Stacking herbicide detoxification and resistant genes improves glyphosate tolerance and reduces phytotoxicity in tobacco (Nicotiana tabacum L.) and rice (Oryza sativa L.)

Glyphosate residues retained in the growing meristematic tissues or in grains of glyphosate-resistant crops affect the plants physiological functions and crop yield. Removing glyphosate residues in the plants is desirable with no penalty on crop yield and quality. We report a new combination of scientific strategy to detoxify glyphosate that reduces the residual levels and improve crop resistance. The glyphosate detoxifying enzymes Aldo-keto reductase (AKR1) and mutated glycine oxidase (mGO) with different modes of action were co-expressed with modified EPSPS, which is insensitive to glyphosate in tobacco (Nicotiana tabacum L.) and rice (Oryza sativa L.). The transgenic tobacco plants expressing individual PsAKR1, mGO, CP4-EPSPS, combinations of PsAKR1:CP4EPSPS, PsAKR1:mGO, and multigene with PsAKR1: mGO: CP4EPSPS genes were developed. The bio-efficacy studies of in-vitro leaf regeneration on different concentrations of glyphosate, seedling bioassay, and spray on transgenic tobacco plants demonstrate that glyphosate detoxification with enhanced resistance. Comparative analysis of the transgenic tobacco plants reveals that double and multigene expressing transgenics had reduced accumulation of shikimic acid, glyphosate, and its primary residue AMPA, and increased levels of sarcosine were observed in all PsAKR1 expressing transgenics. The multigene expressing rice transgenics showed improved glyphosate resistance with yield maintenance. In summary, results suggest that stacking genes with two different detoxification mechanisms and insensitive EPSPS is a potential approach for developing glyphosate-resistant plants with less residual content.

Aldo-keto reductase-1 (AKR1) protect cellular enzymes from salt stress by detoxifying reactive cytotoxic compounds

Cytotoxic compounds like reactive carbonyl compounds such as methylglyoxal (MG), melandialdehyde (MDA), besides the ROS accumulate significantly at higher levels under salinity stress conditions and affect lipids and proteins that inhibit plant growth and productivity. The detoxification of these cytotoxic compounds by overexpression of NADPH-dependent Aldo-ketoreductase (AKR1) enzyme enhances the salinity stress tolerance in tobacco. The PsAKR1 overexpression plants showed higher survival and chlorophyll content and reduced MDA, H2O2, and MG levels under NaCl stress. The transgenic plants showed reduced levels of Na+ levels in both root and shoot due to reduced reactive carbonyl compounds (RCCs) and showed enhanced membrane stability resulted in higher root growth and biomass. The increased levels of antioxidant glutathione and enhanced activity of superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR) suggest AKR1 could protect these enzymes from the RCC induced protein carbonylation by detoxification process. The transgenics also showed higher activity of delta 1-pyrroline-5- carboxylate synthase (P5CS) enzyme resulted in increasedproline levels to maintain osmotic homeostasis. The results demonstrates that the AKR1 protects proteins or enzymes that are involved in scavenging of cytotoxic compounds by detoxifying RCCs generated under salinity stress.

Overexpression of EcbHLH57 Transcription Factor from Eleusine coracana L. in Tobacco Confers Tolerance to Salt, Oxidative and Drought Stress

Basic helix-loop-helix (bHLH) transcription factors constitute one of the largest families in plants and are known to be involved in various developmental processes and stress tolerance. We report the characterization of a stress responsive bHLH transcription factor from stress adapted species finger millet which is homologous to OsbHLH57 and designated as EcbHLH57. The full length sequence of EcbHLH57 consisted of 256 amino acids with a conserved bHLH domain followed by leucine repeats. In finger millet, EcbHLH57 transcripts were induced by ABA, NaCl, PEG, methyl viologen (MV) treatments and drought stress. Overexpression of EcbHLH57 in tobacco significantly increased the tolerance to salinity and drought stress with improved root growth. Transgenic plants showed higher photosynthetic rate and stomatal conductance under drought stress that resulted in higher biomass. Under long-term salinity stress, the transgenic plants accumulated higher seed weight/pod and pod number. The transgenic plants were also tolerant to oxidative stress and showed less accumulation of H202 and MDA levels. The overexpression of EcbHLH57 enhanced the expression of stress responsive genes such as LEA14, rd29A, rd29B, SOD, APX, ADH1, HSP70 and also PP2C and hence improved tolerance to diverse stresses.

Co-expression of AtbHLH17 and AtWRKY28 confers resistance to abiotic stress in Arabidopsis

Stress adaptation in plants involves altered expression of many genes through complex signaling pathways. To achieve the optimum expression of downstream functional genes, we expressed AtbHLH17 (AtAIB) and AtWRKY28 TFs which are known to be upregulated under drought and oxidative stress, respectively in Arabidopsis. Multigene expression cassette with these two TFs and reporter gene GUS was developed using modified gateway cloning strategy. The GUS assay and expression analysis of transgenes in transgenic plants confirmed the integration of multigene cassette. The transgenic lines exhibited enhanced tolerance to NaCl, Mannitol and oxidative stress. Under mannitol stress condition significantly higher root growth was observed in transgenics. Growth under stress and recovery growth was substantially superior in transgenics exposed to gradual long term desiccation stress conditions. We demonstrate the expression of several downstream target genes under various stress conditions. A few genes having either WRKY or bHLH cis elements in their promoter regions showed higher transcript levels than wild type. However, the genes which did not have either of the motifs did not differ in their expression levels in stress conditions compared to wild type plants. Hence co-expressing two or more TFs may result in upregulation of many downstream target genes and substantially improve the stress tolerance of the plants.

Co-expression of AtbHLH17 and AtWRKY28 confers resistance to abiotic stress in Arabidopsis

Stress adaptation in plants involves altered expression of many genes through complex signaling pathways. To achieve the optimum expression of downstream functional genes, we expressed AtbHLH17 (AtAIB) and AtWRKY28 TFs which are known to be upregulated under drought and oxidative stress, respectively in Arabidopsis. Multigene expression cassette with these two TFs and reporter gene GUS was developed using modified gateway cloning strategy. The GUS assay and expression analysis of transgenes in transgenic plants confirmed the integration of multigene cassette. The transgenic lines exhibited enhanced tolerance to NaCl, Mannitol and oxidative stress. Under mannitol stress condition significantly higher root growth was observed in transgenics. Growth under stress and recovery growth was substantially superior in transgenics exposed to gradual long term desiccation stress conditions. We demonstrate the expression of several downstream target genes under various stress conditions. A few genes having either WRKY or bHLH cis elements in their promoter regions showed higher transcript levels than wild type. However, the genes which did not have either of the motifs did not differ in their expression levels in stress conditions compared to wild type plants. Hence co-expressing two or more TFs may result in upregulation of many downstream target genes and substantially improve the stress tolerance of the plants.