Polyphenol oxidase affects normal nodule development in red clover (Trifolium pratense L.)

Abstract

Polyphenol oxidase (PPO) may have multiple functions in tissues depending on its cellular or tissue localization. Here we use PPO RNAi transformants of red clover (Trifolium pratense) to determine the role PPO plays in normal development of plants, and especially in N2-fixing nodules. In red clover, PPO was not essential for either growth or nodule production, or for nodule function in plants grown under optimal, N-free conditions. However, absence of PPO resulted in a more reduced environment in all tissues, as measured by redox potential, and caused subtle developmental changes in nodules. Leaves and, to a lesser extent nodules, lacking PPO tended to accumulate phenolic compounds. A comparison of nodules of two representative contrasting clones by microscopy revealed that nodules lacking PPO were morphologically and anatomically subtly altered, and that phenolics accumulated in different cells and tissues. Developing nodules lacking PPO were longer, and there were more cell layers within the squashed cell layer (SCL), but the walls of these cells were less thickened and the cells were less squashed. Within the N2-fixing zone, bacteroids appeared more granular and were less tightly packed together, and were similar to developmentally compromised bacteroids elicited by catalase mutant rhizobia reported elsewhere.

Rhizobium-Linked Nutritional and Phytochemical Changes Under Multitrophic Functional Contexts in Sustainable Food Systems

“…Rhizobium usually evades such plant defenses in successful infections to induce root nodulation (Tóth and Stacey, 2015;Cao et al, 2017). However, polyphenol oxidase may not be responsible for suppression of Rhizobium nodulation, because the enzyme improves the development, structure, and function of root nodules (Webb et al, 2014). Rhizobium inoculation results in high concentration of nitrogen-based compounds including polyphenol oxidase, while reducing the concentration of phenolic compounds and other carbon-based metabolites (Karoney et al, 2020).…”

“…Rhizobium inoculation results in high concentration of nitrogen-based compounds including polyphenol oxidase, while reducing the concentration of phenolic compounds and other carbon-based metabolites (Karoney et al, 2020). The absence of polyphenol oxidase results in the accumulation of phenolic compounds (Webb et al, 2014), which are not viable sources of organic carbon for Rhizobium development and function compared to the C 4 compounds (fumarate, malate and succinate) (Mitsch et al, 2018).…”

Levels of polyphenol oxidase activity in leaves of Milicia seedlings is indicative of the resistance to Phytolyma lata‐induced gall disease

“…Such localized increase in PPO activity has been demonstrated in several insect-induced plant galls. And in plant cultivars that are resistant to galling, the increase in gall PPO activity induces localized oxidative stress in the gall nutritive tissues, which presumably prevents further feeding by the galling insect and cause the cessation of gall proliferation (Levin, 1971;Kahl, 1982;Araji et al, 2014;Webb et al, 2014). Thus, the increased PPO activity in M. regia galls compared with the surrounding leaf lamina is indicative of a strong post-infestation resistance response against the galling insect in M. regia seedlings.…”

Red Clover HDT, a BAHD Hydroxycinnamoyl-Coenzyme A:L-3,4-Dihydroxyphenylalanine (L-DOPA) Hydroxycinnamoyl Transferase That Synthesizes Clovamide and Other N-Hydroxycinnamoyl-Aromatic Amino Acid Amides

“…Recent work in plants indicates at least two pathways whereby L-DOPA moieties are made, one involving PPO (Araji et al, 2014) and another involving cytochrome P450 enzymes (Sunnadeniya et al, 2016). It is not known how L-DOPA moieties are made in red clover, but silencing of the major PPO genes in red clover does not appear to reduce levels of clovamide ( (Webb et al, 2014) and additional unpublished data), suggesting another pathway, perhaps involving a cytochrome P450, acting on either free L-Tyr or the L-Tyr moiety of a hydroxycinnamoyl-L-Tyr amide. Given that HDT appears to favor use of L-Tyr over L-DOPA, if L-DOPA moieties are formed from L-Tyr and used for clovamide biosynthesis, substrate channeling might be required to favor L-DOPA utilization.…”