Vascular-specific expression of GUS and GFP reporter genes in transgenic grapevine (Vitis vinifera L. cv. Albariño) conferred by the EgCCR promoter of Eucalyptus gunnii

Harnessing tissue-specific genome editing in plants through CRISPR/Cas system: current state and future prospects

In a nutshell, tissue-specific CRISPR/Cas genome editing is the most promising approach for crop improvement which can bypass the hurdle associated with constitutive GE such as off target and pleotropic effects for targeted crop improvement. CRISPR/Cas is a powerful genome-editing tool with a wide range of applications for the genetic improvement of crops. However, the constitutive genome editing of vital genes is often associated with pleiotropic effects on other genes, needless metabolic burden, or interference in the cellular machinery. Tissue-specific genome editing (TSGE), on the other hand, enables researchers to study those genes in specific cells, tissues, or organs without disturbing neighboring groups of cells. Until recently, there was only limited proof of the TSGE concept, where the CRISPR-TSKO tool was successfully used in Arabidopsis, tomato, and cotton, laying a solid foundation for crop improvement. In this review, we have laid out valuable insights into the concept and application of TSGE on relatively unexplored areas such as grain trait improvement under favorable or unfavorable conditions. We also enlisted some of the prominent tissue-specific promoters and described the procedure of their isolation with several TSGE promoter expression systems in detail. Moreover, we highlighted potential negative regulatory genes that could be targeted through TSGE using tissue-specific promoters. In a nutshell, tissue-specific CRISPR/Cas genome editing is the most promising approach for crop improvement which can bypass the hurdle associated with constitutive GE such as off target and pleotropic effects for targeted crop improvement.

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.

Transgenic sweetpotato plants overexpressing tocopherol cyclase display enhanced α-tocopherol content and abiotic stress tolerance

Oxidative stress caused by reactive oxygen species (ROS) under various environmental stresses significantly reduces plant productivity. Tocopherols (collectively known as vitamin E) are a group of lipophilic antioxidants that protect cellular components against oxidative stress. Previously, we isolated five tocopherol biosynthesis genes from sweetpotato (Ipomoea batatas [L.] Lam) plants, including tocopherol cyclase (IbTC). In this study, we generated transgenic sweetpotato plants overexpressing IbTC under the control of cauliflower mosaic virus (CaMV) 35S promoter (referred to as TC plants) via Agrobacterium-mediated transformation to understand the function of IbTC in sweetpotato. Three transgenic lines (TC2, TC9, and TC11) with high transcript levels of IbTC were selected for further characterization. High performance liquid chromatography (HPLC) analysis revealed that α-tocopherol was the most predominant form of tocopherol in sweetpotato tissues. The content of α-tocopherol was 1.6-3.3-fold higher in TC leaves than in non-transgenic (NT) leaves. No significant difference was observed in the tocopherol content of storage roots between TC and NT plants. Additionally, compared with NT plants, TC plants showed enhanced tolerance to multiple environmental stresses, including salt, drought, and oxidative stresses, and showed consistently higher levels of photosystem II activity and chlorophyll content, indicating abiotic stress tolerance. These results suggest IbTC as a strong candidate gene for the development of sweetpotato cultivars with increased α-tocopherol levels and enhanced abiotic stress tolerance.

Overexpression of a Wheat Aquaporin Gene, TdPIP2;1, Enhances Salt and Drought Tolerance in Transgenic Durum Wheat cv. Maali

In this study, we generated transgenic durum wheat cv. Maali overexpressing the wheat plasma membrane aquaporin TdPIP2;1 gene under the control of PrTdPIP2;1 promoter or under the constitutive PrCaMV35S promoter. Histochemical analysis of the fusion PrTdPIP2;1::TdPIP2;1::GusA in wheat plants showed that the β-glucuronidase (GUS) activity was detected in the leaves, stems and roots of stably transformed wheat T3 plants. Our results showed that transgenic wheat lines overexpressing the TdPIP2;1 gene exhibited improved germination rates and biomass production and retained low Na+ and high K+ concentrations in their shoots under high salt and osmotic stress conditions. In a long-term study under greenhouse conditions on salt or drought stress, transgenic TdPIP2;1 lines produced filled grains, whereas wild-type (WT) plants either died at the vegetative stage under salt stress or showed drastically reduced grain filling under drought stress. Performing real time RT-PCR experiments on wheat plants transformed with the fusion PrTdPIP2;1::GusA, we showed an increase in the accumulation of GusA transcripts in the roots of plants challenged with salt and drought stress. Study of the antioxidant defence system in transgenic wheat TdPIP2;1 lines showed that these lines induced the antioxidative enzymes Catalase (CAT) and Superoxide dismutase (SOD) activities more efficiently than the WT plants, which is associated with lower malondialdehyde and hydrogen peroxide contents. Taken together, these results indicate the high potential of the TdPIP2;1 gene for reducing water evaporation from leaves (water loss) in response to water deficit through the lowering of transpiration per unit leaf area (stomatal conductance) and engineering effective drought and salt tolerance in transgenic TdPIP2;1 lines.

