Polyphenol oxidase as a biochemical seed defense mechanism

Abstract

Seed dormancy and resistance to decay are fundamental survival strategies, which allow a population of seeds to germinate over long periods of time. Seeds have physical, chemical, and biological defense mechanisms that protect their food reserves from decay-inducing organisms and herbivores. Here, we hypothesize that seeds also possess enzyme-based biochemical defenses, based on induction of the plant defense enzyme, polyphenol oxidase (PPO), when wild oat (Avena fatua L.) caryopses and seeds were challenged with seed-decaying Fusarium fungi. These studies suggest that dormant seeds are capable of mounting a defense response to pathogens. The pathogen-induced PPO activity from wild oat was attributed to a soluble isoform of the enzyme that appeared to result, at least in part, from proteolytic activation of a latent PPO isoform. PPO activity was also induced in wild oat hulls (lemma and palea), non-living tissues that cover and protect the caryopsis. These results are consistent with the hypothesis that seeds possess inducible enzyme-based biochemical defenses arrayed on the exterior of seeds and these defenses represent a fundamental mechanism of seed survival and longevity in the soil. Enzyme-based biochemical defenses may have broader implications since they may apply to other defense enzymes as well as to a diversity of plant species and ecosystems.

Laccase Properties, Physiological Functions, and Evolution

“…Quinones and other reactive intermediates (e.g., 5,6-dihydroxyindole) are more toxic to herbivores than the original phenolic substrates and may kill microbial pathogens and parasitoids directly [72]. Many PPOs cooperate with peroxidases (PODs; EC 1.11.1.7) [73] and have diverse and overlapping physiological functions in plants, which include involvement in redox metabolism, responses to wound healing, defense against pathogens or insects, synthesis of lignin and suberin, and cross-linking of cell wall components [74,75].…”

The Dead Can Nurture: Novel Insights into the Function of Dead Organs Enclosing Embryos

“…Accordingly, wheat bran was found to contain multiple plant defense-related proteins including oxalate oxidase (OXO), peroxidase (POX), and polyphenol oxidase (PPO) whose activities were highest in the outer layer [41]. Also, when oat DU was incubated with Fusarium avenaceum strain F.a.1, polyphenol oxidase (PPO) was induced in the whole DU as well as in the dissected parts, namely the dead, non-living hulls (lemma and palea) and the caryopses [42].…”

“…The authors suggested that the induction of PPO in the non-living hulls, although surprising, could have resulted from latent forms of PPO that are activated following challenge with F.a.1 [42].…”

Integrating host immune status, Labyrinthula spp. load and environmental stress in a seagrass pathosystem: Assessing immune markers and scope of a new qPCR primer set

“…POXs are multifunctional redox enzymes [20][21] that regulate biochemical processes such as the cross-linking of cell wall components, the production of secondary metabolites and key reactions in the hypersensitive response [21]. PPOs have been characterized as a common component of the plant innate immune response [22] and are responsible for catalyzing the oxidation of monophenols and/or o-diphenols to o-diquinones. Through secondary reactions, o-diquinones can promote quinone toxicity and confer resistance against pathogens [23][24].…”