The rhizosphere supports the development and activity of a huge and diversified microbial community, including microorganisms capable to promote plant growth. Among the latter, plant growth-promoting rhizobacteria (PGPR) colonize roots of monocots and dicots, and enhance plant growth by direct and indirect mechanisms. Modification of root system architecture by PGPR implicates the production of phytohormones and other signals that lead, mostly, to enhanced lateral root branching and development of root hairs. PGPR also modify root functioning, improve plant nutrition and influence the physiology of the whole plant. Recent results provided first clues as to how PGPR signals could trigger these plant responses. Whether local and/or systemic, the plant molecular pathways involved remain often unknown. From an ecological point of view, it emerged that PGPR form coherent functional groups, whose rhizosphere ecology is influenced by a myriad of abiotic and biotic factors in natural and agricultural soils, and these factors can in turn modulate PGPR effects on roots. In this paper, we address novel knowledge and gaps on PGPR modes of action and signals, and highlight recent progress on the links between plant morphological and physiological effects induced by PGPR. We also show the importance of taking into account the size, diversity, and gene expression patterns of PGPR assemblages in the rhizosphere to better understand their impact on plant growth and functioning. Integrating mechanistic and ecological knowledge on PGPR populations in soil will be a prerequisite to develop novel management strategies for sustainable agriculture.

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Plant growth-promoting rhizobacteria and root system functioning.

A variety of soil factors are known to increase nutrient availability and plant productivity. The most influential might be the organisms comprising the soil microbial community of the rhizosphere, which is the soil surrounding the roots of plants where complex interactions occur between the roots, soil, and microorganisms. Root exudates act as substrates and signaling molecules for microbes creating a complex and interwoven relationship between plants and the microbiome. While individual microorganisms such as endophytes, symbionts, pathogens, and plant growth promoting rhizobacteria are increasingly featured in the literature, the larger community of soil microorganisms, or soil microbiome, may have more far-reaching effects. Each microorganism functions in coordination with the overall soil microbiome to influence plant health and crop productivity. Increasing evidence indicates that plants can shape the soil microbiome through the secretion of root exudates. The molecular communication fluctuates according to the plant development stage, proximity to neighboring species, management techniques, and many other factors. This review seeks to summarize the current knowledge on this topic.

http://webdoc.agsci.colostate.edu/soilcrop/course/socr571_reading11.pdf

Manipulating the soil microbiome to increase soil health and plant fertility

Enhancement of drought stress tolerance in crops by plant growth promoting rhizobacteria.

Bacterial volatiles induce systemic resistance in Arabidopsis (vol 134, pg 1017, 2004)

Bacterial interactions with neighboring microorganisms via production of small metabolites enable bacteria to respond and adapt to environmental changes. The study of intercellular interactions primarily focused on soluble metabolites, but bacteria also produce and release into their headspace a wide variety of volatile secondary metabolites, the ecological roles of which have generally been overlooked. However, bacterial volatile compounds are known to contribute to interkingdom interactions (plant, fungi and nematodes), and recent studies also identified their at-a-distance influence on bacterial behavior. The present review describes the biological roles of bacterial volatile compounds in inter- and intraspecies bacterial interactions, a new and yet unexplored research area, with potential clinical and industrial applications.

Role of bacterial volatile compounds in bacterial biology

Choline (Cho) is an essential nutrient for humans as well as the precursor of glycine betaine (GlyBet), an important compatible solute in eukaryotes that protects cells from osmotic stress caused by dehydrating conditions. The key enzyme for plant Cho synthesis is phosphoethanolamine N-methyltransferase (PEAMT), which catalyzes all three methylation steps, including the rate-limiting N-methylation of phosphoethanolamine. Herein, we report that the beneficial soil bacterium Bacillus subtilis (strain GB03) enhances Arabidopsis Cho and GlyBet synthesis associated with enhanced plant tolerance to osmotic stress. When stressed with 100 mM exogenous mannitol, GB03-exposed plants exhibit increased transcript level of PEAMT compared with stressed plants without bacterial exposure. Endogenous Cho and GlyBet metabolite pools were elevated by more than two- and fivefold, respectively, by GB03 treatment, consistent with increased stress tolerance. Moreover, in the xipotl mutant line with reduced Cho production, a loss of GB03-induced drought tolerance is observed. Osmotic-stressed plants with or without GB03 exposure show similar levels of abscsisic acid (ABA) accumulation in both shoots and roots, suggesting that GB03-induced osmoprotection is ABA independent. GB03 treatment also improves drought tolerance in soil-grown plants as characterized by phenotypic comparisons, supported by an elevated accumulation of osmoprotectants. These results provide a biological strategy to enhance Cho biosynthesis in plants and, in turn, increase plant tolerance to osmotic stress by elevating osmoprotectant accumulation.

https://apsjournals.apsnet.org/doi/pdf/10.1094/MPMI-23-8-1097

Cheryl Murzello

Papers

Choline and osmotic-stress tolerance induced in Arabidopsis by the soil microbe Bacillus subtilis (GB03).