Genetic manipulation of crop plants

A study on optimization of pat gene expression cassette for maize transformation

Phosphinothricin acetyltransferase gene (pat) is an important selectable marker and also a key herbicide trait gene in several commercial products. In maize, the transformation frequency (TF) using pat as a selectable marker is the lowest among the commonly used marker options including epsps, pmi or ppo. Low pat transformation efficiency can become a major bottleneck in our ability to efficiently produce large numbers of events, especially for large molecular stack vectors with multiple trait gene cassettes. The root cause of the lower efficiency of pat in maize is not well understood and it is possible that the causes are multifaceted, including maize genotype, pat marker cassette, trait gene combinations and selection system. In this work we have identified a new variant of pat gene through codon optimization that consistently produced a higher transformation frequency (> 2x) than an old version of the pat gene that has codons optimized for expression in dicot plants. The level of PAT protein in all 16 constructs was also found multifold higher (up to 40 fold) over that of the controls. All of the T0 low copy transgenic plants generated from the 16 different constructs showed excellent tolerance to ammonium glufosinate herbicide spray tests at 4x and 8x recommended field application rates (1x = 595 g active ingredient (ai)/hectare of ammonium glufosinate) in the greenhouse.

Developing dual herbicide tolerant transgenic rice plants for sustainable weed management

Herbicides are important constituents of modern integrated weed management system. However, the continuous use of a single herbicide leads to the frequent evolution of resistant weeds which further challenges their management. To overcome this situation, alternating use of multiple herbicides along with conventional weed-management practices is suitable and recommended. The development of multiple herbicide-tolerant crops is still in its infancy, and only a few crops with herbicide tolerance traits have been reported and commercialized. In this study, we developed transgenic rice plants that were tolerant to both bensulfuron methyl (BM) and glufosinate herbicides. The herbicide tolerant mutant variant of rice AHAS (Acetohydroxyacid synthase) was overexpressed along with codon optimized bacterial bar gene. The developed transgenic lines showed significant tolerance to both herbicides at various stages of plant development. The selected transgenic lines displayed an increased tolerance against 100 μM BM and 30 mg/L phosphinothricin during seed germination stage. Foliar applications further confirmed the dual tolerance to 300 μM BM and 2% basta herbicides without any significant growth and yield penalties. The development of dual-herbicide-tolerant transgenic plants adds further information to the knowledge of crop herbicide tolerance for sustainable weed management in modern agricultural system.

A High-Throughput Regeneration and Transformation Platform for Production of Genetically Modified Banana

Banana (Musa spp.) is an important staple food as well as cash crop in tropical and subtropical countries. Various bacterial, fungal, and viral diseases and pests such as nematodes are major constraints in its production and are currently destabilizing the banana production in sub-Saharan Africa. Genetic engineering is a complementary option used for incorporating useful traits in banana to bypass the long generation time, polyploidy, and sterility of most of the cultivated varieties. A robust transformation protocol for farmer preferred varieties is crucial for banana genomics and improvement. A robust and reproducible system for genetic transformation of banana using embryogenic cell suspensions (ECS) has been developed in this study. Two different types of explants (immature male flowers and multiple buds) were tested for their ability to develop ECS in several varieties of banana locally grown in Africa. ECS of banana varieties "Cavendish Williams" and "Gros Michel" were developed using multiple buds, whereas ECS of "Sukali Ndiizi" was developed using immature male flowers. Regeneration efficiency of ECS was about 20,000-50,000 plantlets per ml of settled cell volume (SCV) depending on variety. ECS of three different varieties were transformed through Agrobacterium-mediated transformation using gusA reporter gene and 20-70 independent transgenic events per ml SCV of ECS were regenerated on selective medium. The presence and integration of gusA gene in transgenic plants was confirmed by PCR, dot blot, and Southern blot analysis and expression by histochemical GUS assays. The robust transformation platform was successfully used to generate hundreds of transgenic lines with disease resistance. Such a platform will facilitate the transfer of technologies to national agricultural research systems (NARS) in Africa.

Advances in selectable marker genes for plant transformation

Plant transformation systems for creating transgenics require separate process for introducing cloned DNA into living plant cells. Identification or selection of those cells that have integrated DNA into appropriate plant genome is a vital step to regenerate fully developed plants from the transformed cells. Selectable marker genes are pivotal for the development of plant transformation technologies because marker genes allow researchers to identify or isolate the cells that are expressing the cloned DNA, to monitor and select the transformed progeny. As only a very small portion of cells are transformed in most experiments, the chances of recovering transgenic lines without selection are usually low. Since the selectable marker gene is expected to function in a range of cell types it is usually constructed as a chimeric gene using regulatory sequences that ensure constitutive expression throughout the plant. Advent of recombinant DNA technology and progress in plant molecular biology had led to a desire to introduce several genes into single transgenic plant line, necessitating the development of various types of selectable markers. This review article describes the developments made in the recent past on plant transformation systems using different selection methods adding a note on their importance as marker genes in transgenic crop plants.

Positive, negative and marker-free strategies for transgenic plant selection

In this review, the use of the most common selection marker genes on plant transformation and the effects of their respective selective agents are discussed. These genes could be divided in two categories according their mode of action: genes for positive and negative selection. The retention of the marker gene flow through chloroplast transformation is also discussed. Further, strategies to recover marker-free transgenic plants, involving multi-auto-transformation (MAT), co-transformation, site-specific recombination and intragenomic relocation of transgenes through transposable elements are reviewed.

Expression and inheritance of foreign genes in transgenic peanut plants generated byAgrobacterium-mediated transformation

To evaluate and characterize the stability of traits transferred viaAgrobacterium transformation, foreign gene expression must be examined in sexually derived progeny. The objective of this study was to analyze three transgenic peanut plants, 1-10, 12-1, and 17-1, for the inheritance and expression of their foreign genes. Segregation ratios for the introduced genes in T₂ plants gave either 100% or 3:1 expression of the β-glucuronidase (GUS) gene, demonstrating recovery of both homozygous and heterozygous T₁ plants. Fluorometric GUS assay in T₁ and T₂ generations of all three plants showed that the GUS gene was stably expressed in the progeny. DNA analyses showed 100% concordance between the presence of the foreign gene and enzyme activity. Our results demonstrate that transgenes in peanut introduced byAgrobacterium can be inherited in a Mendelian manner.

Forest tree biotechnology

The forest products industry has traditionally viewed trees as merely a raw, and more or less immutable, natural resource. However, unlike such inanimate resources as metallic ores, trees have the potential to be modified genetically, essentially transmuting lead into gold. Increasingly, modern alchemists are applying the tools of biotechnology in efforts to reduce the biological constraints on forest productivity. Several new methodologies being used to address problems in forest biology are described with respect to their potential impact on forest tree improvement. In addition to addressing problems inherent to the current use of trees for production of pulp and paper or solid wood products, genetic manipulation of trees brings with it the potential to create new industries based on the novel characteristics of transgenic trees, e.g. trees containing transgenes to detoxify specific pollutants could be used in the remediation of sites contaminated with hazardous wastes. Efforts to modify trees through biotechnology are in their infancy, and this review seeks to outline the underpinnings of what will undoubtedly be an area of increased emphasis in the next millennium.