Abstract
Glyphosate is the most widely used herbicide for its low cost and high efficiency. However, it is rarely applied directly in rice field due to its toxicity to rice. Therefore, glyphosate-tolerant rice can greatly decrease the cost of rice production and provide a more effective weed management strategy. Although, several approaches to develop transgenic rice with glyphosate tolerance have been reported, the agronomic performances of these plants have not been well evaluated, and the feasibility of commercial production has not been confirmed yet. Here, a novel glyphosate-tolerant gene cloned from the bacterium Isoptericola variabilis was identified, codon optimized (designated as I. variabilis-EPSPS*), and transferred into Zhonghua11, a widely used japonica rice cultivar. After systematic analysis of the transgene integration via PCR, Southern blot and flanking sequence isolation, three transgenic lines with only one intact I. variabilis-EPSPS* expression cassette integrated into intergenic regions were identified. Seed test results showed that the glyphosate tolerance of the transgenic rice was about 240 times that of wild type on plant medium. The glyphosate tolerance of transgenic rice lines was further evaluated based on comprehensive agronomic performances in the field with T3 and T5generations in a 2-year assay, which showed that they were rarely affected by glyphosate even when the dosage was 8400 g ha−1. To our knowledge, this is the first demonstration of the development of glyphosate-tolerant rice lines based on a comprehensive analysis of agronomic performances in the field. Taken together, the results suggest that the selected glyphosate-tolerant rice lines are highly tolerant to glyphosate and have the possibility of commercial release. I. variabilis-EPSPS* also can be a promising candidate gene in other species for developing glyphosate-tolerant crops.
Highlights
Rice is a major cereal crop and feeds more than half of the world population
To avoid the adverse effect of the high GC content on the transcription and translation of I. variabilis-enzyme 5enolpyruvylshikimate-3-phosphate synthase (EPSPS), the nucleotide sequence of I. variabilis-EPSPS was optimized without changing the final amino acid sequence, resulting in a decrease of the average GC content to 66.7%
ClassI EPSPSs from most plants and Gramnegative bacteria are considered to be sensitive to glyphosate, but with some amino acid mutations they become tolerant to glyphosate as demonstrated by many studies (Comai et al, 1983; Eschenburg et al, 2002; Zhou et al, 2006; Kahrizi et al, 2007; Funke et al, 2009; Tian et al, 2013, 2015)
Summary
Rice is a major cereal crop and feeds more than half of the world population. It is important to enhance rice production to meet the increasing demand for food. Rice production is currently faced with challenges, such as the reduction of arable land, the decrease of farming labor force, and the shortage of water resources (Van Nguyen and Ferrero, 2006). Due to these factors, there is a shift from traditional transplanting to direct seeding of rice in many areas (Farooq et al, 2011). In the direct seeding system, both the yield and quality of rice are severely affected by weeds (Rao et al, 2007; Akbar et al, 2011). Herbicide management requires less labor, and funding and is more efficient, which makes it the most widely adopted approach in rice cultivation (Akbar et al, 2011; Chauhan, 2013; Antralina et al, 2015; Chauhan et al, 2015)
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