Common Cellular Events Implicated in the Regulation of Cold Stress Tolerance and Soft Rot Resistance Induced by Metabolites of Pseudomonas Aeruginosa in Phalaenopsis Orchids

Microbial metabolites induce diverse plant physiological responses. In this study, Pseudomonas aeruginosa strain Y1 was shown to produce metabolites exhibiting a high capacity for metal chelation and moderate reducing power activity. In Phalaenopsis orchids, treatments with Y1 metabolites (Y1M) increased the contents of carbohydrates and lignin and the activity of antioxidant enzymes and phenylalanine ammonia lyase (PAL). Y1M treatments increased the tolerance to cold stress (10°C) by reducing malondialdehyde (MDA) accumulation and electrolyte leakage in a 10°C growth environment and enhanced disease resistance against bacterial soft rot caused by Erwinia chrysanthemi. Protein analyses indicated that, in response to Y1M treatment, the protein levels of antioxidant enzymes, such as catalase (CAT) and ascorbate peroxidase (APX); pathogenesis-related (PR) proteins, such as PR-2 and PR-3; and lipoxygenase 1 (LOX1) were increased in Phalaenopsis orchids. Transcriptome analyses indicated that Y1M increased transcription associated with the iron-deficient response mediated by miRNA, the regulation of reactive oxygen species (ROS) homeostasis, and Jasmonic acid (JA) biosynthesis/perception. Gene groups associated with the induced defense response, including cellular events of pattern-triggered immunity (PTI), hypersensitive response (HR), synthesis of the PR protein, and callose formation, were also increased by the Y1M treatments. Transcription factors involved in regulating the cold stress response, such as C-repeat binding factor 1 (CBF1), and cell strengthening, such as MYB26, also had increased expression. In summary, Y1M can activate cellular pathways implicated in regulating stress tolerance shared by cold stress and bacterial invasion.