Production of Marker-Free Apple Plants Expressing the Supersweet Protein Gene Driven by Plant Promoter

Effect of Gene Silencing of Translation Initiation Factors eIF(iso)4G and eIF(iso)4E on Sour Cherry Rootstock Resistance to Sharka Disease

Sharka disease, caused by the Plum pox virus (PPV), is one of the most harmful, quarantine viral diseases that affect stone fruit crops. The absence of natural resistance to the virus in stone fruits has become a decisive factor for the use of genetic transformation methods to obtain stable forms. The eIF(iso)4G and eIF(iso)4E genes encode translation initiation factors used in the PPV life cycle. In the presented study, the effect of silencing these genes using the RNA interference method on the resistance of sour cherry rootstock 146-2 plants (Prunus pumila L. x Prunus tomentosa Thunb) to the sharka disease was studied. Two vectors have been created for the genetic transformation of plants, with self-complementary sequences of the eIF(iso)4G and eIF(iso)4E gene fragments. The hairpin expression cassette contains a strong promoter of the peach ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCo) gene, as well as an intron and terminator of the same gene. We used the pMF1 vector containing recombinase R and a codA-nptII gene which makes it possible to obtain intragenic marker-free plants. A successful genetic transformation was carried out by the AGL0 strain of A. tumefaciens. Whole leaves of shoots cultivated in vitro were used as a source of explants. Eight independent transgenic lines of rootstock 146-2 were obtained in experiments (sixlines with a hairpin to the eIF(iso)4G gene and two lines with a hairpin to the eIF(iso)4E gene). Their status was confirmed by the PCR and Southern blotting. The obtained plants were acclimatized in a greenhouse. The silencing of the eIF(iso)4G and eIF(iso)4E genes in transgenic plants was confirmed by the quantitative PCR. The presence of specific small interfering (si) RNAs was confirmed by the method of Northern blotting. Plants of all transgenic rootstock lines were infected with PPV by the method of grafting with infected buds. Resistance to the PPV infection of the obtained transgenic plants was carried out by using an enzyme immunoassay. The ELISA results showed that silencing the eIF(iso)4G gene did not lead to increased resistance while silencing the eIF(iso)4E factor gene led to increased resistance to the PPV, and the one line's plants showed no signs of infection for two years after infecting. The work demonstrates a (promising) approach in which the creation of stone cultures resistant to the plum pox virus can be achieved by suppressing the genes of translation initiation factors in clonal rootstocks.

Screening and identification of drought tolerance of spring soybean at seedling stage under climate change

Drought is one of the major abiotic stress factors limiting soybean growth and yield, and it frequently occur globally. Therefore, exploring resistant varieties from soybean germplasm is important under climate change. To screen drought resistant spring soybean varieties at seedling stage, pot experiment was used to detect the Survival percentage after drought stress of 60 soybean varieties at seedling stage, twice drought rehydration treatments on seedlings, to evaluate the drought tolerance of spring soybean. The results showed that at the seedling stage, seven varieties were considered drought tolerant, 17 varieties were considered drought sensitive, and 36 varieties were considered to be moderately drought tolerant. Based on this experiment, number 44 (heinong37), 48 (heinong44), 49 (heinong45), 52 (heinong48) is considered the best drought resistant, and number 3 (dongnong48), 4 (dongnong52), 27 (suinong25), 60 (heinong65) is the most sensitive. These varieties provide a reference for further study on drought tolerance and stress resistance gene screening of soybean at the molecular level. The selected soybean varieties can be planted in areas with suitable climates and frequent drought to meet the local soybean demand. In other regions, although cannot be directly grown, they can still be used as parents of selected varieties or as materials for gene screening and extraction, to assist crop breeding at the molecular level in response to increasingly severe drought stress problems under the current climate trends.

