Identification and characterization of a phosphinothricin N-acetyltransferase from Enterobacter LSJC7

Phosphinothricin (PPT) is a widely used and non-selective herbicide. PPT-resistance genes, especially PPT N-acetyltransferase genes, have been used in the development of transgenic PPT-resistant crops. However, there are only a limited number of available PPT-resistance genes for use in plant biotechnology. In this study, we found that Enterobacter LSJC7 is highly resistant to PPT and can acetylate PPT to N-acetyl phosphinothricin (Ac-PPT). Furthermore, a novel PPT N-acetyltransferase gene, named LsarsN, was identified from LSJC7. When LsarsN was expressed in E. coli AW3110, it confered resistance to PPT. Ac-PPT was detected in both the culture medium and cells of AW3110 expressing the LsarsN-pET22b plasmid. The purified LsArsN protein also showed strong N-acetylation ability in vitro, and its enzymatic kinetic curve was fitted with the Michaelis-Mentan equation. Compared with wild-type LsArsN, both R72A and R74A mutants showed significantly lower PPT N-acetylation ability. In summary, our results systematically characterized LsArsN with strong ability for PPT N-acetylation, which lays the groundwork for future research into the use of this novel gene, LsarsN, to create PPT-resistant crops.

Nepetalactone-rich essential oil mitigates phosphinothricin-induced ammonium toxicity in Arabidopsis thaliana (L.) Heynh

Active ingredient of the commercial herbicide BASTA (B), phosphinothricin, acts as an inhibitor of glutamine synthetase (GS), a key enzyme in ammonium assimilation. The treatment with BASTA leads to an elevation of ammonium levels in plants and further to various physiological alterations, ammonium toxicity and lethality. Results of the present study emphasize the complexity underlying control mechanisms that determine BASTA interaction with essential oil (EO) from Nepeta rtanjensis (NrEO), bioherbicide inducing oxidative stress in target plants. Simultaneous application of NrEO and BASTA, two agents showing differential mode of action, suspends BASTA-induced ammonium toxicity in Arabidopsis thaliana plants. This is achieved through maintaining GS activity, which sustains a sub-toxic and/or sub-lethal ammonium concentration in tissues. As revealed by the present study, regulation of GS activity, as influenced by BASTA and NrEO, occurs at transcriptional, posttranscriptional, and/or posttranslational levels. Two genes encoding cytosolic GS, GLN1;1 and GLN1;3, are highlighted as the main isozymes in Arabidopsis shoots contributing to NrEO-induced overcoming of BASTA-generated ammonium toxicity. The effects of NrEO might be ascribed to its major component nepetalactone, but the contribution of minor EO components should not be neglected. Although of fundamental significance, the results of the present study suggest possible low efficiency of BASTA in plantations of medicinal/aromatic plants such as Nepeta species. Furthermore, these results highlight the possibility of using NrEO as a bioherbicide in BASTA-treated crop fields to mitigate the effect of BASTA residues in contaminated soils.

Reactive oxygen species trigger the fast action of glufosinate

Glufosinate is primarily toxic to plants due to a massive light-dependent generation of reactive oxygen species rather than ammonia accumulation or carbon assimilation inhibition. Glutamine synthetase (GS) plays a key role in plant nitrogen metabolism and photorespiration. Glufosinate (C₅H₁₂NO₄P) targets GS and causes catastrophic consequences leading to rapid plant cell death, and the causes for phytoxicity have been attributed to ammonia accumulation and carbon assimilation restriction. This study aimed to examine the biochemical and physiological consequences of GS inhibition to identify the actual cause for rapid phytotoxicity. Monocotyledonous and dicotyledonous species with different forms of carbon assimilation (C3 versus C4) were selected as model plants. Glufosinate sensitivity was proportional to the uptake of herbicide between species. Herbicide uptake also correlated with the level of GS inhibition and ammonia accumulation in planta even with all species having the same levels of enzyme sensitivity in vitro. Depletion of both glutamine and glutamate occurred in glufosinate-treated leaves; however, amino acid starvation would be expected to cause a slow plant response. Ammonia accumulation in response to GS inhibition, often reported as the driver of glufosinate phytotoxicity, occurred in all species, but did not correlate with either reductions in carbon assimilation or cell death. This is supported by the fact that plants can accumulate high levels of ammonia but show low inhibition of carbon assimilation and absence of phytotoxicity. Glufosinate-treated plants showed a massive light-dependent generation of reactive oxygen species, followed by malondialdehyde accumulation. Consequently, we propose that glufosinate is toxic to plants not because of ammonia accumulation nor carbon assimilation inhibition, but the production of reactive oxygen species driving the catastrophic lipid peroxidation of the cell membranes and rapid cell death.

