Larry Moran posts on glyphosate resistance in weed plants:
Roundup® (glyphosate) has been used to control weeds since 1974 [How Roundup® Works]. In all those years, the number of reported cases of resistant plants has been far below predictions. Only in the past ten years have Roundup®-resistant plants been identified and there are only 11 species of resistant weeds known at last count (Perez-Jones et al. (2007).
We now know from studies of the mechanism of resistance of the C4 EPSP synthase that resistance to glyphosate requires very special circumstances; namely, an enzyme active site that can exclude glyphosate while still allowing phosphoenolpyruvate to bind efficiently [The Molecular Basis of Roundup® Resistance]. Thus, with hindsight, it is perhaps not surprising that so few resistant plants have turned up.
This post and the earlier one outlining the biochemistry of glyphosate are very interesting to me (for reasons that will probably remain obscure!) and make a useful counterpoint to the more common tales of antibiotic resistance in bacteria.
I also found the linked 2004 article from Wired really interesting:
The Mystery of the Coca Plant That Wouldn't Die
by Joshua Davis
Over the past three years, rumors of a new strain of coca have circulated in the Colombian military. The new plant, samples of which are spread out on this table, goes by different names: supercoca, la millonaria. Here in the southern region it's known as Boliviana negra. The most impressive characteristic is not that it produces more leaves - though it does - but that it is resistant to glyphosate. The herbicide, known by its brand name, Roundup, is the key ingredient in the US-financed, billion-dollar aerial coca fumigation campaign that is a cornerstone of America's war on drugs.
One possible explanation: The farmers of the region may have used selective breeding to develop a hardier strain of coca. If a plant happened to demonstrate herbicide resistance, it would be more widely cultivated, and clippings would be either sold or, in many cases, given away or even stolen by other farmers. Such a peer-to-peer network could, over time, result in a coca crop that can withstand large-scale aerial spraying campaigns.
It's a long article, but very compelling. The scenario that comes out as likely in the end is a case of natural adaptive introgression:
Which points back to selective breeding. The implication is that the farmers' decentralized system of disseminating coca cuttings has been amazingly effective - more so than genetic engineering could hope to be. When one plant somewhere in the country demonstrated tolerance to glyphosate, cuttings were made and passed on to dealers and farmers, who could sell them quickly to farmers hoping to withstand the spraying. The best of the next generation was once again used for cuttings and distributed.
This technique - applied over four years - is now the most likely explanation for the arrival of Boliviana negra. By spraying so much territory, the US significantly increased the odds of generating beneficial mutations. There are numerous species of coca, further increasing the diversity of possible mutations. And in the Amazonian region, nature is particularly adaptive and resilient.
Could be true, but I couldn't find any later work confirming the existence of glyphosate resistance; just a lot of references to this one Wired article.