Growth modulation effects of CBM2a under the control of AtEXP4 and CaMV35S promoters in Arabidopsis thaliana, Nicotiana tabacum and Eucalyptus camaldulensis

The expression of cell-wall-targeted Carbohydrate Binding Modules (CBMs) can alter cell wall properties and modulate growth and development in plants such as tobacco and potato. CBM2a identified in xylanase 10A from Cellulomonas fimi is of particular interest for its ability to bind crystalline cellulose. However, its potential for promoting plant growth has not been explored. In this work, we tested the ability of CBM2a to promote growth when expressed using both CaMV35S and a vascular tissue-specific promoter derived from Arabidopsis expansin4 (AtEXP4) in three plant species: Arabidopsis, Nicotiana tabacum and Eucalyptus camaldulensis. In Arabidopsis, the expression of AtEXP4pro:CBM2a showed trends for growth promoting effects including the increase of root and hypocotyl lengths and the enlargements of the vascular xylem area, fiber cells and vessel cells. However, in N. tabacum, the expression of CBM2a under the control of either CaMV35S or AtEXP4 promoter resulted in subtle changes in the plant growth, and the thickness of secondary xylem and vessel and fiber cell sizes were generally reduced in the transgenic lines with AtEXP4pro:CBM2a. In Eucalyptus, while transgenics expressing CaMV35S:CBM2a showed very subtle changes compared to wild type, those transgenics with AtEXP4pro:CBM2a showed increases in plant height, enlargement of xylem areas and xylem fiber and vessel cells. These data provide comparative effects of expressing CBM2a protein in different plant species, and this finding can be applied for plant biomass improvement.

Eucalyptus hairy roots, a fast, efficient and versatile tool to explore function and expression of genes involved in wood formation

Eucalyptus are of tremendous economic importance being the most planted hardwoods worldwide for pulp and paper, timber and bioenergy. The recent release of the Eucalyptus grandis genome sequence pointed out many new candidate genes potentially involved in secondary growth, wood formation or lineage-specific biosynthetic pathways. Their functional characterization is, however, hindered by the tedious, time-consuming and inefficient transformation systems available hitherto for eucalypts. To overcome this limitation, we developed a fast, reliable and efficient protocol to obtain and easily detect co-transformed E. grandis hairy roots using fluorescent markers, with an average efficiency of 62%. We set up conditions both to cultivate excised roots in vitro and to harden composite plants and verified that hairy root morphology and vascular system anatomy were similar to wild-type ones. We further demonstrated that co-transformed hairy roots are suitable for medium-throughput functional studies enabling, for instance, protein subcellular localization, gene expression patterns through RT-qPCR and promoter expression, as well as the modulation of endogenous gene expression. Down-regulation of the Eucalyptus cinnamoyl-CoA reductase1 (EgCCR1) gene, encoding a key enzyme in lignin biosynthesis, led to transgenic roots with reduced lignin levels and thinner cell walls. This gene was used as a proof of concept to demonstrate that the function of genes involved in secondary cell wall biosynthesis and wood formation can be elucidated in transgenic hairy roots using histochemical, transcriptomic and biochemical approaches. The method described here is timely because it will accelerate gene mining of the genome for both basic research and industry purposes.

