Oxidative stress-induced proteolytic activation of polyphenol oxidase triggers an oxidized flavonoids-mediated stress signaling in Camellia sinensis
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Environmental perturbations often increase reactive oxygen species (ROS) production, inducing oxidative damage to various biomolecules. The cellular system utilizes ROS or ROS-generated metabolites to instigate signaling pathways, resulting in acclimation, growth inhibition, or programmed cell death (PCD) for sustaining stress. Various stress-generated or stress-activated signaling components and downstream pathways are identified in model plants; however, they are largely unexplored in non-model plants. Here, we report a stress-induced proteolytic activation of an evolutionarily conserved chloroplast-localized enzyme, polyphenol oxidase (PPO), that oxidizes catechins into theaflavins (TFs) and initiates a stress signaling under drought in Camellia sinensis (Tea). Germplasm-based analysis revealed a heightened proteolytic activation of PPO and consequent TFs accumulation in drought-susceptible genotypes. Transcriptome analysis revealed that PPO activation and TFs accumulation were linked to the activation of UPR(unfolded protein response)-like response, which was reinforced by virus-induced silencing and overexpression of PPO, and direct feeding of TFs in tea plants. Pharmacological treatments revealed that TFs interact with HSP90, activating a canonical IRE-bZIP60-dependent ER stress pathway resulting in PCD. Similar proteolytic activation of PPO and subsequent instigation of stress signaling in other plant species (tomato and wheat) demonstrated that PPO acts as an evolutionarily conserved stress sensor, instigating an inter-organelle communication in plants.
Significance statement
PPO, an evolutionarily conserved and chloroplast/plastid localized enzyme, senses stress and undergoes proteolytic activation to generate oxidized flavonoids/quinones that function as signaling molecules in Tea and likely in other plants. The oxidized flavonoids modulate protein folding machinery and augment folding stress, especially in the ER, inducing canonical IRE1-bZI60-dependent PCD. The finding shows that specific secondary metabolites likely have roles in instigating intra- and intercellular communication to sustain the adversities of stress in different plant species.
