The localization of metabolites on plant surfaces has been problematic because of the limitations of current methodologies. Attempts to localize
glucosinolates, the
sulfur-rich defense compounds of the order Brassicales, on leaf surfaces have given many contradictory results depending on the method employed. Here we developed a matrix-assisted
laser desorption-ionization (MALDI) mass spectrometry protocol to detect surface
glucosinolates on Arabidopsis thaliana leaves by applying the MALDI matrix through sublimation. Quantification was accomplished by
spotting glucosinolate standards directly on the leaf surface. The A. thaliana leaf surface was found to contain approximately 15 nmol of total
glucosinolate per leaf with about 50 pmol mm(-2) on abaxial (bottom) surfaces and 15-30 times less on adaxial (top) surfaces. Of the major compounds detected, 4-methylsulfinylbutylglucosinolate, indol-3-ylmethylglucosinolate, and 8-methylsulfinyloctylglucosinolate were also major components of the leaf interior, but the second most abundant
glucosinolate on the surface,
4-methylthiobutylglucosinolate, was only a trace component of the interior. Distribution on the surface was relatively uniform in contrast to the interior, where
glucosinolates were distributed more abundantly in the midrib and periphery than the rest of the leaf. These results were confirmed by two other mass spectrometry-based techniques,
laser ablation electrospray ionization and liquid extraction surface analysis. The concentrations of
glucosinolates on A. thaliana leaf surfaces were found to be sufficient to attract the specialist feeding lepidopterans Plutella xylostella and Pieris rapae for oviposition. The methods employed here should be easily applied to other plant species and metabolites.