Soil Microbial Communities Affect the Growth and Secondary Metabolite Accumulation in Bletilla striata (Thunb.) Rchb. f

Abstract

Bletilla striata (Thunb.) Rchb.f. is a perennial herb belonging to the Orchidaceae family. Its tubers are used in traditional Chinese medicine to treat gastric ulcers, inflammation, silicosis tuberculosis, and pneumogastric hemorrhage. It has been reported that different soil types can affect the growth of B. striata and the accumulation of secondary metabolites in its tubers, but the biological mechanisms underlying these effects remain unclear. In this study, we compared agronomic traits and the accumulation of secondary metabolites (extractum, polysaccharide, total phenol, militarine) in B. striata grown in sandy loam or sandy clay soil. In addition, we compared physicochemical properties and microbial communities between the two soil types. In pot experiments, we tested how irradiating soil or transplanting microbiota from clay or loam into soil affected B. striata growth and accumulation of secondary metabolites. The results showed that sandy loam and sandy clay soils differed significantly in their physicochemical properties as well as in the structure and composition of their microbial communities. Sandy loam soil had higher pH, SOM, SOC, T-Ca, T-N, T-Mg, T-Mn, T-Zn, A-Ca, A-Mn, and A-Cu than sandy clay soil, but significantly lower T-P, T-K, T-Fe, and A-P content. Sandy loam soil showed 7.32% less bacterial diversity based on the Shannon index, 19.59% less based on the Ace index, and 24.55% less based on the Chao index. The first two components of the PCoA explained 74.43% of the variation in the bacterial community (PC1 = 64.92%, PC2 = 9.51%). Similarly, the first two components of the PCoA explained 58.48% of the variation in the fungal community (PC1 = 43.67%, PC2 = 14.81%). The microbiome associated with sandy clay soil can promote the accumulation of militarine in B. striata tubers, but it inhibits the growth of B. striata. The accumulation of secondary metabolites such as militarine in B. striata was significantly higher in sandy clay than in sandy loam soil. Conversely, B. striata grew better in sandy loam soil. The microbiome associated with sandy loam soil can promote the growth of B. striata, but it reduces the accumulation of militarine in B. striata tubers. Pot experiment results further confirmed that the accumulation of secondary metabolites such as militarine was higher in soil transplanted with loam microbiota than in soil transplanted with clay microbiota. These results may help guide efforts to improve B. striata yield and its accumulation of specific secondary metabolites.

Keywords: Bletilla striata; sandy clay soil; sandy loam soil; secondary metabolites; soil microbiome.

FIGURE 1

Impact of soil type on growth and secondary metabolite content in B. striata. (A) Representative image of sandy clay soil (top left, red bars) and sandy loam soil (top right, blue bars) and Bletilla striata (Thunb.) Rchb.f. plants grown in these soils. Scale bars: 2 or 10 cm. (B) Dry weight of aboveground biomass, belowground biomass, tubers, and fibrous roots of B. striata planted in sandy clay or sandy loam soils. (C) Height of B. striata planted in sandy clay or sandy loam soils. (D) Numbers of tubers, stems, leaves, and flowers of B. striata planted in sandy clay or sandy loam soils. (E) Numbers of tubers, stems, leaves, and flowers on each stem of B. striata planted in sandy clay or sandy loam soils. (F) Ratio of leaf length to width in B. striata planted in sandy clay or sandy loam soils. Content of (G) extractum, (H) polysaccharides, (I) total phenols, and (J) militarine in B. striata planted in sandy clay and sandy loam soils. Data are mean ± SEM (n = 5). *p < 0.05, ^**p < 0.01, and ^***p < 0.005 based on the independent-sample t-test.

FIGURE 2

Physicochemical properties and elemental concentrations in sandy clay and sandy loam soils. (A) Soil pH, (B) soil organic matter (SOM), (C) soil organic carbon (SOC), and (D) ammonium nitrogen concentration in sandy clay soil (red bars) and sandy loam soil (blue bars). (E,F) Total nutrient element content in sandy clay and sandy loam soils: total calcium (T-Ca), total phosphorus (T-P), total nitrogen (T-N), total potassium (T-K), total iron (T-Fe), total magnesium (T-Mg), total manganese (T-Mn), total zinc (T-Zn), and total copper (T-Cu). (G,H) Available element content in sandy clay and sandy loam soils: available calcium (A-Ca), available phosphorus (A-P), available potassium (A-K), available iron (A-Fe), available manganese (A-Mn), available zinc (A-Zn), and available copper (A-Cu). Data are mean ± SEM (n = 5). *p < 0.05, ^**p < 0.01, and ^***p < 0.005 based on the independent-sample t-test.

FIGURE 3

Composition and structure of microbial communities in sandy clay and sandy loam soils. (A,B) Alpha diversity at the level of OTUs in (A) bacterial and (B) fungal communities in sandy clay soil (red) and sandy loam soil (blue). Microbial diversity was quantified using Chao’s, Ace’s, and Shannon’s diversity indices (n = 5). (C,D) Principal component analysis (PCoA) of beta diversity in (C) bacterial and (D) fungal communities in sandy clay and sandy loam soils, based on Bray-Curtis distances. (E,F) Relative abundances of (E) bacteria and (F) fungi in sandy clay and sandy loam soils. Data are mean ± SEM (n = 5). *p < 0.05 and ^***p < 0.005 based on the independent-sample t-test.

FIGURE 4

Relative abundances of microbial genera in sandy loam and sandy clay soils. Relative abundances of (A) fungal genera and (B) bacterial genera in sandy loam and sandy clay soils. Data are mean ± SEM (n = 5). *p < 0.05, ^**p < 0.01, and ^***p < 0.005 based on the independent-sample t-test.

FIGURE 5

Effects of microbial community on growth and secondary metabolite content in B. striata. (A) Schematic depicting planting of B. striata seedlings in experimental pots containing irradiated soil (CK), clay-microbiota transplanted soil (CMTS), or loam-microbiota transplanted soil (LMTS). (B) Representative photographs of B. striata plants grown under the three conditions. (C–I) Post-treatment quantification of the following B. striata parameters: (C) Aboveground dry weight, (D) belowground dry weight, (E) height, (F) ratio of leaf length to width, (G) number of roots, (H) root length, and (I) root width. (J–M) Post-treatment quantification of the following B. striata parameters: (J) Extractum content, (K) polysaccharide content, (L) militarine content, and (M) total phenol content in B. striata. Data are mean ± SEM (n = 5). *p < 0.05 and **p < 0.01 based on the independent-sample t-test.