Abstract
Despite growing evidence that plant growth-promoting bacteria can be used to improve crop vigor, a comparison of the different methods of delivery to determine which is optimal has not been published. An optimal inoculation method ensures that the inoculant colonizes the host plant so that its potential for plant growth-promotion is fully evaluated. The objective of this study was to compare the efficacy of three seed coating methods, seedling priming, and soil drench for delivering three bacterial inoculants to the sorghum rhizosphere and root endosphere. The methods were compared across multiple time points under axenic conditions and colonization efficiency was determined by quantitative polymerase chain reaction (qPCR). Two seed coating methods were also assessed in the field to test the reproducibility of the greenhouse results under non-sterile conditions. In the greenhouse seed coating methods were more successful in delivering the Gram-positive inoculant (Terrabacter sp.) while better colonization from the Gram-negative bacteria (Chitinophaga pinensis and Caulobacter rhizosphaerae) was observed with seedling priming and soil drench. This suggested that Gram-positive bacteria may be more suitable for the seed coating methods possibly because of their thick peptidoglycan cell wall. We also demonstrated that prolonged seed coating for 12 h could effectively enhance the colonization of C. pinensis, an endophytic bacterium, but not the rhizosphere colonizing C. rhizosphaerae. In the field only a small amount of inoculant was detected in the rhizosphere. This comparison demonstrates the importance of using the appropriate inoculation method for testing different types of bacteria for their plant growth-promotion potential.
Highlights
Plants and soil microbiomes have interacted and co-evolved for over a million years
Chitinophaga pinensis isolated from sorghum root endosphere, as well as Terrabacter sp. and C. rhizosphaerae isolated from soil where sorghum was growing, were used for inoculation in this study
The colonization of C. rhizosphaerae in the rhizosphere was greater (104–105 copies per ng of rhizosphere DNA) when the seedling priming and soil drench method were used as compared to the seed coating approaches (102–103 copies per ng of rhizosphere DNA; Figure 1A)
Summary
Many soil-inhabiting microbes are capable of improving plant growth (Delaux and Schornack, 2021). To facilitate close interactions with these microbes, plants release rhizodeposits from plant roots into the rhizosphere, a soil layer adhering to the root, to serve as carbon sources and as signaling cues to these microbes. Apart from interacting with plants, some microbes can further colonize the inner root zone termed endosphere and those microbes are known as root endophytes. The intimate association between root endophytes and root tissues may enhance the exchange of nutrients between plants and microbes (Harman and Uphoff, 2019). The colonization of rhizobia inside root nodules allows these bacteria to fix N more efficiently due to the hypoxic conditions in the nodules, the fixed N is supplied to the host plant in exchange for carbon (Ledermann et al, 2021)
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