Defense enzyme responses in dormant wild oat and wheat caryopses challenged with a seed decay pathogen

Authors: JAMES, MATTHEW - Washington State University; POLLARD, ANNE - Washington State University; Okubara, Patricia; FUERST, E - Washington State University

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/27/2017
Publication Date: 1/22/2018
Citation: James, M.S., Pollard, A.T., Okubara, P.A., Fuerst, E.P. 2018. Defense enzyme responses in dormant wild oat and wheat caryopses challenged with a seed decay pathogen. Frontiers in Plant Science. 8:2259.

Interpretive Summary: Weed seeds persist in the soil through dormancy and resistance to seed decay organisms and herbivores. Seeds have well-established physical, chemical, and biological defense mechanisms that protect them from decay-inducing organisms and herbivores. To address a knowledge gap in the biochemical mechanisms underlying resistance, we quantified the defense enzyme activities in dormant wild oat seed. We showed that these seeds release three defense enzymes into their environmental upon interaction with a seed-decay fungus, and that a fourth enzyme that is active in leaf and root defense did not play a role in seed defense. Our findings indicate that dormant weed seeds are capable of mounting complex biochemical defenses against pathogens.

Technical Abstract: Seed dormancy and resistance to seed decay organisms are fundamental ecological strategies for weed seed persistence in the weed seed-bank. Seeds have well-established physical, chemical, and biological defense mechanisms that protect their food reserves from decay-inducing organisms and herbivores. However, there are few studies demonstrating biochemical responses of dormant weed seeds to pathogens. Wild oat and wheat caryopses were incubated with the pathogenic Fusarium avenaceum strain F.a. 1 for two to three days. In whole caryopsis assays, the defense enzymes polyphenol oxidase (PPO), exochitinase (CHI), and peroxidase (POD) were induced but oxalate oxidase (OxO) was reduced in both wild oat and wheat in response to the pathogen. Leachates were made from wild oat and wheat caryopses incubated ± F.a. 1 to characterize the soluble fraction of these activities that was released into the environment. The level of induction by F.a. 1 was greater in the leachates than in whole caryopsis assays for PPO, CHI, and POD, suggesting that much of the induced enzyme activity was released into the surrounding environment. OxO levels were very low in the leachates indicating that most OxO activity was in some way bound to the caryopsis surface. Comparisons of enzyme activities in caryopsis and mycelia leachates suggested that the caryopsis was the primary source of soluble PPO and POD, whereas the mycelia were a major source of soluble CHI. Results are consistent with the hypothesis that dormant weed seeds are capable of mounting complex biochemical defenses against pathogens. These biochemical defenses may apply to a greater diversity of defense enzymes, plant species, and ecosystems.