Background
The authors write "there has been considerable recent controversy over whether plants produce methane," with some studies indicating they emit it (Keppler et al., 2006), but with others indicating they do not (Dueck et al., 2007).

What was done
In an attempt to resolve the controversy, Nisbet et al. "conducted further experiments on plants grown in controlled conditions" and "re-analyzed the previously published data."

What was learned
The fourteen researchers (thirteen from the UK and one from Sweden) were able to demonstrate that "plants do not contain a biochemical mechanism for methanogenesis," and that they "cannot produce methane as an end-product or by-product of their metabolism." However, they determined that "when plants transpire, any methane that is already dissolved in the water derived from the soil will be released into the atmosphere," and that "under high stress conditions, such as high UV radiation, methane is released as part of the cellular breakdown process."

What it means
In light of the findings noted above, plus "a new analysis of global methane levels from satellite retrievals," Nisbet et al. rightly conclude that "plants are not a major source of the global methane production." On the other hand, they acknowledge "the role of plants in moving methane about," and indicate their importance in the global cycling of methane, but not its production.

References
Dueck, T.A., de Visser, R., Poorter, H., Persijn, S., Gorissen, A., de Visser, W., Schapendonk, A., Verhagen, J., Snel, J., Harren, F.J.M., Ngai, A.K.Y., Verstappen, F., Bouwmeester, H., Voesenek, L.A.C. and van der Werf, A. 2007. No evidence for substantial aerobic methane emission by terrestrial plants: a ¹³C-labelling approach. New Phytologist 175: 29-35.

Keppler, F., Hamilton, J.T.G., Brass, M. and Rockmann, T. 2006. Methane emissions from terrestrial plants under aerobic conditions. Nature 439: 187-191.