Non-destructive Raman spectroscopy for early monitoring of virus infection in plants

Plant viruses are obligate biotrophic pathogens infecting a wide variety of plant species. To support their propagation and proliferation, they reprogram the plant cell metabolism, causing morphological and physiological alterations in the infected hosts, eventually leading to reduced crop yield and poor product quality. Among the wide variety of symptoms induced by plant viruses, the most common is leaf chlorosis, associated to depleted photosynthetic activity. Early virus detection can contribute to limit virus spread and economic impact. However, currently used diagnostic procedures, such as immunological or nucleic acid-based techniques are time-consuming, destructive and expensive. Raman spectroscopy (RS) is a non-destructive procedure that generates a chemical fingerprinting of a sample, at low operating costs. Here, we used RS combined to chemometric analysis to evaluate the metabolic changes occurring during the infection of an economically relevant crop, tomato, by two dangerous viral pathogens, Tomato yellow leaf curl Sardinia virus (TYLCSV) and Tomato spotted wilt virus (TSWV). Coherent modifications of the entire spectral profile were detected in regions associated with chlorophylls as well as carotenoids, carbohydrates, and phenolic compounds, allowing to discriminate healthy plants from specimens infected by both viruses, in a time frame of 4 weeks after virus inoculum. RS could not only detect metabolic changes occurring at early and still asymptomatic stages of infection (with >70% accuracy at 14 days after inoculation for TYLCSV and >85% at 8 days for TSWV), but allowed also to differentiate spectral variations induced by the two viruses. This investigation provides interesting clues to test the efficacy of RS to monitor the response of plants to biotic or abiotic stimuli, with particular attention to the photosynthetic process.