A proposed mechanism for nitrogen acquisition by grass seedlings through oxidation of symbiotic bacteria

Abstract

In this paper we propose and provide evidence for a mechanism, oxidative nitrogen scavenging (ONS), whereby seedlings of some grass species may extract nitrogen from symbiotic diazotrophic bacteria through oxidation by plant-secreted reactive oxygen species (ROS). Experiments on this proposed mechanism employ tall fescue (Festuca arundinaceae) seedlings to elucidate features of the oxidative mechanism. We employed 15N2 gas assimilation experiments to demonstrate nitrogen fixation, direct microscopic visualization of bacteria on seedling surfaces to visualize the bacterial oxidation process, reactive oxygen probes to test for the presence of H2O2 and cultural experiments to assess conditions under which H2O2 is secreted by seedlings. We also made surveys of the seedlings of several grass species to assess the distribution of the phenomenon of microbial oxidation in the Poaceae. Key elements of the proposed mechanism for nitrogen acquisition in seedlings include: 1) diazotrophic bacteria are vectored on or within seeds; 2) at seed germination bacteria colonize seedling roots and shoots; 3) seedling tissues secrete ROS onto bacteria; 4) bacterial cell walls, membranes, nucleic acids, proteins and other biological molecules are oxidized; 5) nitrates and/or smaller fragments of organic nitrogen-containing molecules resulting from oxidation may be absorbed by seedling tissues and larger peptide fragments may be further processed by secreted or cell wall plant proteases until they are small enough for transport into cells. Hydrogen peroxide secretion from seedling roots and bacterial oxidation was observed in several species in subfamily Pooideae where seeds possessed adherent paleas and lemmas, but was not seen in grasses that lacked this feature or long-cultivated crop species.