|
|
|
|||||||||||||||||||||||
|
Herbicide Tolerant Crops - Impacts on agriculture and environmentTable of Contents:
Download this article as pdf here:
Herbicide Tolerant Crops
( IntroductionCrop growing is not feasible without measures to control weeds. Weeds are
plants which compete with the planted crop for space, light water and
nutrients. They could furthermore cause difficulties during harvest or in
the harvested product. Methods to generate herbicide tolerant plantsIn recent years several approaches have proved practicable of
circumventing this problem of selectivity in crops. One way
is through biotechnology. It is now possible for instance
to transfer herbicide inactivating traits into important crop plants.
The responsible genes have been shown to be stably integrated and
inherited. Glufosinate Ammonium - a modern herbicideGlufosinate Ammonium is the ammonium salt of the amino acid Phosphinothricin which has been derived from the natural compound L-phosphinothricyl -L-alanine-L-alanine. This tripetide was obtained from Streptomyces viridochromogenes by Bayer et al (1972) in Germany and from Streptomyces hygroscopicus by Kondo et al (1973) in Japan and got the common name Bialaphos. The amino acid Phosphinothricin was found to be the biologically active moiety of the tripetide. It proved to have strong herbicidal efficacy on mono- and dicotyledonons plants and was patented by Hoechst AG (Rupp et al, 1977). Mode of herbicidal efficacyThe broad herbicidal activity of Glufosinate Ammonium is the result of a specific inhibition of glutamine synthetase. In plants this is an essential enzyme for the assimilation of primary ammonia as well as for the reassimilation of ammonia released by metabolic processes. The inhibition of glutamine synthetase causes increasing concentrations of ammonia in plant cells. Finally they reach a level which is phytotoxic and destroys the plant. Mechanism of Tolerance to Glufosinate AmmoniumThe Streptomyces species producing the tripeptide Bialaphos containing the
herbicidal active amino acid Phosphinothricin possess an enzyme - and thus
a gene - which acetylates the amino group of Phosphinothricin. It protects
the producer strain against intoxication by the own metabolite. Metabolism and ResiduesGlufosinate tolerant plantsThe metabolism of Glufosinate-Ammonium was tested in the following genetically modified crops: maize, soybean, rape, tomato. Due to the presence of the PAT gene in these plants the herbicidal active isomer (L-Glufosinate) is rapidly acetylated within as fast as it penetrates into the plant. The high initial residue level (residues in the transgenic plants normally consist of N-acetyl-L-Glufosinate, D-Glufosinate and with lesser amounts 3-methylphosphinico-propionic acid, Hoe061517) decreases when the plants are growing. From residue trials it is documented that at the silage or harvest stage no residues remain (below limit of quantification) in maize and oilseed rape. SoilIn soil the active substance Glufosinate-ammonium is rapidly degraded with half-life (DT-50) of 3-20 days and DT-90 values of 10-30 days. The new metabolite from the transgenic plants N-acetyl-L-Glufosinate is also rapidly degraded. The deacetylation (first degradation step) takes place within one day after which the degradation pathway and velocity is the same as for the parent substance Glufosinate-Ammonium. Due to this rapid degradation, the risk of leaching and groundwater contamination is minimized. In lysimeter studies neither Glufosinate nor its metabolites were detectable in the leachate. MammalsMammals excrete both Glufosinate-Ammonium and N-acetyl-Glufosinate very rapidly. More than 90% of the administered dose are excreted within the first 48-h intervall after admininstration. (90% of that amount via faeces and normally less than 10% with the urine after oral dose). N-acetyl-L-Glufosinate is stable in the animal: it passes the stomach of rats without deacetylation, in the urine only the N-acetyl compound is excreted. Deacetylation takes place in the faeces presumably by the intestinal bacteria. Depending on the initial dose between 1 and 10% are deactylated. This process is reversible. After dosage of Glufosinate-Ammonium, the acetylated compound was found in the rat faeces. Product safety and environmental effectsToxicological propertiesAcute, subchronic and chronic toxicity studies showed very low toxicity
of Glufosinate-Ammonium. As a manufacturing use product Glufosinate-Ammonium
possesses no toxic properties which would render it a dangerous substance as
defined in the Regulation on Hazardous Materials. Assessment of hazard to typical non-target organismsAssessments of possible ecological risks for the use of Glufosinate-Ammonium were conducted by Dorn et al (1992). They have shown that as far as can be known at present no hazard is to be expected neither to aquatic organisms as algae, waterfleas and fishes nor to terrestrial organisms as soil microorganisms, earthworms, honeybees, beneficial arthropods, birds and mammals. RegistrationGlufosinate Ammonium is registered as a non selective herbicide in all major countries of the world. It is known for its favorable toxicological and ecotoxicological profile. In spring 1995 Canadian authorities granted the first registration for use as a selective herbicide in Glufosinate tolerant oilseed rape. Safety Assessments of PAT Gene and ProteinSpecificity of the PAT ProteinIncubation of purified enzyme (PAT Protein) with 14C Glutamate did not result in acetylation of Glutamate. A 1000 fold higher concentration of Glutamate (or other proteingenic amino acids) could not outcompete 14C labelled Glufosinate as a substrate. The extremely high substrate specifity of the PAT protein indicates that no other proteinogenic amino acid can be acetylated. DegradationDegradation studies of the PAT gene in digestive fluids from pork, chicken
and cow showed that the DNA is completely degraded within 1 hour
(at 37°C and pH 1,5). Degradation studies of the PAT Protein in digestive fluids
from pork, chicken, cow and in simulated human gastric fluid resulted in an
immediate breakdown of the protein and its enzymatic activity within seconds. AllergenicityA thorough comparison of the PAT protein with other known protein sequences
was conducted. No homology to known allergenic or toxic protein could be
detected. Safety Assessment of transgene dispersalWith the advent of transgenic crops, the danger of gene introgression from
