GENOMICS, MOLECULAR GENETICS & BIOTECHNOLOGY Transgenic Cotton with Improved Resistance to Glyphosate Herbicide

G rowers have overwhelmingly adopted glyphosate- resistant (Roundup Ready, Monsanto Co., Ches- Glyphosate (N-(phosphonomethyl)glycine) herbicide can be topi- terfield, MO) cotton cultivars since introduction in 1997. cally applied twice at rates as high as 0.84 kg a.e. (acid-equivalent) In 2002, about 72% of the U.S. hectarage was planted to ha 1 to glyphosate-resistant cotton (Gossypium hirsutum L.) cultivars until the fourth true leaf stage, with the requirement of at least 10 d cultivars containing the Monsanto glyphosate resistance and two nodes of growth between applications. But, such cultivars gene alone, or combined with Monsanto's Bollgard gene are not reproductively resistant to glyphosate applied topically or (derived from Bacillus thuringiensis var. kurstaki) con- imprecisely directed after the four-leaf stage because glyphosate can ferring protection from certain lepidopteran pest insects curtail pollen development and ovule fertilization, which potentially (Perlak et al., 1991; USDA-AMS, 2002). The popularity reduces yield. Extending glyphosate resistance past the four-leaf stage of glyphosate-resistant cotton cultivars reflects the broad- would provide growers with additional weed management options. spectrum weed control possible with glyphosate, plus Our objective was to test under field conditions glyphosate resistance capability to farm cotton on more hectares compared of cotton germplasm transformed with gene constructs previously with traditional weed management approaches (Cul- shown to impart extended glyphosate resistance in the greenhouse. pepper and York, 1998; York, 1997). Four or six transgenic cotton lines containing one of several constructs The limitation of the current cotton glyphosate resis- conferring extended glyphosate resistance, plus the current glypho-