Background

Symbiosis between soil microbes and crops is a key pillar of agricultural productivity, influencing nitrogen fixation, nutrient uptake, and pest defense. To meet the demand for reducing chemical fertilizers and pesticides, continuous efforts have been made to supply microbial agents with beneficial traits to the soil.

However, selectively controlling genetically modified microbes once released into outdoor fields has been nearly impossible. Spraying specific chemicals across entire soil areas is not only costly but also imposes unexpected burdens on the soil ecosystem. The risk of biological containment failure—where engineered bacteria escape target crops to proliferate around weeds or non-target plants, or spread to rivers via drainage—has been a persistent challenge.

Researchers sought a solution in establishing a molecular communication network where crops themselves regulate the physiological activity of beneficial bacteria. The concept involves using non-natural amino acids, which do not exist in nature, as a signal code to design microbes that synthesize designated proteins only around the roots of target plants.

Key Findings

A research team led by Professor Jennifer A. Brophy (Jennifer A. N. A research team led by Professor Brophy at Stanford University announced the establishment of an artificial signaling system that controls protein expression in soil bacteria within plants using Genetic Code Expansion (GCE) technology based on non-canonical amino acids (ncAAs). The research results were published in the latest issue of the international journal Nature Biotechnology.

The chemical signal selected by the researchers is O-methyl-L-tyrosine (OMe-Tyr), a tyrosine analog. To create this orthogonal molecule that does not participate in plant metabolism, the team introduced the OMe-Tyr biosynthetic enzyme pathway into the model plant Arabidopsis thaliana, a representative horticultural crop tomato, and a woody plant poplar. The root tissues of each plant synthesized OMe-Tyr and secreted it externally, providing a stable supply to the rhizosphere soil.

Bacillus subtilis, an agriculturally useful bacterium, was selected as the target for receiving the signal. The research team introduced an orthogonal aminoacyl-tRNA synthetase/tRNA pair specific to OMe-Tyr into the B. subtilis genome and inserted the UAG (amber) stop codon within the target protein gene sequence to induce expression.

In this system, B. subtilis recognizes the UAG codon as OMe-Tyr and produces intact proteins only in environments where the OMe-Tyr secreted by plant roots is present. In general soil environments lacking the OMe-Tyr signal, protein synthesis immediately halts because the ribosome stops translation at the UAG position.

In actual symbiotic conditions, experiments targeting Green Fluorescent Protein (GFP) revealed that only bacteria colonizing plant roots secreting OMe-Tyr exhibited strong fluorescence. The researchers noted that the rate of bacterial protein expression induction increased more than 10-fold compared to non-secreting control plants or general soil conditions.

Significance and Outlook

This study is considered the first instance of directly mediating communication between different kingdoms—plants and microbes—using a synthetic molecular language.

Existing microbial fertilizers often suffer from diluted concentrations due to wind, rain, or soil erosion after application, or they disperse to the roots of surrounding weeds, failing to provide sufficient efficacy for the target crop. Using this developed technology, it is possible to induce engineered bacteria to produce nitrogen-fixing enzymes or pathogen-inhibiting substances only when they precisely reach the rhizosphere of the target crop. Essentially, the crop directly controls the switch for microbial production.

A breakthrough has also been achieved to address safety concerns regarding the release of genetically modified organisms into the wild. By embedding a UAG codon into an enzyme gene essential for bacterial survival, bacteria that leave the rhizosphere of the designated crop secreting OMe-Tyr will fail to synthesize proteins and die. This enables a strict biological containment mechanism to function without the need for artificial chemical spraying.

However, clear hurdles remain before commercialization in agricultural fields. Long-term demonstration cultivation is required to ensure that the process of synthesizing and secreting OMe-Tyr does not cause metabolic burdens, such as growth delays or yield reductions, in plants. The potential for indigenous microbes in complex soil environments to degrade or modify OMe-Tyr, thereby reducing signal strength, is also a challenge that must be addressed in future studies.