Mass spectrometry imaging of glucosinolates in Arabidopsis flowers and siliques

Abstract

Glucosinolates are multi-functional plant secondary metabolites which play a vital role in plant defence and are, as dietary compounds, important to human health and livestock well-being. Knowledge of the tissue-specific regulation of their biosynthesis and accumulation is essential for plant breeding programs. Here, we report that in Arabidopsis thaliana, glucosinolates are accumulated differentially in specific cells of reproductive organs. Using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI), distribution patterns of three selected compounds, 4-methylsulfinylbutyl (glucoraphanin), indol-3-ylmethyl (glucobrassicin), and 4-benzoyloxybutyl glucosinolates, were mapped in the tissues of whole flower buds, sepals and siliques. The results show that tissue localization patterns of aliphatic glucosinolate glucoraphanin and 4-benzoyloxybutyl glucosinolate were similar, but indole glucosinolate glucobrassicin had different localisation, indicating a possible difference in function. The high resolution images obtained by a complementary approach, cryo-SEM Energy Dispersive X-ray analysis (cryo-SEM-EDX), confirmed increased concentration of sulphur in areas with elevated amounts of glucosinolates, and allowed identifying the cell types implicated in accumulation of glucosinolates. High concentration of sulphur was found in S-cells adjacent to the phloem in pedicels and siliques, indicating the presence of glucosinolates. Moreover, both MALDI MSI and cryo-SEM-EDX analyses indicated accumulation of glucosinolates in cells on the outer surface of the sepals, suggesting that a layer of glucosinolate-accumulating epidermal cells protects the whole of the developing flower, in addition to the S-cells, which protect the phloem. This research demonstrates the high potential of MALDI MSI for understanding the cell-specific compartmentation of plant metabolites and its regulation.

Matrix assisted laser desorption/ionization-mass spectrometry imaging (MALDI-MSI) for direct visualization of plant metabolites in situ

“…This approach has its foundation in earlier studies like those described by Jun thaliana flowers and siliques [34], secondary metabolites in trichomes [35], defense induced synthesis of pinoresinol monogulcoside in pea pod endocarp [36], and the antibiome of roots with a developing biofilm of Bacillus amyloliquefaciens [37]. Due to the relative ease sample preparation, and the applicability of MSI to surface analysis by a variety of methods, imaging surfaces of plants should be a prolific area of research in years to come.…”

Integration of Biosynthesis and Long-Distance Transport Establish Organ-Specific Glucosinolate Profiles in VegetativeArabidopsis

“…The observed localization of GTR1 and GTR2 in cells outside the stele in the lateral root branching points of the main root ( Figures 6E and 6H) suggests that GTR1-and GTR2-mediated retention may occur via import of GLS into storage cells in these areas. In Arabidopsis, GLS accumulates to concentrations up to 100 mM in sulfur-rich cells (S-cells) localized adjacent to the vasculature in aboveground tissues (Koroleva et al, 2000(Koroleva et al, , 2010Koroleva and Cramer, 2011;Sarsby et al, 2012). While S-cells in roots have previously been described in the inner periderm of the main root in field-grown B. napus (McCully et al, 2008), it remains to be investigated whether cells outside the stele expressing GTR1 and GTR2 represent S-cells in Arabidopsis roots.…”

Mass Spectrometry Imaging of Biomolecular Information

“…Other recent MSI studies of plant tissue described toxic glycoalkaloids in potatoe tuber 188 , symbiosis of plants with nitrogen fixing microorganisms 189 , anthocyanines in rice pericarp 190 and glucosinolates in Arabidopsis flowers and siliques 191 .…”

“…The methodology employed for plant MSI is well spread over the available imaging techniques MALDI 186,[188][189][190][191][192][193][194][195][196][197][198][199] , DESI [200][201][202][203][204][205] , SIMS 206 and LDI 207,208 .…”