Millions of hectares are being planted with T-HRCs, with insect resistance in second place, with both traits often "stacked" in the same seeds to enhance their value. Farmers perceive a utility in T-HRCs, as they have many alternatives and still repeatedly purchase seed of T-HRCs. The real values of T-HRCs come from instances in which there really are no viable weed control methods (including the situation due to evolved herbicide resistances in weeds) and the fact that such T-HRCs could lead to more sustainable world food production. T-HRCs are the subject of a book (Duke, 1996) as well as in-depth reviews of the molecular aspects (Cole and Rodgers, 2000; Gressel, 2000, 2002).
The easiest way to obtain selectivity among closely related species is to engineer resistance to a general herbicide into the crop. For example, it has already been shown that red rice (Oryza sativa) is easily controlled by glufosinate in transgenic rice (Sankula et al., 1997) bearing the bar gene that confers resistance to this herbicide. The immediate answer to multiple resistance problems in weeds of wheat in major growing areas is to engineer resistances to inexpensive herbicides (Gressel, 1988, 2002). Neither the chemical nor the biotechnological industries have shown particular interest in generating T-HRCs in wheat and rice. Because too little profit is perceived to come from wheat or rice seed or even from generic herbicides, it may be necessary to have wheat and rice engineered by the public sector. Glufosinate resistance has been engineered into wheat, more as a marker gene than for agronomic utility (Weeks et al., 1993). A request has been made for field use of glufosinate-resistant rice in the United States (Anonymous, 1999a), which has been allowed, despite the fact that it is well known that the gene can introgress into conspecific red rice (Sankula et al., 1998).
BD-HR wheats and rices may be an answer to the major problems of these crops. Insert a gene into wheat or rice conferring resistance to a broad-spectrum herbicide and you can control weeds that evolved resistance in wheat and even closely related grasses, including red, weedy, and other wild rices (Gressel, 1999a, 1999b, 1999c). The transgenes will allow overcoming the problems of resistance that have evolved, especially the problems of cross-resistances (where one evolutionary step conferred resistance to a variety of chemicals) and multiple resistance (where a sequence of evolutionary steps with different selectors conferred resistance to a variety of chemicals). The use of nonplant transgenes may also allow farmers to overcome the natural resistances in weeds closely related to the crop. One can even choose bacterial genes for inexpensive long-established herbicides such as dalapon (Buchanan-Wollaston et al., 1992), modify them for plant codon usage, and transform them into grain crops (Gressel, 1997a). Such genes have the advantage that it is harder, albeit not impossible, for the weeds to mimic bacteria than to mimic plants in evolving resistance.
The problems of interclass cross-resistances and multiple resistances in wheat have necessitated considering generating BD-HR oilseed rape, especially in Australia (Gressel, 1999b). Oilseed rape has become an excellent rotational crop, alternating with wheat in many places where wheat is grown. There are many agronomic advantages to rotating a dicot with a monocot, especially vis-à-vis weeds. It should be far easier to eliminate grass weeds in oilseed rape than in wheat, as there are more selective graminicides available for use in dicot crops. This is usually correct except in Australia, where Lolium rigidum has successfully evolved widespread target site and inter-group cross-resistances as well as multiple resistances to all these graminicides (although not in all fields). It is additionally necessary to generate BD-HR oilseed rape (Brassica napus) to allow control of closely related Brassica weeds including those such as Raphanus raphanistrum. Here we can make the case for the need for management strategies with T-HRCs as with any selective herbicide. Atrazine-resistant BD-HR oilseed rape was widely and exclusively used to control R. raphanistrum in Australia. It was generated by backcrossing the gene in from a mutant B. campestris that evolved in Canada (Souza-Machado, 1982). Inevitably, there are reports that this weed has evolved resistance to triazines (Cheam et al., 2000) as well as resistance to ALS inhibitors (Cheam et al., 1999) and to diflufenican (Cheam et al., 2000). Thus, one must engineer resistance into oilseed rape that allows control of both Lolium rigidum and Raphanus raphanistrum, with all their resistances. To delay further resistances to the few remaining herbicides, perhaps gene stacking and using herbicide mixtures are necessary (see later).
Only the first generations of BD-HCRs have been released, and considerable improvements are to be expected. There are far too few concrete molecular and biochemical data published about the properties of these crops, and thus there are problems in evaluating their properties to allow suggestions for improvements. Thus, some of what will be said here should be considered speculative. Two types of genes have been used to generate herbicide-resistant crops: (1) those whose gene product detoxifies the herbicide and (2) those for which the herbicide target has been modified such that it no longer binds the herbicide. One could envisage other types such as exclusionary mechanisms or sequestration, but they have yet to be found and thus not utilized. The resulting T-HRCs with each type of resistance are rather different and thus will be discussed separately.
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