Accumulation of PrLeg, a Perilla legumin protein in potato tuber results in enhanced level of sulphur-containing amino acids

Potato is the fourth staple food in the world, following rice, wheat, and maize, whereas tubers contain high quality of starch, relatively high amounts of vitamin C and many other important substances. It also contains relatively good quality of protein (about 3 to 6% of the dried weight) and patatin, and 11S globulin is a major storage protein with high level of lysine. However, tuber protein contains relatively low amounts of sulphur-containing amino acids, which may result in low nutritional value. Recently, we cloned a gene encoding PrLeg polypeptide, a seed storage protein from Perilla, which contains relatively higher levels of sulphur-containing amino acids. We transformed PrLeg cDNA into a potato plant to over-express under the direction of the tuber-specific promoter, patatin. Most of the transgenic lines identified through PCR and RT-PCR analyses were able to accumulate high amount of prLeg transcript in their tuber tissue, while very little or no transcript that were detected in their leaf tissues. The level of methionine content was elevated up to three-fold compared to non-transgenic parental line, without any significant changes in other amino acids, suggesting that further research is required to get a deeper insight into their nutritional value.

Isolation and Characterization of a Theta Glutathione S-transferase Gene from Panax ginseng Meyer

Plants have versatile detoxification systems to encounter the phytotoxicity of the wide range of natural and synthetic compounds present in the environment. Glutathione S-transferase (GST) is an enzyme that detoxifies natural and exogenous toxic compounds by conjugation with glutathione (GSH). Recently, several roles of GST giving stress tolerance in plants have demonstrated, but little is known about the role of ginseng GSTs. Therefore, this work aimed to provide further information on the GST gene present in Panax ginseng genome as well as its expression and function. A GST cDNA (PgGST) was isolated from P. ginseng cDNA library, and it showed the amino acid sequence similarity with theta type of GSTs. PgGST in ginseng plant was induced by exposure to metals, plant hormone, heavy metals, and high light irradiance. To improve the resistance against environmental stresses, full-length cDNA of PgGST was introduced into Nicotiana tabacum. Overexpression of PgGST led to twofold increase in GST-specific activity compared to the non-transgenic plants, and the GST overexpressed plant showed resistance against herbicide phosphinothricin. The results suggested that the PgGST isolated from ginseng might have a role in the protection mechanism against toxic materials such as heavy metals and herbicides.

Herbicide phosphinothricin causes direct stimulation hormesis

Herbicide phosphinothricin (PPT) inhibits glutamine synthetase (GS), a key enzyme in nitrogen assimilation, thus causing ammonia accumulation, glutamine depletion and eventually plant death. However, the growth response of Lotus corniculatus L. plants immersed in solutions with a broad range of PPT concentrations is biphasic, with pronounced stimulating effect on biomass production at concentrations ≤ 50 μM and growth inhibition at higher concentrations. The growth stimulation at low PPT concentrations is a result of activation of chloroplastic isoform GS2, while the growth suppression is caused by inhibition of both cytosolic GS1 and GS2 at higher PPT concentrations. Since the results are obtained in cell-free system (e.g. protein extracts), to which the principles of homeostasis are not applicable, this PPT effect is an unambiguous example of direct stimulation hormesis. A detailed molecular mechanism of concentration-dependent interaction of both PPT and a related GS inhibitor, methionine sulfoximine, with GS holoenzymes is proposed. The mechanism is in concurrence with all experimental and literature data.

