Transacetalization of Chitosan with Trehalose to Enhance Rhizobacteria Preservation and Boost Roots Colonization

The application of plant-growth-promoting rhizobacteria in open-field agriculture is challenged by the lack of adequate technologies to preserve them in anhydrous conditions and deliver them to the rhizosphere. Here, the transacetalization of chitosan with trehalose allows the synthesis of a new biopolymer, namely chito_tre, that can support the encapsulation, storage, and delivery of plant growth-promoting rhizobacteria (PGPRs) in a seed-coating format. In the liquid phase, chito_tre preserves largely used PGPRs such as Rhizobium tropici, Azorhizobium caulinodans, Bradyrhizobium japonicum, Klebsiella variicola, and Pseudomonas fluorescens, with a log reduction <1 at 48 h and at room temperature. In the solid phase, chito_tre enables the preservation of PGPRs up to 28 days, with a log reduction at room temperature circa equal to 2 (K. variicola), 4 (B. japonicum, A. caulinodans, P. fluorescens), and 5 (R. tropici), depending on the microorganism considered. When applied as a seed coating, chito_tre loaded with PGPRs facilitates root colonization in Cicer arietinum (chickpea), Glycine max (soybean), Sesbania sesban (Egyptian riverhemp), and Zea mays (corn), boosts root development, and enables a synergistic strategy to enhance plant growth. Together, these results demonstrate the functionalization of largely available biopolymers with osmoprotectants to establish a new class of seed-coating materials that can enhance plant growth.