cultivated plant species into the natural flora is widely discussed. Herbicide
resistance genes are in this respect an ideal approach for ecological
monitoring. If the postulated gene flux from cultivated crops towards the wild
flora would occur it would be easily detectable. Herbicide tolerant
crops therefore can help to answer the questions:
It is evident that no general answer can be drawn on this topic. Therefore a species by species consideration is necessary. It is accepted that a transgene in maize cannot be transmitted via cross pollination to other plant species in most parts of the world with the exception of Southern Mexico and Central America where wild relatives occur. Soybean is also a safe crop in this respect. Only in Northern China wild relatives occur and the plant is a notorius self pollinator. Sugarbeets can cross freely with wild beets (Beta Vulgaris ssp maritima). This wild plant grows along the coasts of Western Europe and the Mediterranean bassin. In these areas outcrossing is posssible. Using transgenic beets as females or by avoiding seed production areas where wild beets occur, the outcrossing into wild beet populations can be avoided. In farmers fields sugar beets do normally not flower. The few flowering contaminants which arose either from weed beets or from crosses with wild beets in some seed production areas (Broomberg et al 1995) should be erased before seed setting anyway. Rapeseed is considered as a crop which interbreeds with
related wild species from the Brassicaceae family. Therefore in this crop the
probability for outcrossing is higher than in all other agricultural crops. If the rapeseed field will not be treated with the complementary herbicide and outcrossing into B. campestris can occur, the consequence is similar to the situation of a non contaminated rapeseed field. Good agricultural practice will suppress and eliminate the emerging seedlings in the next crop. Crosses of rapeseed and the two most common related weeds Sinapis
arvensis and Raphanus raphanistrum do not occur under agricultural conditions
even if both weeds grow in close vicinity of B. napus. If pollen from the wild
species are brought on a stigma o B. napus together with B. napus pollen,
the B. napus pollen grows through the pistill faster and will fertilize the egg
cell. As a consequence only non hybrid seed will develop The same is true
in reciprocal crosses where the weeds are used as females. Also there the
pollen of the same species fertilizes the ovaries (Kerlan et al 1992). The obtained interspecific F1-hybrids showed besides morphological abnormalities a reduced fertility. It is extremely unlikely that under agricultural conditions these plants can compete with well adopted fully fertile species. These mentioned artificial conditions under which the Raphanus-Brassica hybrids were created do not correspond to conditions in agriculture, because a pollinator with B napus pollen was completely excluded. In presence of B. napus pollen this would have outcompeted the pollen of the related species as described above. In order to set the remote probability of intergeneric crosses in the right relation to agricultural reality it is helpful to remember that in a rapeseed field at harvest 3-5% of the seeds are falling on the ground. This corresponds to 100-150kg seeds/ha which is equivalent to 2.000-3.000 seeds/m2 or 20-30 mio seeds/ha. Ever since rapeseed is grown as a crop, farmers have to handle this problem. The rapeseed seedlings which emerge from the lost seeds behave like a weed in the following crop. Appropriate cultivation practice as well as herbicide rotation solved this problem in the past as they will do in the future. Weed Resistance to Glufosinate AmmoniumDue to the mode of action of Glufosinate Ammonium it is very unlikely that weeds become resistant. The reason is that this would require a mutation of the target enzyme glutamine synthetase. However, mutated glutamine synthetase which lost its binding affinity for Glufosinate Ammonium simultaneously lost its binding affinity for glutamate, a structural analogue of Glufosinate Ammonium. A mutated enzyme therefore could not catalyze the amidation of glutamate to glutamine, the essential detoxification step for ammonia. Such a mutation would be lethal. Therefore it is extremely unlikely that weed will develop a spontaneous resistance towards Glufosinate Ammonium. This hypothesis is well supported:
Impacts on weed management - herbicide use patternSince the first field tests with Glufosinate Ammonium tolerant tobacco plants in 1989 in France there was proof for the applicability of the new approach for selective weed control with Glufosinate Ammonium. In the following years field trial work was extended tremendously in Europe and particularly in North America. Glufosinate Ammonium tolerant Oilseed Rape, Maize, Soybeans and more recently Sugar Beets have been grown in over 1.100 field trials. Glufosinate Ammonium has been sprayed post emergence with single and sequential applications of 150 to 600 g ai/ha. The timings of treatments were choosen primarily according to the following growth stages of weeds: early post (2-4 leaves) mid post (3-5 leaves), late post (5-8 leaves). Generally broad spectrum weed control of all major weeds was achieved with one to two treatments of up to 600 g ai/ha depending on one hand on weed pressure and on the other hand on duration of weed emergence and row closure ot the crop. Crop safety for Glufosinate Ammonium was excellent. Yield assessments revealed high yields confirming the concept of superior weed control with post emergent application of Glufosinate Ammonium. The impact on weed management can be summarized as
follows:
ConclusionThere is a proof now that genetically modified herbicide tolerant crops become reality in agriculture. For the first time Glufosinate Ammonium tolerant oilseed rape varities have been registered and launched in Canada 1995. This new approach will substantially contribute to further improved cultivation of major crops. It allows the safe use of herbicides with well accepted favourable profiles. This enables farmers to control weeds as competitors of crops with as little side effects as possible.
|
||||||||||||||||||||||||
Dieser Artikel ist leider nicht in deutscher Sprache verfügbar. 
pdf 500 Kb)
E. Rasche, G. Donn
Suchen Sie bei antikoerper-online.de.
Passende Antikörper aus
über  Produkten.
|