Plant cell cultures herbicide-resistant

Figure 9. Somatic cell selection for herbicide resistance. Bottom left, a flask of alfalfa cells in suspension. Top left, addition of herbicide to the cells. Center, cells plated onto solid medium containing herbicide a resistant callus growing on herbicide-containing medium. Top right, resistant plantlets regenerating. Bottom right, tolerant plants selected from tissue culture growing in the field after being sprayed with the herbicide.

Many different bioassays and biochemical or genetic tests have been developed to identify resistant weeds. However, these are normally conducted after the suspected development of resistance, not in a proactive or preventive manner. The potential for evolution of resistance to a new herbicide can be examined in several ways wild-type populations can be screened for resistant individuals, model plant populations can be muta-genized and screened for resistance, resistant cells can be selected in culture, with or without prior exposure to the herbicide, or biochemical or genetic assays can be used to identify known resistance mechanisms. However, more complex or obscure resistance mechanisms may exist, and certain mechanisms may only be expressed in whole plants, not in cell cultures. More recent techniques focused on rapid genetic evolution can also provide a clue to the relative ease with which resistance can be generated, but still require a large investment. However, as in many predictive studies, it is often difficult to relate the results of such experiments to resistance evolution in the field. 

A GST enzyme from pea Is very effective In catalyzing GSH conjugation of the herbicide fluorodlfen ( ). This enzyme has a pH optimum and other properties that are comparable to mammalian GST enzymes (85). This enzyme activity was observed In other plant species, but fluorodlfen resistant species appeared to have higher levels of this enzyme than susceptible species ( ). Additional studies with pea Indicated the presence of two soluble GST Isozymes, one that utilized fluorodlfen and one that utilized -cinnamic acid as substrates (W7). These Isozymes appeared to form aggregates during purification. In addition, a microsomal GST was detected In pea that utilized both -cinnamic acid and benzo(a)pyrene as substrates (WT.). Soybean cell suspension cultures metabolized -clnnamlc acid In a 6% yield to a product that corresponded to the GSH conjugate of t-clnnamlc acid by paper chromatography (107). 

Herbicide-resistant plant varieties have proven to be valuable experimental tools In determining the molecular mode of action of herbicides (2,8). In addition, such varieties are likely to be an Important source of selectable markers for use In plant molecular genetics and In the engineering of resistant crop species ( ) The earliest herbicide-resistant blotypes described arose spontaneously from weed populations which had been repeatedly exposed to a herbicide (10,11). More recently, mutagenesis and selection on defined media have been used to Isolate herbicide-resistant or herbicide-tolerant mutants of higher plants from populations of cells In tissue culture (4,5.7,12). While the use of plant tissue culture has proven useful for Isolation of some types of mutants, the... 

Cell Culture Approaches for Obtaining Herbicide-Resistant Chloroplasts in Crop Plants... 

Genetic techniques for obtaining chloroplast-encoded herbicide resistance in crop plants are reviewed. These are based on the use of cell culture methods and Include mutant selection in cultured cells, chloroplast transfer via protoplast fusion, and genetic recombination between chloroplast genomes. The first two methods are described in detail using trlazine resistance as an example. 

Cell-culture techniques can be applied in which billions of cells are allowed to grow in a medium and develop mutants which, for example, might be resistant to particular vimses or herbicides or have other desirable qualities. If the cells with the desired qualities can be regenerated into whole plants, results can be obtained that might have taken decades using conventional plant-breeding techniques. 

The data for this field experiment are summarized in Table 1. Data are presented here for 13 lines which were derived from culture and field-evaluated for resistance to Roundup herbicide. Each of these lines was significantly more tolerant to Roundup than the regenerated non-selected control B74. However, the level of resistance in these 13 cell lines was not commercially significant. Nevertheless, this does indicate that resistant plants can be derived by selections at the cellular level. 

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