.

In vitro regeneration and Agrobacterium-mediated genetic transformation of Euonymus alatus

An in vitro plant regeneration method and an Agrobacterium tumefaciens-mediated genetic transformation protocol were developed for Euonymus alatus. More than 60% of cotyledon and 70% of hypocotyl sections from 10-day-old seedlings of E. alatus produced 2-4 shoots on woody plant medium (WPM) supplemented with 5.0 mg/l 6-benzylaminopurine (BA) plus 0.2 mg/l alpha-naphthalene acetic acid (NAA), and 77% of shoots produced roots on WPM medium with 0.3 mg/l NAA and 0.5 mg/l Indole-3-butyricacid (IBA). On infection with Agrobacterium tumefaciens strain EHA105 harboring a gusplus gene that contained a plant recognizable intron from the castor bean catalase gene to ensure plant-specific beta-glucuronidase (GUS) expression, 16% of cotyledon and 15% of hypocotyl explants produced transgenic shoots using kanamycin as a selection agent, and 67% of these shoots rooted. Stable insertion of T-DNA into the host genome was determined with organ- and tissue-specific expression of the gusplus gene and further confirmed with a PCR-based molecular analysis.