Plants cannot use inert nitrogen (N) from the atmosphere, and for most crops, N additions via synthetic fertilizers is required. Developing practices that move away from synthetic nitrogen fertilizer will contribute to sustainable food production and food security1. Certain crops, such as legumes, enter into a beneficial relationship with bacteria, known as rhizobia. In specialized root structures known as nodules, rhizobia chemically transform inert nitrogen into usable, bioreactive nitrogen - a process referred to as N-fixation. Previous attempts to engineer crops to perform N-fixation have failed2, in part because the bacterial enzymes that perform N- fixation require the biochemical conditions in root nodules3.
ARSX 2020 prize-winning SYMBIONT technology is a new way to deliver crop therapeutics. SYMBIONTs rely on another type of plant-associated bacteria, referred to as Agrobacterium. In nature, Agrobacterium are responsible for crown gall disease on plants, which you may have previously seen as a large growth on a tree while hiking in a forest. Using genetic engineering, we modify Agrobacterium galls into beneficial SYMBIONTs. Even though SYMBIONTS are made on the plant stem, we unexpectedly observed striking parallels between the environment inside SYMBIONTs and root nodules, suggesting the SYMBIONT environment may support N-fixation.
Our 2023 ARSX project will determine whether SYMBIONTS can be used to fix nitrogen. With two independent strategies, we will: (1) identify pairs of gall-forming Agrobacterium and nodule-forming Rhizobium that can productively coexist in SYMBIONTs and (2) express the bacterial enzymes responsible for nitrogen fixation (the nitrogenase complex) directly within the SYMBIONT. Strategy 1 will use strains from the ARS NRRL collection and could yield an entirely ‘natural’ solution, while Strategy 2 will leverage existing designs for nitrogenase expression in plants.
