Mass spectrometric imaging (MSI)

So far, chemicals were extracted from whole plants, insects, bacterial colonies, and the structures of putative chemical signals were studied for whole organisms or for their larger sections. It is obvious to expect that the source of chemical signals will be localized, and co-localization of compounds with putative biosynthetic or storing glands/structures will be indicative for finding the function of the detected compounds.
In the past three years we have initiated a program aimed at developing mass-spectrometry based methods for small molecule imaging (determining their topological-coded intensity maps). A pioneering study was performed in 2007-2008 in cooperation with the Department of Biochemistry (J. Gershenzon) on A. thaliana (Shroff et al. 2008 PNAS). We were able to observe non-uniform distribution of glucosinolates (Gls, major pro-phytoalexins in the thale cress), validate the MALDI images using another analytical method, and to correlate the distribution with insect larvae feeding. Recently, a quantitative study of surface-abundant Gls on A. thaliana leaves was finished. Results show that Gls are present on leaf surfaces at a concentration that could be detected by Plutella xylostella females. In both studies we used a MALDI matrix (9-aminoacridine) to see the compounds of interest. However, if compounds exhibit strong adsorptions of UV laser light used in the MALDI experiments, we can eliminate the matrix and so make the imaging method more simple without needs of extensive sample preparation. Using this approach, we can study surface compounds at cellular spatial resolution (Hölscher et al. 2009 Plant J.).
Employing the previously developed LiDHB MALDI matrix for analysis of neutral lipids and hydrocarbons, localization of hydrocarbons and wax esters on insect wings and plant leaves was studied (Vrkoslav et al. 2010 J. Amer. Soc. Mass Spectrom.).
Applications of mass spectrometry imaging was recently reviewed (Svatos, 2010 Trends in Biotechnol.).