Promoter isolation and characterization of GhAO-like1, a Gossypium hirsutum gene similar to multicopper oxidases that is highly expressed in reproductive organs

Cotton is one of the most economically important cultivated crops. It is the major source of natural fiber for the textile industry and an important target for genetic modification for both biotic stress and herbicide tolerance. Therefore, the characterization of genes and regulatory regions that might be useful for genetic transformation is indispensable. The isolation and characterization of new regulatory regions is of great importance to drive transgene expression in genetically modified crops. One of the major drawbacks in cotton production is pest damage; therefore, the most promising, cost-effective, and sustainable method for pest control is the development of genetically resistant cotton lines. Considering this scenario, our group isolated and characterized the promoter region of a MCO (multicopper oxidase) from Gossypium hirsutum, named GhAO-like1 (ascorbate oxidase-like1). The quantitative expression, together with the in vivo characterization of the promoter region reveals that GhAO-like1 has a flower- and fruit-specific expression pattern. The GUS activity is mainly observed in stamens, as expected considering that the GhAO-like1 regulatory sequence is enriched in cis elements, which have been characterized as a target of reproductive tissue specific transcription factors. Both histological and quantitative analyses in Arabidopsis thaliana have confirmed flower (mainly in stamens) and fruit expression of GhAO-like1. In the present paper, we isolated and characterized both in silico and in vivo the promoter region of the GhAO-like1 gene. The regulatory region of GhAO-like1 might be useful to confer tissue-specific expression in genetically modified plants.

Precision breeding of grapevine (Vitis vinifera L.) for improved traits

This review provides an overview of recent technological advancements that enable precision breeding to genetically improve elite cultivars of grapevine (Vitis vinifera L.). Precision breeding, previously termed "cisgenic" or "intragenic" genetic improvement, necessitates a better understanding and use of genomic resources now becoming accessible. Although it is now a relatively simple task to identify genetic elements and genes from numerous "omics" databases, the control of major agronomic and enological traits often involves the currently unknown participation of many genes and regulatory machineries. In addition, genetic evolution has left numerous vestigial genes and sequences without tangible functions. Thus, it is critical to functionally test each of these genetic entities to determine their real-world functionality or contribution to trait attributes. Toward this goal, several diverse techniques now are in place, including cell culture systems to allow efficient plant regeneration, advanced gene insertion techniques, and, very recently, resources for genomic analyses. Currently, these techniques are being used for high-throughput expression analysis of a wide range of grapevine-derived promoters and disease-related genes. It is envisioned that future research efforts will be extended to the study of promoters and genes functioning to enhance other important traits, such as fruit quality and vigor.

Functional analysis of the durum wheat gene TdPIP2;1 and its promoter region in response to abiotic stress in rice

In a previous work, we demonstrated that expression of TdPIP2;1 in Xenopus oocytes resulted in an increase in Pf compared to water injected oocytes. Phenotypic analyses of transgenic tobacco plants expressing TdPIP2;1 generated a tolerance phenotype towards drought and salinity stresses. To elucidate its stress tolerance mechanism at the transcriptional level, we isolated and characterized the promoter region of the TdPIP2;1 gene. A 1060-bp genomic fragment upstream of the TdPIP2;1 translated sequence has been isolated, cloned, and designated as the proTdPIP2;1 promoter. Sequence analysis of proTdPIP2;1 revealed the presence of cis regulatory elements which could be required for abiotic stress responsiveness, for tissue-specific and vascular expression. The proTdPIP2;1 promoter was fused to the β-glucuronidase (gusA) gene and the resulting construct was transferred into rice (cv. Nipponbare). Histochemical analysis of proTdPIP2;1::Gus in rice plants revealed that the GUS activity was observed in leaves, stems and roots of stably transformed rice T3 plants. Histological sections prepared revealed accumulation of GUS products in phloem, xylem and in some cells adjacent to xylem. The transcripts were up-regulated by dehydration. Transgenic rice plants overexpressing proTdPIP2;1 in fusion with TdPIP2;1, showed enhanced drought tolerance, while wild type plants were more sensitive and exhibited symptoms of wilting and chlorosis. These findings suggest that expression of the TdPIP2;1 gene regulated by its own promoter achieves enhanced drought tolerance in rice.

Development of an intra-molecularly shuffled efficient chimeric plant promoter from plant infecting Mirabilis mosaic virus promoter sequence

