The expression of the Saccharomyces cerevisiae HAL1 gene increases salt tolerance in transgenic watermelon [Citrullus lanatus (Thunb.) Matsun. & Nakai.]

Approaches Involved in the Vegetable Crops Salt Stress Tolerance Improvement: Present Status and Way Ahead

Salt stress is one of the most important abiotic stresses as it persists throughout the plant life cycle. The productivity of crops is prominently affected by soil salinization due to faulty agricultural practices, increasing human activities, and natural processes. Approximately 10% of the total land area (950 Mha) and 50% of the total irrigated area (230 Mha) in the world are under salt stress. As a consequence, an annual loss of 12 billion US$ is estimated because of reduction in agriculture production inflicted by salt stress. The severity of salt stress will increase in the upcoming years with the increasing world population, and hence the forced use of poor-quality soil and irrigation water. Unfortunately, majority of the vegetable crops, such as bean, carrot, celery, eggplant, lettuce, muskmelon, okra, pea, pepper, potato, spinach, and tomato, have very low salinity threshold (ECt, which ranged from 1 to 2.5 dS m-1 in saturated soil). These crops used almost every part of the world and lakes' novel salt tolerance gene within their gene pool. Salt stress severely affects the yield and quality of these crops. To resolve this issue, novel genes governing salt tolerance under extreme salt stress were identified and transferred to the vegetable crops. The vegetable improvement for salt tolerance will require not only the yield influencing trait but also target those characters or traits that directly influence the salt stress to the crop developmental stage. Genetic engineering and grafting is the potential tool which can improve salt tolerance in vegetable crop regardless of species barriers. In the present review, an updated detail of the various physio-biochemical and molecular aspects involved in salt stress have been explored.

Highly efficient, genotype-independent transformation and gene editing in watermelon (Citrullus lanatus) using a chimeric ClGRF4-GIF1 gene

Efficient genetic transformation has the potential to advance research and breeding in watermelon (Citrullus lanatus), but regeneration from tissue culture remains challenging. Previous work showed that expressing a fusion of two interacting transcription factors, GROWTH-REGULATING FACTOR4 (GRF4) and GRF-INTERACTING FACTOR1 (GIF1), improved regeneration in wheat (Triticum aestivum). By overexpressing a chimeric fusion of ClGRF4 and ClGIF1, we achieved highly efficient transformation in watermelon. Mutating the mi396 microRNA target site in ClGRF further boosted the transformation efficiency up to 67.27% in a genotype-independent manner. ClGRF4-GIF1 can also be combined with clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) genome editing tools to achieve highly efficient gene editing in watermelon, which we used to successfully create diploid seedless watermelon. This research thus puts forward a powerful transformation tool for future watermelon research and breeding.

Transgenic crops for the agricultural improvement in Pakistan: a perspective of environmental stresses and the current status of genetically modified crops

Transgenic technologies have emerged as a powerful tool for crop improvement in terms of yield, quality, and quantity in many countries of the world. However, concerns also exist about the possible risks involved in transgenic crop cultivation. In this review, literature is analyzed to gauge the real intensity of the issues caused by environmental stresses in Pakistan. In addition, the research work on genetically modified organisms (GMOs) development and their performance is analyzed to serve as a guide for the scientists to help them select useful genes for crop transformation in Pakistan. The funding of GMOs research in Pakistan shows that it does not follow the global trend. We also present socio-economic impact of GM crops and political dimensions in the seed sector and the policies of the government. We envisage that this review provides guidelines for public and private sectors as well as the policy makers in Pakistan and in other countries that face similar environmental threats posed by the changing climate.

Phenotypic and genetic characterization of tomato mutants provides new insights into leaf development and its relationship to agronomic traits

BACKGROUND

Tomato mutants altered in leaf morphology are usually identified in the greenhouse, which demands considerable time and space and can only be performed in adequate periods. For a faster but equally reliable scrutiny method we addressed the screening in vitro of 971 T-DNA lines. Leaf development was evaluated in vitro in seedlings and shoot-derived axenic plants. New mutants were characterized in the greenhouse to establish the relationship between in vitro and in vivo leaf morphology, and to shed light on possible links between leaf development and agronomic traits, a promising field in which much remains to be discovered.

RESULTS

Following the screening in vitro of tomato T-DNA lines, putative mutants altered in leaf morphology were evaluated in the greenhouse. The comparison of results in both conditions indicated a general phenotypic correspondence, showing that in vitro culture is a reliable system for finding mutants altered in leaf development. Apart from providing homogeneous conditions, the main advantage of screening in vitro lies in the enormous time and space saving. Studies on the association between phenotype and nptII gene expression showed co-segregation in two lines (P > 99%). The use of an enhancer trap also allowed identifying gain-of-function mutants through reporter expression analysis. These studies suggested that genes altered in three other mutants were T-DNA tagged. New mutants putatively altered in brassinosteroid synthesis or perception, mutations determining multiple pleiotropic effects, lines affected in organ curvature, and the first tomato mutant with helical growth were discovered. Results also revealed new possible links between leaf development and agronomic traits, such as axillary branching, flower abscission, fruit development and fruit cracking. Furthermore, we found that the gene tagged in mutant 2635-MM encodes a Sterol 3-beta-glucosyltransferase. Expression analysis suggested that abnormal leaf development might be due to the lack-off-function of this gene.

