Replant disease caused by continuous cropping commonly occurs in yam with consecutive monoculture. However, little is known about how the continuous cropping of yam affects the rhizospheric soil bacterial community structure. In this study, the effects of continuous cropping on rhizospheric soil characteristics, bacterial diversity, and community structure were investigated in the Yongfeng yam fields under monoculture for 1, 5, 10, 15, and 20 years. Long-term monoculture caused soil acidification and increased the concentration of available potassium (AK) and available phosphorus (AP), and soil bacterial richness, but decreased the soil bacterial diversity. An exception was for the field under monoculture for 20 years as it showed the highest bacterial diversity. The relative abundance of beneficial bacteria, such as Proteobacteria, Actinobacteria, and Chloroflexi decreased while the relative abundance of harmful bacteria, including Gemmatimonadetes and Acidobacteria, increased with an extended continuous cultivation time. The networks varied among yams with different cultivation years and became complex with the increase in cultivation years. However, after time in monoculture, the bacterial network decreased gradually and existed stably. These changes in bacterial community composition and co-occurrence of networks may increase the potential risk of soil-borne disease and reduce the yield and quality of Yongfeng yam.

A novel esterase gene was isolated from a soil metagenomic library. The gene encoded a protein of 520 amino acids which contained a 21 aa signal peptide. Primary structure analysis of the protein sequence revealed that it contained a conserved active site motif (SxSxG) and a structural motif (CS-D-HC). Then the esterase gene was cloned and expressed inEscherichia coli BL21(DE3). SDS-PAGE analysis of the purified esterase showed that it was expressed in a highly soluble form and its molecular mass was estimated to be 55 kDa. Characterization of the esterase revealed that it exhibited high activity towardp-nitrophenyl esters with short acyl chains and especiallyp-nitrophenyl acetate, suggesting that it was a typical carboxylesterase rather than a lipase. Withp-nitrophenyl acetate as substrate, the enzyme showed its optimal activity at pH 7.0 and 30 °C, and it was stable at a broad pH range from 4.5 to 10.0 and temperature not higher than 50 °C. Furthermore, the enzyme showed different substrate specificity from known esterase, it was not only hydrolyzing againstp-nitrophenyl esters, but also hydrolyzing all hydroxybenzoic esters and hydroxycinnamic ester assayed. As it was an enzyme active on a broad range of phenolic esters, simultaneously possessing feruloyl esterase, chlorogenate esterase and tannase activities, it could serve as a valuable candidate for applications in biotechnology.

Soil microbes are an abundant source of enzymes with unique properties that may be useful for industrial applications. As most wild-type strains show low chlorogenic acid esterase expression and activity, and most microbes cannot be cultured in the laboratory, a metagenomic approach provides methods of identifying new enzymes. In this study, a gene encoding a chlorogenic acid esterase, named Tan410, was isolated from a soil metagenomic library and overexpressed in Escherichia coli BL21 (DE3). The recombinant enzyme, with a predicted molecular weight of 54.88 kDa, was purified to homogeneity. The K_m and V_max values for Tan410 were 1.26 mM and 0.33 mM min^–1, respectively, with chlorogenic acid as the substrate. Its optimum temperature and pH for reaction were 30 °C and 7.5, respectively. The enzyme exhibited moderate thermostability and broad pH stability (3.0–10.0). Tan410 was also able to hydrolyse ethyl ferulate, methyl caffeate, propyl gallate, ethyl gallate, methyl vanillate, methyl benzoate, ethyl benzoate, methyl 2,5-dihydroxybenzoate, and methyl 3,5-dihydroxybenzoate, and it released caffeic and ferulic acids from agricultural byproducts (destarched wheat bran and coffee pulp). Tan140 has potential for industrial application in biomass valorization.

Meloidogyne incognita is one of the most detrimental root-knot nematodes (RKNs) globally. The restricted application of chemical nematicides has resulted in an increasing inclination towards environment-friendly alternatives. In this study, a strain of Pseudomonas aeruginosa XR2-39, which was isolated from compost fermentation of edible fungus residue, exhibited effective biocontrol activity against M. incognita. In vitro experiments employing the fermentation filtrate of strain XR2-39 achieved high nematicidal efficacy of second-stage juveniles (J2s), resulting in corrected mortality rates of 97.12% and 100% after 24 h and 48 h, respectively. The fermentation filtrate also demonstrated a high relative hatching inhibition for egg masses (97.87%) and free eggs (100%). In addition, strain XR2-39 exhibited strong capabilities in indole-3-acetic acid (IAA) production (with a yield of 33.01 mg/L), siderophore production (with a yield of 71.45% unit), and phosphate solubilization (with a dissolved amount of 831.15 mg/L). Pot experiments indicated that the incubation of tomato roots with 20% fermentation broth led to an increase in fresh shoot weight, root weight, shoot length, root length, and stem diameter by 448.57%, 136.36%, 179.29%, 49.39%, and 57.14%, respectively, when compared to the water control. Moreover, the inoculation with 20% fermentation broth significantly decreased tomato root galls, resulting in a gall index of 37.00, which was significantly lower than that of the water-control treatment (77.50). Furthermore, the active compound in the fermentation filtrate remained stable within the pH range of 7.0 to 11.0, maintaining a corrected mortality rate of over 89.0%. It also demonstrated thermostability, as the boiled fermentation filtrate (treated at 120 °C for 2 h) showed a high corrected mortality rate against J2s. Additionally, the active substance displayed strong UV tolerance and storage stability. These characteristics of active compounds make strain XR2-39 a promising biocontrol agent for M.incognita.