Revealing the Chemical Composition of Birch Pollen Grains by Raman Spectroscopic Imaging

Abstract

The investigation of the biochemical composition of pollen grains is of the utmost interest for several environmental aspects, such as their allergenic potential and their changes in growth conditions due to climatic factors. In order to fully understand the composition of pollen grains, not only is an in-depth analysis of their molecular components necessary but also spatial information of, e.g., the thickness of the outer shell, should be recorded. However, there is a lack of studies using molecular imaging methods for a spatially resolved biochemical composition on a single-grain level. In this study, Raman spectroscopy was implemented as an analytical tool to investigate birch pollen by imaging single pollen grains and analyzing their spectral profiles. The imaging modality allowed us to reveal the layered structure of pollen grains based on the biochemical information of the recorded Raman spectra. Seven different birch pollen species collected at two different locations in Germany were investigated and compared. Using chemometric algorithms such as hierarchical cluster analysis and multiple-curve resolution, several components of the grain wall, such as sporopollenin, as well as the inner core presenting high starch concentrations, were identified and quantified. Differences in the concentrations of, e.g., sporopollenin, lipids and proteins in the pollen species at the two different collection sites were found, and are discussed in connection with germination and other growth processes.

Patterns of Phenolic Compounds in Betula and Pinus Pollen

“…The biochemical composition of gymnosperm and angiosperm pollen is similar [ 29 , 34 , 35 ]. The pollen cell wall contains cellulose, hemicellulose, pectin, proteins, long-chain wax esters and phenylpropanoids [ 34 , 35 , 36 , 37 ]. There exist differences in the amount of these molecules [ 34 ] or they are distributed differently in the pollen cell wall tubes [ 29 ].…”

“…The methods and extraction solvents used for the extraction of phenolic compounds under laboratory conditions depend greatly on the types of phenolic compounds to be extracted. In order to speed up the extraction process, pollen is grated by mixers [ 39 ], and processed by cold [ 36 , 40 ]. In pollen, due to the stability of sporopollenin, the exine is resistant to both alkalis and concentrated inorganic acids [ 41 ].…”

Special Issue on “Raman Spectroscopy for Chemical and Structural Characterization in Biology”

“…In biology, it is particularly appropriate for the identification of biomolecules and the specific spectral signatures of cells [ 1 ]. In this Special Issue, six experimental studies and a review explore the performance of Raman spectroscopy to probe animal [ 2 , 3 , 4 , 5 , 6 ], vegetal [ 7 ], and fungal [ 8 ] cells, obtaining original information via data processing using multivariate analysis methods such as principal component analysis (PCA) [ 3 , 5 , 8 ], hierarchical cluster analysis (HCA) [ 3 , 7 ], and multiple curve resolution (MCR) [ 7 ] approaches.…”

“…Thanks to its high spatial resolution of a few hundreds of nanometers, Raman spectroscopy is an ideal label-free tool to characterize single cells [ 4 , 7 ]. Confocal Raman microscopy allows single birch pollen grains to be distinguished from each other depending on their collection site due to differences in their sporopollenin, lipid, and protein composition [ 7 ].…”

“…Thanks to its high spatial resolution of a few hundreds of nanometers, Raman spectroscopy is an ideal label-free tool to characterize single cells [ 4 , 7 ]. Confocal Raman microscopy allows single birch pollen grains to be distinguished from each other depending on their collection site due to differences in their sporopollenin, lipid, and protein composition [ 7 ]. It also reveals chemical groups reflecting the occurrence of DNA repair processes in living cells [ 4 ] and the presence of ancient organic residues (e.g., proteins and nucleic acids) in fossilized bone tissues [ 6 ].…”