Changes in the metabolomic profiling of Allium cepa L. (onion) plants infected with Stemphylium vesicarium

The Allium cepa L. (onion) crop is affected by leaf blight disease caused by Stemphylium vesicarium; the shifts that S. vesicarium infection causes in the primary metabolism of onion plants are not known. In this study, the profiling of primary metabolites obtained by nuclear magnetic resonance (¹H NMR) spectroscopy of organs from onion plants uninfected (control) and infected with S. vesicarium were compared. Twenty-eight metabolites were identified in the ¹H NMR spectra of onion bulbs, thirty in leaves and thirty-eight in roots. Twenty-four metabolites were the same in bulbs, leaves and roots (fructose, glucose, myo-inositol, sucrose, alanine, asparagine, 4-aminobutyrate, glutamine, glutamic acid, isoleucine, leucine, phenylalanine, tryptophan, tyrosine, valine, citric acid, formic acid, lactic acid, malic acid, pyruvic acid, adenosine, guanosine, and uridine), but galactose, proline and acetoin were only identified in leaves; and trehalose, methionine, cytidine and methanol were only found in roots. Compared to uninfected plants, the concentration of sucrose, succinic, citric and formic acids and several amino acids that participate in the tricarboxylic acid cycle were reduced; which suggest that the processes involved in energy production are reduced by the infection with S. vesicarium. The concentration of formic acid was reduced in bulbs and leaves, which indicates that might infection induces the oxidation of formic acid to CO₂ under conditions of low sucrose availability in onion plants. The increases of pyruvic acid, amino acids derived from pyruvic acid (alanine, leucine and valine), glutamine, glutamic acid, nucleosides of pyrimidine and purine, and GABA could be used for the successful growth of the pathogen.