Shifts Between and Among Populations of Wheat Rhizosphere Pseudomonas, Streptomyces and Phyllobacterium Suggest Consistent Phosphate Mobilization at Different Wheat Growth Stages Under Abiotic Stress

Abstract

Climate change models predict more frequent and prolonged drought events in Central Europe, which will exert extraordinary pressure on agroecosystems. One of the consequences is drought-related nutrient limitations for crops negatively affecting agricultural productivity. These effects can be mitigated by beneficial plant growth promoting rhizobacteria. In this study, we investigated the potential of cultivable bacterial species for phosphate solubilization in the rhizosphere of winter wheat at two relevant growth stages - stem elongation and grain filling stages. Rhizosphere samples were collected in the Global Change Experimental Facility in Central Germany, which comprises plots with conventional and organic farming systems under ambient and future climate. Phosphate-solubilizing bacteria were selectively isolated on Pikovskaya medium, phylogenetically classified by 16S rRNA sequencing, and tested for in vitro mineral phosphate solubilization and drought tolerance using plate assays. The culture isolates were dominated by members of the genera Phyllobacterium, Pseudomonas and Streptomyces. Cultivation-derived species richness and abundance of dominant taxa, especially within the genera Phyllobacterium and Pseudomonas, as well as composition of Pseudomonas species were affected by wheat growth stage. Pseudomonas was found to be more abundant at stem elongation than at grain filling, while for Phyllobacterium the opposite pattern was observed. The abundance of Streptomyces isolates remained stable throughout the studied growth stages. The temporal shifts in the cultivable fraction of the community along with considerable P solubilization potentials of Phyllobacterium and Pseudomonas species suggest functional redundancy between and among genera at different wheat growth stages. Phosphate-solubilizing Phyllobacterium species were assigned to Phyllobacterium ifriqiyense and Phyllobacterium sophorae. It is the first time that phosphate solubilization potential is described for these species. Since Phyllobacterium species showed the highest drought tolerance along all isolates, they may play an increasingly important role in phosphate solubilization in a future dryer climate.

Keywords: PGPR; agriculture; climate change; drought tolerance; phosphate solubilization; wheat.

FIGURE 1

Identity, abundance and activity levels of cultivable mineral phosphate-solubilizing bacteria in the wheat rhizospheres. Taxa have been summarized on genus level and the most dominant ones, with a relative abundance ≥2% within the collection, are shown in the pie charts at (A) stem elongation stage in May 2015 and (B) grain filling stage in July 2015. The predominantly isolated taxa of the genus Phyllobacterium, Pseudomonas and Streptomyces were clustered (identity level of 99.5%) and (C) phylogenetic trees were created for dominant clusters (number of isolates >3) with reference sequences from NCBI. Information on abundance of the dominant clusters within treatment levels of growth stage (stem elongation in May, grain filling in July), climate (A-ambient, F-future) and land use (CF-conventional farming, OF-organic farming) are given as circles, whereas circle size is related to the number of isolates. Moreover, the average phosphate solubilizing potentials for each bacterial 16S rRNA gene cluster were calculated for PSI 1 and PSI 2 and differences in expression levels are indicated using a heatmap.

FIGURE 2

Effect of time on composition of isolated rhizobacterial taxa among and within the dominating genera Phyllobacterium, Pseudomonas and Streptomyces. Running betadisper function based on Bray-Curtis dissimilarity (A) dispersion patterns of the isolated taxa are presented in an ordination plot. Diversity indices (B) species richness and (C) Shannon-Weaver-Index were calculated, followed by ANOVA, testing for differences in isolated species composition of the three dominant genera between the two sampling points, May (stem elongation stage, BBCH 37-39) and July (grain filling stage, BBCH 75-77) 2015, respectively.

FIGURE 3

Phosphate-solubilizing potentials of the three dominant genera. Comparison of the two indices, (A) PSI 1 and (B) PSI 2 for Phyllobacterium, Pseudomonas und Streptomyces isolates at stem elongation in May and at grain filling in July 2015 sampled from wheat rhizosphere. Standard errors are given in the bars. Significant differences were calculated for the both indices separately and are marked by different letters (ANOVA and Tukey HSD Test).