Attachment Issues: Microbes, Minerals, and the Persistence of Soil Organic Matter

The remnants of microorganisms are now understood to account for the majority of organic matter in many mineral soils. Despite the significance of this microbial necromass for soil carbon storage, we know relatively little about how the traits of microorganisms interact with soil minerals to determine the stability of microbe derived carbon in soil. Soil minerals differ in their surface area and chemistry potentially influencing microbial attachment, biofilm formation, and the persistence of microbial necromass. To address this knowledge gap, we grew twelve bacterial species from four broad groups of varying cell wall morphology (Gram positive, Gram negative, filamentous actinobacteria, and capsule-forming bacteria) in 13C-enriched minimal media with soil minerals (sand, clay, goethite-coated sand, and goethite-coated clay). The decomposition of heat-killed and dried necromass-mineral preparations was then traced in a 28-day soil microcosm experiment. Over the incubation period 20–80% of the necromass carbon was respired depending upon both cell wall morphology and mineral chemistry. In general, the necromass carbon from Gram-positive bacteria persisted longer than that of Gram-negative bacteria. Goethite coating on clay tended to reduce decomposition, especially for Gram-positive bacterial necromass (as only ~30% of the C was respired). This may be a consequence of anionic teichoic acids in the cell wall of Gram-positive bacteria adhering to positively charged iron oxides coating the clay mineral surface. Necromass decomposition was greatest for Gram-negative bacteria grown in the presence of sand (50–80% of the necromass C was respired) suggesting that these cells have difficulty forming stable attachments to sand surfaces. Taken together this work suggests that interactions between the surface chemistry of microbial cells and soil minerals may provide new insights into how microbes and minerals interact to influence soil organic matter persistence

Decrypting the multi-functional biological activators and inducers of defense responses against biotic stresses in plants

Plant diseases are still the main problem for the reduction in crop yield and a threat to global food security. Additionally, excessive usage of chemical inputs such as pesticides and fungicides to control plant diseases have created another serious problem for human and environmental health. In view of this, the application of plant growth-promoting rhizobacteria (PGPR) for controlling plant disease incidences has been identified as an eco-friendly approach for coping with the food security issue. In this review, we have identified different ways by which PGPRs are capable of reducing phytopathogenic infestations and enhancing crop yield. PGPR suppresses plant diseases, both directly and indirectly, mediated by microbial metabolites and signaling components. Microbial synthesized anti-pathogenic metabolites such as siderophores, antibiotics, lytic enzymes, hydrogen cyanide, and several others act directly on phytopathogens. The indirect mechanisms of reducing plant disease infestation are caused by the stimulation of plant immune responses known as initiation of systemic resistance (ISR) which is mediated by triggering plant immune responses elicited through pathogen-associated molecular patterns (PAMPs). The ISR triggered in the infected region of the plant leads to the development of systemic acquired resistance (SAR) throughout the plant making the plant resistant to a wide range of pathogens. A number of PGPRs including Pseudomonas and Bacillus genera have proven their ability to stimulate ISR. However, there are still some challenges in the large-scale application and acceptance of PGPR for pest and disease management. Further, we discuss the newly formulated PGPR inoculants possessing both plant growth-promoting activities and plant disease suppression ability for a holistic approach to sustaining plant health and enhancing crop productivity