The provenance, half-life and biological activity of malondialdehyde (MDA) were 
investigated in Arabidopsis thaliana. We provide genetic confirmation of the 
hypothesis that MDA originates from fatty acids containing more than two 
methylene-linked double bonds, showing that tri-unsaturated fatty acids are the 
in vivo source of up to 75% of MDA. The abundance of the combined pool of free 
and reversibly bound MDA did not change dramatically in stress, although a 
significant increase in the free MDA pool under oxidative conditions was 
observed. The half-life of infiltrated MDA indicated rapid metabolic 
turnover/sequestration. Exposure of plants to low levels of MDA using a recently 
developed protocol powerfully upregulated many genes on a cDNA microarray with a 
bias towards those implicated in abiotic/environmental stress (e.g. ROF1 and 
XERO2). Remarkably, and in contrast to the activities of other reactive 
electrophile species (i.e. small vinyl ketones), none of the 
pathogenesis-related (PR) genes tested responded to MDA. The use of structural 
mimics of MDA isomers suggested that the propensity of the molecule to act as a 
cross-linking/modifying reagent might contribute to the activation of gene 
expression. Changes in the concentration/localisation of unbound MDA in vivo 
could strongly affect stress-related transcription.