From Genome Sequencing to CRISPR-Based Genome Editing for Climate-Resilient Forest Trees

Due to the economic and ecological importance of forest trees, modern breeding and genetic manipulation of forest trees have become increasingly prevalent. The CRISPR-based technology provides a versatile, powerful, and widely accepted tool for analyzing gene function and precise genetic modification in virtually any species but remains largely unexplored in forest species. Rapidly accumulating genetic and genomic resources for forest trees enabled the identification of numerous genes and biological processes that are associated with important traits such as wood quality, drought, or pest resistance, facilitating the selection of suitable gene editing targets. Here, we introduce and discuss the latest progress, opportunities, and challenges of genome sequencing and editing for improving forest sustainability.

Methylglyoxal-glyoxalase system as a possible selection module for raising marker-safe plants in rice

Methylglyoxal (MG) is ubiquitously produced in all living organisms as a byproduct of glycolysis, higher levels of which are cytotoxic, leading to oxidative stress and apoptosis in the living systems. Though its generation is spontaneous but its detoxification involves glyoxalase pathway genes. Based on this understanding, the present study describes the possible role of MG as a novel non-antibiotic-based selection agent in rice. Further, by metabolizing MG, the glyoxalase pathway genes viz. glyoxalase I (GLYI) and glyoxalase II (GLYII), may serve as selection markers. Therefore, herein, transgenic rice harboring GLYI-GLYII genes (as selection markers) were developed and the effect of MG as a selection agent was assessed. The 3 mM MG concentration was observed as optimum for the selection of transformed calli, allowing efficient callus induction and proliferation along with high regeneration frequency (55 ± 2%) of the transgenic calli. Since the transformed calli exhibited constitutively higher activity of GLYI and GLYII enzymes compared to the wild type calli, the rise in MG levels was restricted even upon exogenous addition of MG during the selection process, resulting in efficient selection of the transformed calli. Therefore, MG-based selection method is a useful and efficient system for selection of transformed plants without significantly compromising the transformation efficiency. Further, this MG-based selection system is bio-safe and can pave way towards better public acceptance of transgenic plants.

A Strategy for the Production and Molecular Validation of Agrobacterium-Mediated Intragenic Octoploid Strawberry

Intragenesis is an all-native engineering technology for crop improvement. Using an intragenic strategy to bring genes from wild species to cultivated strawberry could expand the genetic variability. A robust regeneration protocol was developed for the strawberry cv. ‘Shanghai Angel’ by optimizing the dose of Thidiazuron and identifying the most suitable explants. The expression cassette was assembled with all DNA fragments from F. vesca, harboring a sugar transporter gene FvSTP8 driven by a fruit-specific FvKnox promoter. Transformed strawberry was developed through an Agrobacterium-mediated strategy without any selectable markers. Other than PCR selection, probe-based duplex droplet digital PCR (ddPCR) was performed to determine the T-DNA insert. Four independent transformed shoots were obtained with a maximum of 5.3% efficiency. Two lines were confirmed to be chimeras, while the other two were complete transformants with six and 11 copies of the intragene, respectively. The presence of a vector backbone beyond the T-DNA in these transformants indicated that intragenic strawberries were not obtained. The current work optimized the procedures for producing transformed strawberry without antibiotic selection, and accurately determined the insertion copies by ddPCR in the strawberry genome for the first time. These strategies might be promising for the engineering of ‘Shanghai Angel’ and other cultivars to improve agronomic traits.

Strategies to produce T-DNA free CRISPRed fruit trees via Agrobacterium tumefaciens stable gene transfer

Genome editing via CRISPR/Cas9 is a powerful technology, which has been widely applied to improve traits in cereals, vegetables and even fruit trees. For the delivery of CRISPR/Cas9 components into dicotyledonous plants, Agrobacterium tumefaciens mediated gene transfer is still the prevalent method, although editing is often accompanied by the integration of the bacterial T-DNA into the host genome. We assessed two approaches in order to achieve T-DNA excision from the plant genome, minimizing the extent of foreign DNA left behind. The first is based on the Flp/FRT system and the second on Cas9 and synthetic cleavage target sites (CTS) close to T-DNA borders, which are recognized by the sgRNA. Several grapevine and apple lines, transformed with a panel of CRISPR/SpCas9 binary vectors, were regenerated and characterized for T-DNA copy number and for the rate of targeted editing. As detected by an optimized NGS-based sequencing method, trimming at T-DNA borders occurred in 100% of the lines, impairing in most cases the excision. Another observation was the leakage activity of Cas9 which produced pierced and therefore non-functional CTS. Deletions of genomic DNA and presence of filler DNA were also noticed at the junctions between T-DNA and genomic DNA. This study proved that many factors must be considered for designing efficient binary vectors capable of minimizing the presence of exogenous DNA in CRISPRed fruit trees.

