Insights into the changes in chemical compositions of the cell wall of pear fruit infected by Alternaria alternata with confocal Raman microspectroscopy

In this paper, fresh pear fruits and those infected by Alternaria alternata (A. alternata) were studied by confocal Raman microspectroscopy (CRM) to illustrate the changes in chemical compositions of cell wall. Firstly, Raman spectra of the cell wall of both fresh and infected fruits were collected with spatial resolution at micron level, and then label-free in situ imaging of chemical compositions in the cell wall were mapped. The results showed that there were significant changes in the signal intensity of cell wall, especially in the later stage of A. alternata infection. After 8 days of infection at room temperature, the signal intensities at 1086 and 871 cm−1, which were associated with cellulose and pectin, were decreased by 58.50% and 58.67%, respectively, revealing changes in the main components of the infected cell wall infected. Meanwhile, the chemical images of the cell wall of both fresh and infected fruits were compared, indicating that A. alternata infection caused the alterations of morphological structure and chemical compositions in the cell wall in a time-dependent manner. Our results confirmed that CRM is a useful tool for the identification of compositional changes in the cell wall caused by fungal infection without the need for any chemical treatment. For the first time, the current research applied CRM in phytopathology for investigating interactive relationship between a pathogen and its host, thus offering a new way for in-depth study of pathogen-host interactions at cellular level.