The acquisition of thin sections of açaí (Euterpe oleracea Mart.) seed with elevated potassium content for molecular mapping by mass spectrometry imaging.

Abstract

Matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS) of tissues became popular in the last decade. Consequently, adapting sample preparation methods for different materials turned out to be a pivotal step for successful analysis due to the requirement of sample slices of 12-20 μm thickness. However, acquiring thin sections compatible with MALDI-IMS for unusual samples is challenging, as existing histological protocols may not be suitable, thus requiring new methods. Açaí (Euterpe oleracea Mart.) seed is an example of a challenging material due to its toughness and resistance to crack, therefore our goal was to develop a methodology to obtain thin (12-20 μm) and entire sections of açaí seeds for MALDI-IMS analysis. Different strategies were evaluated for obtaining thin sections of seeds, and the combination of the following steps was found to be the most suitable option: (i) softening of seeds by water immersion for 24 h; (ii) transversal cut of seeds to obtain half-seeds using a razor blade and a hammer; (iii) half-seeds imbibition in gelatin; (iv) samples sectioning using a cryostat at -20°C to obtain samples with 12-20 μm thickness; (v) collection of samples in an indium tin oxide-coated glass slide covered by double-sided copper tape to avoid sample wrapping and ensure adhesion after unfreezing; and (vi) storage of samples in a -80°C freezer, if necessary. This adapted sample preparation method enabled the analysis of açaí seeds by MALDI-IMS, providing spatial distribution of carbohydrates in the endosperm. The adaptations developed for sample preparation will help investigate the metabolic and physiological properties of açaí seeds in future studies.