Visualization of cyanogenic glycosides in floral tissues

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) is a particularly useful tool to determine the localization of metabolites in plant tissues enabling the exploration of spatial metabolic processes in plant biology. In this case study, we examined cyanogenic glycosides (CNglycs), in the delicate floral tissues of highly cyanogenic Grevillea robusta (southern silky oak) and Macadamia tetraphylla (macadamia nut) collected in the field. CNglycs are canonical defence metabolites, but more recent work indicates they may play a much broader role in plant metabolism. We quantified CNglycs in floral tissues by chemical assay and visualized their spatial distribution using MALDI-MSI to explore their biochemical and ecological roles. The species examined contained two tyrosine-derived CNglycs, the monoglycoside dhurrin, and diglycoside proteacin, identified using HP-TLC and LC-MS/MS analyses. Within florets, the most cyanogenic tissue was the style for both species. MALDI-MSI revealed different distributions of dhurrin and proteacin within florets of both species and differences in the ratios of dhurrin to proteacin between specific floral tissues. Dhurrin tended to be mostly localized in epidermal layers, consistent with a role in defence, while proteacin was distributed more evenly throughout other floral tissues indicating other potential roles.