We developed an efficient chimeric promoter, MUASMSCP, with enhanced activity and salicylic acid (SA)/abscisic acid (ABA) inducibility, incorporating the upstream activation sequence (UAS) of Mirabilis mosaic virus full-length transcript (MUAS, -297 to -38) to the 5' end of Mirabilis mosaic virus sub-genomic transcript (MSCP, -306 to -125) promoter-fragment containing the TATA element. We compared the transient activity of the MUASMSCP promoter in tobacco/Arabidopsis protoplasts and in whole plant (Petunia hybrida) with the same that obtained from CaMV35S and MUAS35SCP promoters individually. The MUASMSCP promoter showed 1.1 and 1.5 times stronger GUS-activities over that obtained from MUAS35SCP and CaMV35S promoters respectively, in tobacco (Xanthi Brad) protoplasts. In transgenic tobacco (Nicotiana tabacum, var. Samsun NN), the MUASMSCP promoter showed 1.1 and 2.2 times stronger activities than MUAS35SCP and CaMV35S(2) promoters respectively. We observed a fair correlation between MUASMSCP-, MUAS35SCP- and CaMV35S(2)-driven GUS activities with the corresponding uidA-mRNA level in transgenic plants. X-gluc staining of transgenic germinating seed-sections and whole seedlings also support above findings. Protein-extracts made from tobacco protoplasts expressing GFP and human-IL-24 genes driven individually by the MUASMSCP promoter showed enhanced expression of the reporters compared to that obtained from the CaMV35S promoter. Furthermore, MUASMSCP-driven protoplast-derived human IL-24 showed enhanced cell inhibitory activity in DU-145 prostate cancer cells compared to that obtained from the CaMV35S promoter. We propose chimeric MUASMSCP promoter developed in the study could be useful for strong constitutive expression of transgenes in both plant/animal cells and it may become an efficient substitute for CaMV35S/CaMV35S(2) promoter.

Development of vascular tissue and stress inducible hybrid-synthetic promoters through dof-1 motifs rearrangement

A Caulimovirus-based hybrid-promoter, EFCFS, was derived by fusing the distal region (-227 to -54, FUAS) of Figwort mosaic virus full-length transcript promoter (F20) with the core promoter (-151 to +12, FS3CP) domain of Figwort mosaic virus sub-genomic transcript promoter (FS3). The hybrid-promoter (EFCFS) showed enhanced activity compared to the CaMV35S, F20 and FS3 promoters; while it showed equivalent activity with that of the CAMV35S(2) promoter in both transient protoplast (Nicotiana tabacum cv. Xanthi Brad) and transgenic plants (Nicotiana tabacum; Samsun NN). Further, we have engineered the EFCFS promoter sequence by inserting additional copies of the stress-inducible 'AAAG' cis-motif (Dof-1) to generate a set of three hybrid-synthetic promoters namely; EFCFS-HS-1, EFCFS-HS-2 and EFCFS-HS-3-containing 10, 11 and 13 'AAAG' motif, respectively. Transgenic plants expressing these hybrid synthetic promoters coupled to the GUS reporter were developed and their transcriptional activities were compared with F20, FS3, 35S and 35S(2) promoters, respectively. The relative levels of uidA-mRNA accumulation in transgenic plants driven by above promoters individually were compared by qRT-PCR. Localization of GUS reporter activity in plant tissue was assayed by histochemical approach. CLSM-based study revealed that hybrid-synthetic promoters namely; EFCFS-HS-1, EFCFS-HS-2 and EFCFS-HS-3 showed enhanced activity in vascular tissue compared to the CaMV35S promoter. In the presence of abiotic stress elicitors, salicylic acid and jasmonic acid, the EFCFS-HS-1 promoters showed enhanced activity compared to the 35S promoter. Newly derived hybrid-synthetic promoter/s with enhanced activity and stress inducibility could become efficient tools for advancement of plant biotechnology.

Genetic transformation of fruit trees: current status and remaining challenges

Genetic transformation has emerged as a powerful tool for genetic improvement of fruit trees hindered by their reproductive biology and their high levels of heterozygosity. For years, genetic engineering of fruit trees has focussed principally on enhancing disease resistance (against viruses, fungi, and bacteria), although there are few examples of field cultivation and commercial application of these transgenic plants. In addition, over the years much work has been performed to enhance abiotic stress tolerance, to induce modifications of plant growth and habit, to produce marker-free transgenic plants and to improve fruit quality by modification of genes that are crucially important in the production of specific plant components. Recently, with the release of several genome sequences, studies of functional genomics are becoming increasingly important: by modification (overexpression or silencing) of genes involved in the production of specific plant components is possible to uncover regulatory mechanisms associated with the biosynthesis and catabolism of metabolites in plants. This review focuses on the main advances, in recent years, in genetic transformation of the most important species of fruit trees, devoting particular attention to functional genomics approaches and possible future challenges of genetic engineering for these species in the post-genomic era.