Came here to make a similar point: not only could you make a plant that produces enzymes that cleave the herbicide, you could make a plant that expresses efflux pumps that pump the herbicide out of the cells, making the plant immune as well. This mechanism is used by cancer tumors and bacteria to resist all the crap we throw at them trying to kill them. I suspect there are a multitude of ways to make Round-up resistant crops.