CONCLUSION

In vitro culture is a quick, efficient and reliable tool for identifying tomato mutants altered in leaf morphology. The characterization of new mutants in vivo revealed new links between leaf development and some agronomic traits. Moreover, the possible implication of a gene encoding a Sterol 3-beta-glucosyltransferase in tomato leaf development is reported.

Efficient CRISPR/Cas9-based gene knockout in watermelon

CRISPR/Cas9 system can precisely edit genomic sequence and effectively create knockout mutations in T0 generation watermelon plants. Genome editing offers great advantage to reveal gene function and generate agronomically important mutations to crops. Recently, RNA-guided genome editing system using the type II clustered regularly interspaced short palindromic repeats (CRISPR)-associated protein 9 (Cas9) has been applied to several plant species, achieving successful targeted mutagenesis. Here, we report the genome of watermelon, an important fruit crop, can also be precisely edited by CRISPR/Cas9 system. ClPDS, phytoene desaturase in watermelon, was selected as the target gene because its mutant bears evident albino phenotype. CRISPR/Cas9 system performed genome editing, such as insertions or deletions at the expected position, in transfected watermelon protoplast cells. More importantly, all transgenic watermelon plants harbored ClPDS mutations and showed clear or mosaic albino phenotype, indicating that CRISPR/Cas9 system has technically 100% of genome editing efficiency in transgenic watermelon lines. Furthermore, there were very likely no off-target mutations, indicated by examining regions that were highly homologous to sgRNA sequences. Our results show that CRISPR/Cas9 system is a powerful tool to effectively create knockout mutations in watermelon.

Role of transgenic plants in agriculture and biopharming

At present, environmental degradation and the consistently growing population are two main problems on the planet earth. Fulfilling the needs of this growing population is quite difficult from the limited arable land available on the globe. Although there are legal, social and political barriers to the utilization of biotechnology, advances in this field have substantially improved agriculture and human life to a great extent. One of the vital tools of biotechnology is genetic engineering (GE) which is used to modify plants, animals and microorganisms according to desired needs. In fact, genetic engineering facilitates the transfer of desired characteristics into other plants which is not possible through conventional plant breeding. A variety of crops have been engineered for enhanced resistance to a multitude of stresses such as herbicides, insecticides, viruses and a combination of biotic and abiotic stresses in different crops including rice, mustard, maize, potato, tomato, etc. Apart from the use of GE in agriculture, it is being extensively employed to modify the plants for enhanced production of vaccines, hormones, etc. Vaccines against certain diseases are certainly available in the market, but most of them are very costly. Developing countries cannot afford the disease control through such cost-intensive vaccines. Alternatively, efforts are being made to produce edible vaccines which are cheap and have many advantages over the commercialized vaccines. Transgenic plants generated for this purpose are capable of expressing recombinant proteins including viral and bacterial antigens and antibodies. Common food plants like banana, tomato, rice, carrot, etc. have been used to produce vaccines against certain diseases like hepatitis B, cholera, HIV, etc. Thus, the up- and down-regulation of desired genes which are used for the modification of plants have a marked role in the improvement of genetic crops. In this review, we have comprehensively discussed the role of genetic engineering in generating transgenic lines/cultivars of different crops with improved nutrient quality, biofuel production, enhanced production of vaccines and antibodies, increased resistance against insects, herbicides, diseases and abiotic stresses as well as the safety measures for their commercialization.

An insight into functional genomics of transgenic lines of tomato cv Rio Grande harbouring yeast halotolerance genes

Tomato cv Rio Grande plants were transformed with yeast halotolerance genes (HAL I or HAL II) using pPM7HAL I or pJRM16HAL II, with p35GUSINT as control. Transformation efficiency varied in the three constructs, with highest transformation found with p35GUSINT. Final selection of the transgenic plants was made on the basis of PCR. Transgene integration and copy number were assessed by Southern hybridisation. The primary transformants were allowed to self-pollinate and the expected Mendelian ratios were studied in second-generation progeny. Five independent homozygous lines each of HAL I and HAL II, as well as the control, were characterised to study inter-transformant expression variability. The transformants showed considerable variability in expression of the respective genes, as shown by salt tolerance assays, chlorophyll content and peroxidase activity. The transgene expression in transgenic lines was analysed by semi-quantitative RT-PCR. In response to different salt concentrations, transgenic plants over-expressing HAL I and HAL II had significantly (α=0.05) better performance than the control This study presents the comparative responses of the three constructs under the same transformation conditions and suggests possible mechanisms governed by yeast HAL I and HAL II genes, which seem to work in a coordinated manner by relatively decreasing osmotic and oxidative shock at different rates. Our results suggest that the yeast HAL I increases K(+) /Na(+) selectivity and has a more functional role than HAL II in improving salt tolerance of the tomato cv Rio Grande grown in Pakistan.

