ABSTRACT
There is more to biotechnology than developing herbicide-resistant crops. Engineering the crop directly to be resistant to weeds without a herbicide is a concept that is mainly a glint in the eyes of researchers. One excep tion is the case of parasitic weeds, where genes have actually been found by transposon tagging that confer resistance, both in corn and in its wild relatives, and are being engineered into corn.526 In this respect, parasitic weeds are closer to plant pathogens in their relationship with the crop, and thus it can be expected that there will be resistance genes to parasitic weeds, just as there are resistance genes to pathogens. Additionally, it was found that marigold (a non host) turns on a number of genes when suc cessfully warding off attempted attack by StHga asiatica.468 The gene prod ucts were identified after PCR-based differential cDNA amplification. One gene NRSA-1 is a homolog of disease-resistant genes from other species, and could possibly play a role in resistance,468 if transformed into suscepti ble species with wound inducible promoters. Are there genes for better competition with other weeds? There must be, but how well have we looked? Various researchers have proposed engineering allelochemical production into crops. One might question the metabolic cost of such production against final crop yield, and whether it will be too complex due to the number of genes involved or whether it is economical, ques tions that can only be answered in the field. There is also a possibility that allelochemicals will have undesirable mammalian toxicity. As a larger arsenal of herbicide-resistant crops is needed to manage weeds, it is also worth considering ways to modify weeds to lesser virulence as outlined below or engineering biocontrol agents to greater virulence, as discussed in Chapter 10.
