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.

Keywords: seed decay; seed defense; seed longevity; weed seed bank.

FIGURE 1

Seeds and pathogen: experimental materials. (A) Wild oat isoline ‘M73’: seed (left), dissected (middle) into lemma with awn, and palea, and caryopsis (right). We refer to lemma and palea as “hulls.” When we refer to wild oat “seeds” we are referring to intact seeds, not to “caryopses.” (B) “Model system,” method of incubation of wild oat caryopses on Fusarium avenaceum strain F.a.1. (left) and untreated control (right). Wild oat M73 was chosen as the subject based on extreme dormancy level (Naylor and Fedec, 1978), allowing extended incubations.

FIGURE 2

Model for simultaneous activation and release of constitutive polyphenol oxidase (PPO) by proteolysis on the surface of a wild oat caryopsis following pathogen challenge. Such a mechanism would not exclude PPO induction by other mechanisms, such as transcriptional induction. Both o-quinones and melanins are hypothesized to have anti-microbial properties (Constabel and Barbehenn, 2008).