(17f) Scale-up of Protist-Facilitated Biotechnology for Transporting Agrochemicals and Beneficial Bacteria Along Plant Roots

Protists are adapted to move rapidly through unsaturated soils and are known to chemotact towards bacterial prey abundant at growing root tips. Filter-feeding soil protists accumulate bacteria-sized particles from their environment, and the technology for nano- and micro-encapsulating agrochemicals is now available. Protists may therefore serve as a vehicle for transporting encapsulated agrochemicals and plant growth promoting rhizobacteria (PGPR) through soil and targeting their delivery to the growing tips of plants roots. Four different soil protists were isolated from soil and established in laboratory culture. A transport assay comprised of an emulated soil micromodel (a PDMS-based microfluidic device) was developed to screen candidate soil protists for performance in taking up, transporting, and depositing fluorescent microspheres or polymer-encapsulated nanoparticles through the physical structure of sandy loam soil. Custom tracking software was developed to quantify the abundance of both ingested and deposited fluorescent particles with position and time. Emulated soil micromodels were shown to be highly effective in differentiating protist-facilitated transport from passive particle movement via Brownian motion, with no flow-induced artifacts. A filter-feeding, ciliated, protist species belonging to the Colpoda genus was found to be a high-performing organism in the emulated soil micromodel transport assay. Then, a second scaled-up transport assay was created that contained real sandy loam soil and a live Medicago truncatula plant contained within a 3D-printed growth chamber. These mesoscale devices are modeled after larger “rhizobox” chambers and are referred to here as “rhizoslides”. The most promising protist candidate from the emulated soil micromodel transport assay was studied using the rhizoslides. Here, position of fluorescent particles or fluorescent protein-expressing PGPR (bacteria) was tracked weekly in replicate rhizoslides that either contained or did not contain live protists. Inclusion of Colpoda was shown to facilitate transport of fluorescent particles and of bacteria along growing plant roots. Also, rhizoslides containing protists exhibited bacterial colonies located further down plant roots compared with bacteria-only rhizoslides. Even though protists are the natural predators of the bacteria, the inclusion of protists enhances transport of bacteria to the distal region of plant roots: the portion most in need of colonization by PGPR. Facilitating transport and targeting delivery of valuable agrochemicals and bacteria to the tips of growing plant roots will greatly facilitate the implementation of more sustainable no-till farming practices, reduce overall agrochemical use, and improve surface water quality.