Wild Wheat Rhizosphere-Associated Plant Growth-Promoting Bacteria Exudates: Effect on Root Development in Modern Wheat and Composition

Abstract

Diazotrophic bacteria isolated from the rhizosphere of a wild wheat ancestor, grown from its refuge area in the Fertile Crescent, were found to be efficient Plant Growth-Promoting Rhizobacteria (PGPR), upon interaction with an elite wheat cultivar. In nitrogen-starved plants, they increased the amount of nitrogen in the seed crop (per plant) by about twofold. A bacterial growth medium was developed to investigate the effects of bacterial exudates on root development in the elite cultivar, and to analyze the exo-metabolomes and exo-proteomes. Altered root development was observed, with distinct responses depending on the strain, for instance, with respect to root hair development. A first conclusion from these results is that the ability of wheat to establish effective beneficial interactions with PGPRs does not appear to have undergone systematic deep reprogramming during domestication. Exo-metabolome analysis revealed a complex set of secondary metabolites, including nutrient ion chelators, cyclopeptides that could act as phytohormone mimetics, and quorum sensing molecules having inter-kingdom signaling properties. The exo-proteome-comprised strain-specific enzymes, and structural proteins belonging to outer-membrane vesicles, are likely to sequester metabolites in their lumen. Thus, the methodological processes we have developed to collect and analyze bacterial exudates have revealed that PGPRs constitutively exude a highly complex set of metabolites; this is likely to allow numerous mechanisms to simultaneously contribute to plant growth promotion, and thereby to also broaden the spectra of plant genotypes (species and accessions/cultivars) with which beneficial interactions can occur.

Agricultural intensification reduces selection of putative plant growth-promoting rhizobacteria in wheat

“…Microbiota of crop ancestors may offer a way to enhance sustainable food production, as discussed by the rewilding hypothesis [ 98 ]. Future work will involve performing what is known as a microbiome transplant whereby the microbes we have identified as being lost through domestication will be inoculated to modern wheat varieties to identify whether beneficial effects from these microbes can be re-established, which has already been demonstrated [ 99 ], analogous to fecal microbiota transplantation to redirect the dysbiotic composition of the human microbiome. Alternatively, the reintroduction of key genetic elements to modern plants from their ancestors, by selective breeding or gene editing, for the selection of beneficial microbiota from the bulk soil reservoir is another promising strategy.…”