Abstract
Biotic and abiotic stresses cause substantial changes in plant biochemistry. These changes are typically revealed by high-performance liquid chromatography (HPLC) and mass spectroscopy-coupled HPLC (HPLC-MS). This information can be used to determine underlying molecular mechanisms of biotic and abiotic stresses in plants. A growing body of evidence suggests that changes in plant biochemistry can be probed by Raman spectroscopy, an emerging analytical technique that is based on inelastic light scattering. Non-invasive and non-destructive detection and identification of these changes allow for the use of Raman spectroscopy for confirmatory diagnostics of plant biotic and abiotic stresses. In this study, we couple HPLC and HPLC-MS findings on biochemical changes caused by Candidatus Liberibacter spp. (Ca. L. asiaticus) in citrus trees to the spectroscopic signatures of plant leaves derived by Raman spectroscopy. Our results show that Ca. L. asiaticus cause an increase in hydroxycinnamates, the precursors of lignins, and flavones, as well as a decrease in the concentration of lutein that are detected by Raman spectroscopy. These findings suggest that Ca. L. asiaticus induce a strong plant defense response that aims to exterminate bacteria present in the plant phloem. This work also suggests that Raman spectroscopy can be used to resolve stress-induced changes in plant biochemistry on the molecular level.
Highlights
Raman spectroscopy (RS) is a label-free, non-invasive, nondestructive spectroscopic technique that provides information about the chemical structure of analyzed specimens (Cardona, 1975)
HLB-associated changes in the spectra of citrus trees were in the vibrational bands centered at 1,000, 1,155–1,226, and 1,525 cm−1, as well as in 1,601–1,630 cm−1 (Figures 1, 2, Sanchez et al, 2019c)
It should be noted that in the Raman spectrum collected from the leaves of healthy citrus trees, these two bands were centered at 1,606 and 1,630 cm−1, whereas in the spectrum of HLBinfected trees, the bands were found at 1,601 and 1,630 cm−1 (Figures 1, 2 and Supplementary Figure 2)
Summary
Raman spectroscopy (RS) is a label-free, non-invasive, nondestructive spectroscopic technique that provides information about the chemical structure of analyzed specimens (Cardona, 1975). Several groups independently demonstrated that spectroscopic analysis of plant leaves and seeds could be used for confirmatory diagnostics of diseases, nitrogen, phosphorus, and potassium deficiencies as well as salinity stress in plans (Yeturu et al, 2016; Egging et al, 2018; Farber and Kurouski, 2018; Mandrile et al, 2019; Sanchez et al, 2019a,c, 2020b,c; Huang et al, 2020). Such diagnostics are based on changes in the biochemistry of the plant (Farber et al, 2019a). We performed systematic comparison of RS and reported HPLC (Killiny and Nehela, 2017) and HPLC-MS (Hijaz et al, 2013)-based biochemical changes associated with HLB, otherwise known as citrus greening disease
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