Herbicide resistance level

Llewellyn, R.S. and S.B. Powles (2001). High levels of herbicide resistance in rigid ryegrass (Lolium rigidum) in the wheat belt of Western Australia. Weed Technol., 15 242-248. 

Figure 10 shows, in considerably more detail, the sequence of mutagenesis and selection which has been actually used to develop herbicide resistant lines. Calli were initiated from a healthy alfalfa plant. After 4-8 weeks, these calli were broken up into suspensions, and either treated with a mutagenizing agent or screened simply by selection for spontaneous mutations. After either procedure, the selected lines, i.e., the lines which survived exposure to the herbicide, were then regenerated, and the plants were evaluated in a number of schemes. In addition, plants selected at the cellular level for resistance were recycled through the entire system of mutagenesis and selection to enhance the desired resistance trait. 

The Hra gene has also been used to transform a number of heterologous species, and selectable levels of resistance have been obtained in each case. The Hra gene can probably be used to confer useful levels of herbicide resistance in most plant species. 

While most plants are susceptible to paraquat, some paraquat-resistant horseweed (Erigeron sp. and Conyza sp.) biotypes are apparently insensitive to the herbicide due either to elevated levels of superoxide dismutase and other enzymes in a pathway detoxifying oxygen radicals or to differential sequestration of paraquat in the weed (8, 9). Data on the mechanism of most other types of herbicide resistance in weeds are still not complete. 

Triazine Resistance We attempted to answer the previous four questions using data and examples derived from the study of the best documented case of herbicide resistance, triazine resistance. Two kinds of mechanisms may be responsible for this triazine resistance first is the presence of detoxification metabolic pathways, as seen in corn (11). This also may occur in weed populations, especially Panicoideae, but a low heritability makes its study complex. The second mechanism of triazine resistance is the loss of herbicide binding at the level of the chloroplast. 

Selection for Herbicide Resistance at the Whole-Plant Level... 

We have Isolated single, nuclear, dominant mutations In whole plants of the small crucifer Arabldopsls thallana which confer a high level of resistance to a sulfonylurea herbicide. The ease and rapidity with which such mutations can be Isolated using A.thallana suggests that the approach may be of broad utility for studies of herbicide resistance and mode of action. 

Herbicide-resistant GM crops may have been modified such that they are able to metabolize the particular herbicide, rendering it innocuous for the crop plant. Alternatively, enzymes targeted by the herbicide may have been rendered insensitive to the herbicide by introduction of mutated versions or equivalents derived from other donor organisms. Both these modihcations may lead to altered levels of the herbicide and its metabolites being present in the crop. The issue of the safety of the altered herbicide residue prohle usually is considered as part of the pre-market assessment of the registration of the herbicide, which in many countries follow a regulatory procedure different from that for GM crops. 

There are no commercial herbicide resistant crops that function by increased expression of the protein target, although some level of plant resistance has been reported for glyphosate, glufosinate, some DPEs and inhibitors of hydroxy-phenylpyruvate dioxygenase. Similarly, cellular sequestration of the herbicide from the target has been reported with some DPEs, auxins and photosystem I inhibitors, but none have been developed commercially [3]. 

Fluorescence decay after a flash The kinetics of reoxidation monitors the fluorescence decay after a flash. The initial part of the decay was the same in herbicide resistant and susceptible Syneohooystis 6714 but the quasi stationary fluorescence level was higher in DCMU-II, DCMU-IIg and AzV. The same type of decays were obtained for resistant and susceptible thylakoids ot Cheno podium aVbim (Fig. 1) Fast initial decay, higher plateau level in the resistant thylakoids as compared to the wild type. 

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