Insights Into Genetic and Molecular Elements for Transgenic Crop Development

Climate change and the exploration of new areas of cultivation have impacted the yields of several economically important crops worldwide. Both conventional plant breeding based on planned crosses between parents with specific traits and genetic engineering to develop new biotechnological tools (NBTs) have allowed the development of elite cultivars with new features of agronomic interest. The use of these NBTs in the search for agricultural solutions has gained prominence in recent years due to their rapid generation of elite cultivars that meet the needs of crop producers, and the efficiency of these NBTs is closely related to the optimization or best use of their elements. Currently, several genetic engineering techniques are used in synthetic biotechnology to successfully improve desirable traits or remove undesirable traits in crops. However, the features, drawbacks, and advantages of each technique are still not well understood, and thus, these methods have not been fully exploited. Here, we provide a brief overview of the plant genetic engineering platforms that have been used for proof of concept and agronomic trait improvement, review the major elements and processes of synthetic biotechnology, and, finally, present the major NBTs used to improve agronomic traits in socioeconomically important crops.

Production of selectable marker gene-free Cavendish banana (Musa spp.) using a steroid-inducible recombinase platform

Genetic improvement of commercially accepted banana cultivars is strongly reliant on the ability to introduce genes that encode important agro-traits such as disease resistance. In most cases this can only be achieved using a transgenic approach. Public and regulatory acceptance of these events would greatly increase with “clean” single copy integration events free of the selectable marker gene and extraneous vector backbone. This would also allow for the successive addition of new genes and traits as they become available. In this study, we used the pMarker Free 1 (pMF1) vector containing the green fluorescent protein (gfp) reporter gene to assess the effectiveness of steroid-inducible recombination and positive/negative dual selection to regenerate transgenic Cavendish banana plants that were potentially free of the selectable marker gene. By examining the interaction of two different Agrobacterium strains with two different cultivars of Cavendish banana, namely Williams and Grand Naine, we describe a transformation and regeneration strategy that successfully produced marker-free, single transgene copy, gfp-expressing events. The system will provide a useful means of serially improving banana into the future.

Functional characterization of a strong promoter of the early light-inducible protein gene from tomato

The tomato ELIP gene promoter is mainly active in the ripening fruit. Considering its high activity, the promoter could be used for molecular breeding of plants in the future. The ability to obtain new varieties of transgenic plants with economically valuable traits relies on a high level of target gene expression, which is largely controlled by a gene promoter. Hence, research aimed at finding and characterizing new tissue-specific promoters that direct gene expression in specific plant tissues or at certain developmental stages has become the most important field of plant biotechnology. Here, we cloned and characterized the promoter of the early light-inducible protein (ELIP) gene from tomato (Solanum lycopersicum cv. Yalf). ELIPs are produced in the presence of light and putatively function in the chloroplast-to-chromoplast conversion, playing a photorepairing role in the photosynthetic system. Analysis of the promoter sequence revealed multiple cis-acting elements related to light responsiveness, and other motifs involved in plant hormone response and circadian control. To determine the functionality of the promoter, seven 5'-deletion variants were fused with the β-glucuronidase (GUS) reporter gene and introduced into tomato. Histochemical analysis of transgenic tomato plants revealed different levels of GUS activity in most analyzed tissues, depending on the promoter fragment used. The intensity of staining was considerably higher in ripening fruits than in unripe and non-fruit tissues. Quantitative analysis indicated that the level of GUS activity with the longest (full-length) version of the ELIP promoter in ripened fruits was comparable to that in plants expressing the constitutive CaMV35S promoter. Further, the location of both negative and positive regulatory motifs was identified. The described ELIP promoter is a potential tool for various applications in plant biotechnology.