Figure 1.
Pairwise correlations between (-)-catechin concentration, monthly precipitation, and bacterial populations.
(A) Correlation between the concentration of (-)-catechin in soil of R. formosanum and monthly precipitation (r = 0.919, P < 0.0001, n = 14). (B) Correlation between the concentration of (-)-catechin and bacterial populations in soil of R. formosanum (r = -0.714, P = 0.0041, n = 14).
Figure 2.
Bacterial flora (A) and catechin utilizing bacteria (B) in the rhizosphere of a Rhododendron formosanum plantation.
(-)-Catechin was the only carbon source added to the medium that was used for microbial isolation. After 3 months of incubation, Pseudomonas spp., Burkholderia spp., Variovorax spp. Stenotrophomonas spp., and Pandoraea spp. were isolated and identified as dominant catechin-utilizing bacteria.
Figure 3.
Metabolic pathway for (-)-catechin transformed by Pseudomonas sp. CRF3-Ps-1 was analysed by the LC-ESI-MS/MS method (A).
(-)-Catechin (CAT) was transformed into taxifolin (Tax) via ketone formation during the first 24 h. Subsequently, C-ring hydrolysis occurred and generated protocatechuic acid (PCA) and glycerol (Gly). Finally, (-)-catechin was transformed into glycerol 72 h after incubation. The possible transformation hypothesis is also illustrated (B).
Figure 4.
Relative concentrations of (-)-catechin and protocatechuic acid, and the bacterial population in the medium, during 120 h incubation with Pseudomonas sp. CRF3-Ps-1.
(●) Relative concentration of catechin (r = -0.958, P = 0.0025, n = 6); (○) Bacterial population of Pseudomonas CRF3-Ps-1 (r = 0.974, P = 0.001, n = 6); (△) Relative concentration of protocatechuic acid (r = 0.874, P = 0.0226, n = 6). (B) Correlations between the concentration of protocatechuic acid and bacterial populations in the soil of R. formosanum (r = 0.734, P = 0.0066, n = 12).
Figure 5.
The phytotoxic effects of (-)-catechin on the seed germination (A) and radicle growth (B) of Lactuca sativa at different concentrations in combination with 0 µg, 10 µg and 50 µg protocatechuic acid.
Error bars represent the standard errors of the mean.
Figure 6.
Hypothetical scheme of allelopathic interactions between Rhododendron formosanum and dominant Pseudomonas species.
Initially, (-)-catechin from the leaves of R. formosanum accumulates in the soil via leaching. After a period of time, the population of catechin-utilizing Pseudomonas spp. increases and the (-)-catechin is converted into protocatechuic acid through biotransformation. The protocatechuic acid exhibited synergistic inhibitory effects with the original (-)-catechin on the seed germination of the plant. Finally, (-)-catechin is transformed into glycerol and utilized by microorganisms as a carbon source. Thus, interactions between R. formosanum and the dominant Pseudomonas species are established.