Generation of transgenic watermelon resistant to Zucchini yellow mosaic virus and Papaya ringspot virus type W

Zucchini yellow mosaic virus (ZYMV) and Papaya ringspot virus type W (PRSV W) are major limiting factors for production of watermelon worldwide. For the effective control of these two viruses by transgenic resistance, an untranslatable chimeric construct containing truncated ZYMV coat protein (CP) and PRSV W CP genes was transferred to commercial watermelon cultivars by Agrobacterium-mediated transformation. Using our protocol, a total of 27 putative transgenic lines were obtained from three cultivars of 'Feeling' (23 lines), 'China baby' (3 lines), and 'Quality' (1 line). PCR and Southern blot analyses confirmed that the chimeric construct was incorporated into the genomic DNA of the transformants. Greenhouse evaluation of the selected ten transgenic lines of 'Feeling' cultivar revealed that two immune lines conferred complete resistance to ZYMV and PRSV W, from which virus accumulation were not detected by Western blotting 4 weeks after inoculation. The transgenic transcript was not detected, but small interfering RNA (siRNA) was readily detected from the two immune lines and T(1) progeny of line ZW 10 before inoculation, indicating that RNA-mediated post-transcriptional gene silencing (PTGS) is the underlying mechanism for the double-virus resistance. The segregation ratio of T(1) progeny of the immune line ZW10 indicated that the single inserted transgene is nuclearly inherited and associated with the phenotype of double-virus resistance as a dominant trait. The transgenic lines derived from the commercial watermelon cultivars have great potential for control of the two important viruses and can be implemented directly without further breeding.

Establishment of a transgenic hairy root system in wild and domesticated watermelon (Citrullus lanatus) for studying root vigor under drought

Root vigor is an important trait for the growth of terrestrial plants, especially in water-deficit environments. Although deserts plants are known for their highly developed root architecture, the molecular mechanism responsible for this trait has not been determined. Here we established an efficient protocol for the genetic manipulation of two varieties of watermelon plants: a desert-grown wild watermelon that shows vigorous root growth under drought, and a domesticated cultivar showing retardation of root growth under drought stress. Agrobacterium rhizogenes-mediated transgenic hairy roots were efficiently induced and selected from the hypocotyls of these plants. Transgenic GUS expression was detected in the roots by RT-PCR and histochemical GUS staining. Moreover, a liquid culture system for evaluating their root growth was also established. Interestingly, growth of the hairy roots derived from domesticated variety of watermelon strongly inhibited under high osmotic condition, whereas the hairy roots derived from wild variety of watermelon retained substantial growth rates under the stress condition. The new protocol presented here offers a powerful tool for the comparative study of the molecular mechanism underlying drought-induced root growth in desert plants.

Vegetables

The conscious promotion of health by an appropriate, balanced diet has become an important social request. Vegetable thereby possesses a special importance due to its high vitamin, mineral and dietary fibre content. Major progress has been made over the past few years in the transformation of vegetables. The expression of several genes has been inhibited by sense gene suppression, and new traits caused by new gene constructs are stably inherited. This chapter reviews advances in various traits such as disease resistance, abiotic stress tolerance, quality improvement, pharmaceutical and industrial application. Results are presented from most important vegetable families, like Solanaceae, Brassicaceae, Fabaceae, Cucurbitaceae, Asteraceae, Apiaceae, Chenopodiaceae and Liliaceae. Although many research trends in this report are positive, only a few transgenic vegetables have been released from confined into precommercial testing or into use.

The HAL1 function on Na+ homeostasis is maintained over time in salt-treated transgenic tomato plants, but the high reduction of Na+ in leaf is not associated with salt tolerance

To achieve a deeper knowledge on the function of HAL1 gene in tomato (Solanum lycopersicum) plants submitted to salt stress, in this study, we studied the growth and physiological responses to high salt stress of T3 transgenic plants (an azygous line without transgene and both homozygous and hemizygous lines for HAL1) proceeding from a primary transformant with a very high expression level of HAL1 gene. The homozygous plants for HAL1 gene did not increase their salt tolerance in spite of an earlier and higher reduction of the Na(+) accumulation in leaves, being moreover the Na(+) homeostasis maintained throughout the growth cycle. The greater ability of the homozygous line to regulate the Na(+) transport to the shoot to long term was even shown in low accumulation of Na(+) in fruits. By comparing the homozygous and hemizygous lines, a higher salt tolerance in the hemizygous line, with respect to the homozygous line, was observed on the basis of fruit yield. The Na(+) homeostasis and osmotic homeostasis were also different in homozygous and hemizygous lines. Indeed, the Na(+) accumulation rate in leaves was greater in hemizygous than in homozygous line after 35 days of 100 mM NaCl treatment and only at the end of growth cycle did the hemizygous line show leaf Na(+) levels similar to those found in the homozygous line. With respect to the osmotic homeostasis, the main difference between lines was the different contribution of inorganic and organic solutes to the leaf osmotic balance. Taken together, these results suggest that the greater Na(+) exclusion ability of the homozygous line overexpressing HAL1 induces a greater use of organic solutes for osmotic balance, which seems to have an energy cost and hence a growth penalty that reverts negatively on fruit yield.