Production of phosphinothricin-tolerant rice (Oryza sativa L.) through the application of phosphinothricin as growth regulator

 A novel procedure has been developed to produce rice (Oryza sativa L.) tolerant to the herbicide phosphinothricin (PPT) by means of in vitro selection. First, sublethal and lethal concentrations of PPT on 7-day-old seedlings were determined and morphogenetic events in response to the PPT treatment evaluated. Differentiation of 6-30 microshoots on 5-40% of the treated plant material was observed on a hormone-free culture medium supplemented with a sublethal concentration of PPT. We proved that PPT is morphogenetically active, similar to the action of many other herbicides, showing cytokinin-like effects in rice tissue culture. Fertile plants were grown from those microshoots having PPT tolerance under greenhouse conditions. To the best of our knowledge, this is the first report on the production of rice plants tolerant to this herbicide without genetic transformation. Since PPT is a competitive inhibitor of glutamine synthetase (GS), total GS activity in PPT-tolerant and PPT-sensitive plants was examined comprehensively in order to decide whether this enzyme has any role in PPT tolerance. An elevated GS activity was detected in PPT-tolerant plant material which could result in an elevated PPT tolerance at unchanged concentrations of the herbicide.

Glutamine synthetase from mesophyll and bundle sheath maize cells: isoenzyme complements and different sensitivities to phosphinothricin

 Anion-exchange FPLC has been used to resolve the isoforms of glutamine synthetase (GS, EC 6.3.1.2) from Zea mays mesophyll (MC) and bundle sheath cells (BSC). Two different isoforms were detected in both types of photosynthetic cells. The predominantly active isoform was GS1 (61%) in MC and GS2 (67%) in BSC. The relative contribution of GS1 and GS2 to the overall GS activity in BSC in maize here reported resembles the proportion described for most C3 plants. Differences among these isoforms in terms of their susceptibility to phosphinothricin (PPT), an analogue of glutamate and known inhibitor of GS, were found. The GS1 isoenzyme from MC was the most sensitive form, being inhibited by 50% at approximately 2.0 μM DL-PPT, whereas the GS2 from BSC presented the highest tolerance to the inhibitor (I₅₀=30 μM). The transferase-to-semibiosynthetic activity ratio for the MC isoforms, which was higher than the ratio for the BSC isoforms, and the differences shown by the isoforms in susceptibility to PPT predict important differences in the biochemical properties and regulation of GS isoenzymes. In this regard, the cytoplasmic isoenzymes, and especially the one in MC, due to its relatively high contribution to mesophyll cell GS activity, could play a vital role in nitrogen metabolism in maize.

An efficient particle bombardment system for the genetic transformation of asparagus (Asparagus officinalis L.)

The microprojectile bombardment method was used to transfer DNA into embryogenic callus of asparagus (Asparagus officcinalis L.) and to produce stably transformed asparagus plants. Embryogenic callus, derived from UC 157 and UC72 asparagus cultivars, was bombarded with tungsten particles coated with plasmid DNA that contained genes encoding hygromycin phosphotransferase, phosphinothricin acetyl transferase and β-glucuronidase. Putatively transformed calli were identified from the bombarded tissue after 4 months selection on 25 mg/L hygromycin B plus 4 mg/L phosphinothricin (PPT). By selecting embryogenic callus on hygromycin plus PPT the overall transformation and selection efficiencies were substantially improved over selection with hygromycin or PPT alone, where no transgenic clones were recovered. The transgenic nature of the selected material was demonstrated by GUS histochemical assays and Southern blot hybridization analysis. Transgenic asparagus plants were found to withstand the prescribed levels of the PPT-based herbicide BASTATM for weed control.