Figure 3. Overlap between Plant Responses to Singlet Oxygen and to Biotic Stressors. SO oxidizes β-carotene to generate the retrograde signaling molecule β-cyclocitral, which is also induced by simulated beet armyworm feeding (mechanical damage paired with caterpillar oral secretions). In addition, SO oxidizes membrane lipids, giving rise to hydroxy fatty acids including 10-HO-FA and 15-HO-FA. These two HO-FAs, which are considered diagnostic markers of SO accumulation, are also induced in Arabidopsis by Pseudomonas syringae infection. Gene expression profiles induced in host plants by P. syringae , the fungal pathogen Peronospora parasitica , or by treatment with effectors associated with other pathogens (e.g. flagellin, chitin, oligogalacturonides) also show considerable overlap with transcriptional reprogramming by SO. This figure was made in Biorender.
Figure 1. Chemical Properties of Singlet Oxygen. Whereas ground state molecular oxygen is in a triplet state, with two unpaired, spin-parallel electrons (A), singlet oxygen has two valence electrons spin-paired in a single orbital and a second orbital left empty. Singlet oxygen is more reactive than triplet oxygen, and can perform ene reactions and Diels-Alder cycloadditions (C) that triplet oxygen cannot.