Biofilm-overproducing Bacillus subtilis B12ΔYwcc decreases Cd uptake in Chinese cabbage through increasing Cd-immobilizing related gene abundance and root surface colonization

Biofilms formation in plant growth-promoting bacteria for alleviating agro-environmental stress

Biofilm formation represents a pivotal and adaptable trait among microorganisms within natural environments. This attribute plays a multifaceted role across diverse contexts, including environmental, aquatic, industrial, and medical systems. While previous research has primarily focused on the adverse impacts of biofilms, harnessing their potential effectively could confer substantial advantages to humanity. In the face of escalating environmental pressures (e.g., drought, salinity, extreme temperatures, and heavy metal pollution), which jeopardize global crop yields, enhancing crop stress tolerance becomes a paramount endeavor for restoring sufficient food production. Recently, biofilm-forming plant growth-promoting bacteria (PGPB) have emerged as promising candidates for agricultural application. These biofilms are evidence of microorganism colonization on plant roots. Their remarkable stress resilience empowers crops to thrive and yield even in harsh conditions. This is accomplished through increased root colonization, improved soil properties, and the synthesis of valuable secondary metabolites (e.g., ACC deaminase, acetin, 2,3-butanediol, proline, etc.). This article elucidates the mechanisms underpinning the role of biofilm-forming PGPB in bolstering plant growth amidst environmental challenges. Furthermore, it explores the tangible applications of these biofilms in agriculture and delves into strategies for manipulating biofilm formation to extract maximal benefits in practical crop production scenarios.

Metal-immobilizing Pseudomonas taiwanensis WRS8 reduces heavy metal accumulation in Coriandrum sativum by changing the metal immobilization-related bacterial population abundances

Metal-immobilizing bacteria play a critical role in metal accumulation in vegetables. However, little is known concerning the mechanisms involved in bacteria-induced reduced metal availability and uptake in vegetables. In this study, the impacts of metal-immobilizing Pseudomonas taiwanensis WRS8 on the plant biomass, Cd and Pb availability and uptake in two coriander (Coriandrum sativum L.) cultivars, and bacterial community structure were investigated in the polluted soil. Strain WRS8 increased the biomass of two coriander cultivars by 25-48% and reduced Cd and Pb contents in the edible tissues by 40-59% and available Cd and Pb contents in the rhizosphere soils by 11.1-15.2%, compared with the controls. Strain WRS8 significantly increased the pH values and relative abundances of the dominant populations of Sphingomonas, Pseudomonas, Gaiellales, Streptomyces, Frankiales, Bradyrhizobium, and Luteimonas, while strain WRS8 significantly decreased the relative abundances of the dominant populations of Gemmatimonadaceae, Nitrospira, Haliangium, Paenibacillus, Massilia, Bryobacter, and Rokubacteriales and the rare bacterial populations of Enterorhabdus, Roseburia, Luteibacter, and Planifilum in the rhizosphere soils, compared with the controls. Significantly negative correlations were observed between the available metal concentrations and the abundances of Pseudomonas, Luteimonas, Frankiales, and Planifilum. These results implied that strain WRS8 could affect the abundances of the dominant and rare bacterial populations involved in metal immobilization, resulting in increased pH values and decreased metal availability and uptake in the vegetables in the contaminated soil.

Involvement of exogenous arsenic-reducing bacteria in root surface biofilm formation promoted phytoextraction of arsenic

Root surface biofilm (RSB) is the last window for pollutants entering plant roots and thus plays a critical role in the phytoextraction of pollutants. Exogenous arsenic-reducing bacteria (EARB) have been adopted to enhance the phytoextraction of arsenic (As). However, whether EARB would be involved in RSB formation together with indigenous bacteria and the role of EARB involvement in As phytoextraction are still unknown. Herein, two EARB strains and two phytoextractors (wheat and maize) were selected to investigate the involvement of EARB in RSB formation and its role in As phytoextraction. Results showed that EARB successfully participated in RSB formation together with indigenous bacteria, attributing to their strong chemotaxis and biofilm formation abilities induced by root exudates. The involvement of EARB in RSB formation significantly enhanced As accumulation in plant roots, since more arsenite (As(III)) caused by arsenate (As(V)) reduction in RSB was absorbed by roots. Its underlying mechanism was further elucidated. EARB involvement increased phylum Proteobacteria to produce more siderophores in RSB. Siderophores then improved photosynthesis by increasing catalase and peroxidase activities and decreasing the malondialdehyde of plants. These actions further raised the shoot fresh weight to enhance As accumulation in plant roots. Moreover, mesophyll cell in wheat has a stronger As(V) reduction ability than that in maize, resulting in opposite distribution patterns of As(III) and As(V) in wheat and maize shoots. This study provides a new understanding of phytoextraction enhanced by exogenous bacteria and fills the gap in the role of EARB in As phytoextraction from the perspective of the RSB microregion.

The potential of Bacillus subtilis and phosphorus in improving the growth of wheat under chromium stress

AIM

Hexavalent chromium (Cr⁺⁶ ) is one of the most toxic heavy metals that have deteriorating effects on the growth and quality of the end product of wheat. Consequently, this research was designed to evaluate the role of Bacillus subtilis and phosphorus fertilizer on wheat facing Cr⁺⁶ stress.

METHODS AND RESULTS

The soil was incubated with Bacillus subtilis and phosphorus fertilizer before sowing. The statistical analysis of the data showed that the co-application of B. subtilis and phosphorus yielded considerably more significant (p < 0.05) results compared with an individual application of the respective treatments. The co-treatment improved the morphological, physiological and biochemical parameters of plants compared with untreated controls. The increase in shoot length, root length, shoot fresh weight and root fresh weight was 38.17%, 29.31%, 47.89% and 45.85%, respectively, compared with untreated stress-facing plants. The application of B. subtilis and phosphorus enhanced osmolytes content (proline 39.98% and sugar 41.30%), relative water content and stability maintenance of proteins (86.65%) and cell membranes (66.66%). Furthermore, augmented production of antioxidants by 67.71% (superoxide dismutase), 95.39% (ascorbate peroxidase) and 60.88% (catalase), respectively, were observed in the Cr⁺⁶ - stressed plants after co-application of B. subtilis and phosphorus.

CONCLUSION

It was observed that the accumulation of Cr⁺⁶ was reduced by 54.24%, 59.19% and 90.26% in the shoot, root and wheat grains, respectively. Thus, the combined application of B. subtilis and phosphorus has the potential to reduce the heavy metal toxicity in crops.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study explored the usefulness of Bacillus subtilis and phosphorus application on wheat in heavy metal stress. It is a step toward the combinatorial use of plant growth-promoting rhizobacteria with nutrients to improve the ecosystems' health.