A Microbial fuel cell which taps into small amounts of sugars and acids given off by plant roots.

Principle of the Plant-MFC Plants transform solar radiation and carbon dioxide into organic compounds. Part of the organic compounds is excreted by the roots of the plant (=rhizodeposition). These excreted organic compounds are oxidized by electrochemically active bacteria. During the oxidation, the bacteria donate the electrons to an anode which flow via an energy user to the cathode; hence electricity is generated.

http://www.ete.wur.nl/UK/Research/Renewable+Energy/Green+electricity+production+by+living+plants+in+a+microbial+fuel+cell/