Bioinspired Rhamnolipid Protects Wheat Against Zymoseptoria tritici Through Mainly Direct Antifungal Activity and Without Major Impact on Leaf Physiology

Nanoemulsions of synthetic rhamnolipids act as plant resistance inducers without damaging plant tissues or affecting soil microbiota

Plant pathogens and pests can cause significant losses in crop yields, affecting food security and the global economy. Many traditional chemical pesticides are used to combat these organisms. This can lead to the development of pesticide-resistant strains of pathogens/insects and negatively impact the environment. The development of new bioprotectants, which are less harmful to the environment and less likely to lead to pesticide-resistance, appears as a sustainable strategy to increase plant immunity. Natural Rhamnolipids (RL-Nat) are a class of biosurfactants with bioprotectant properties that are produced by an opportunistic human pathogen bacterium. RL-Nat can act as plant resistance inducers against a wide variety of pathogens. Recently, a series of bioinspired synthetic mono-RLs produced by green chemistry were also reported as phytoprotectants. Here, we explored their capacity to generate novel colloidal systems that might be used to encapsulate bioactive hydrophobic compounds to enhance their performance as plant bioprotectants. The synthetic mono-RLs showed good surfactant properties and emulsification power providing stable nanoemulsions capable of acting as bio-carriers with good wettability. Synthetic RLs-stabilized nanoemulsions were more effective than RLs suspensions at inducing plant immunity, without causing deleterious effects. These nanoemulsions were innocuous to native substrate microbiota and beneficial soil-borne microbes, making them promising safe bio-carriers for crop protection.

Impact of Rhamnolipids (RLs), Natural Defense Elicitors, on Shoot and Root Proteomes of Brassica napus by a Tandem Mass Tags (TMTs) Labeling Approach

The rapeseed crop is susceptible to many pathogens such as parasitic plants or fungi attacking aerial or root parts. Conventional plant protection products, used intensively in agriculture, have a negative impact on the environment as well as on human health. There is therefore a growing demand for the development of more planet-friendly alternative protection methods such as biocontrol compounds. Natural rhamnolipids (RLs) can be used as elicitors of plant defense mechanisms. These glycolipids, from bacteria secretome, are biodegradable, non-toxic and are known for their stimulating and protective effects, in particular on rapeseed against filamentous fungi. Characterizing the organ responsiveness to defense-stimulating compounds such as RLs is missing. This analysis is crucial in the frame of optimizing the effectiveness of RLs against various diseases. A Tandem Mass Tags (TMT) labeling of the proteins extracted from the shoots and roots of rapeseed has been performed and showed a differential pattern of protein abundance between them. Quantitative proteomic analysis highlighted the differential accumulation of parietal and cytoplasmic defense or stress proteins in response to RL treatments with a clear effect of the type of application (foliar spraying or root absorption). These results must be considered for further use of RLs to fight specific rapeseed pathogens.

Biosurfactants' multifarious functional potential for sustainable agricultural practices

Increasing food demand by the ever-growing population imposes an extra burden on the agricultural and food industries. Chemical-based pesticides, fungicides, fertilizers, and high-breeding crop varieties are typically employed to enhance crop productivity. Overexploitation of chemicals and their persistence in the environment, however, has detrimental effects on soil, water, and air which consequently disturb the food chain and the ecosystem. The lower aqueous solubility and higher hydrophobicity of agrochemicals, pesticides, metals, and hydrocarbons allow them to adhere to soil particles and, therefore, continue in the environment. Chemical pesticides, viz., organophosphate, organochlorine, and carbamate, are used regularly to protect agriculture produce. Hydrophobic pollutants strongly adhered to soil particles can be solubilized or desorbed through the usage of biosurfactant/s (BSs) or BS-producing and pesticide-degrading microorganisms. Among different types of BSs, rhamnolipids (RL), surfactin, mannosylerythritol lipids (MELs), and sophorolipids (SL) have been explored extensively due to their broad-spectrum antimicrobial activities against several phytopathogens. Different isoforms of lipopeptide, viz., iturin, fengycin, and surfactin, have also been reported against phytopathogens. The key role of BSs in designing and developing biopesticide formulations is to protect crops and our environment. Various functional properties such as wetting, spreading, penetration ability, and retention period are improved in surfactant-based formulations. This review emphasizes the use of diverse types of BSs and their source microorganisms to challenge phytopathogens. Extensive efforts seem to be focused on discovering the innovative antimicrobial potential of BSs to combat phytopathogens. We discussed the effectiveness of BSs in solubilizing pesticides to reduce their toxicity and contamination effects in the soil environment. Thus, we have shed some light on the use of BSs as an alternative to chemical pesticides and other agrochemicals as sparse literature discusses their interactions with pesticides. Life cycle assessment (LCA) and life cycle sustainability analysis (LCSA) quantifying their impact on human activities/interventions are also included. Nanoencapsulation of pesticide formulations is an innovative approach in minimizing pesticide doses and ultimately reducing their direct exposures to humans and animals. Some of the established big players and new entrants in the global BS market are providing promising solutions for agricultural practices. In conclusion, a better understanding of the role of BSs in pesticide solubilization and/or degradation by microorganisms represents a valuable approach to reducing their negative impact and maintaining sustainable agricultural practices.

Comparative study on antimicrobial activity of mono-rhamnolipid and di-rhamnolipid and exploration of cost-effective antimicrobial agents for agricultural applications

Background

Chemical pesticides have defects in crop diseases control, such as narrow antimicrobial spectrum, chemicals residue risk and harm to farmland ecosystem. Antimicrobial agents from microbial sources are highly interested in agriculture. Studies showed that rhamnolipid biosurfactants possessed certain antimicrobial activity. The structural differences in rhamnolipid inevitably affect their activities. But the antimicrobial effect of mono-rhamnolipid and di-rhamnolipid is unknown. Rhamnolipid with unique structure can be produced using specific microbial cell factory.

Results

Different types of rhamnolipid were produced from different Pseudomonas aeruginosa strains. Rha-C₁₀-C₁₀ and Rha-Rha-C₁₀-C₁₀ were the main homologues in the separated mono-rhamnolipid and di-rhamnolipid, respectively. Both mono-rhamnolipid and di-rhamnolipid exhibited certain antimicrobial activity against the tested microbial strains, especially the fungi and Gram-positive bacteria. But mono-rhamnolipid was superior to di-rhamnolipid, with inhibition zone diameters larger than 25 mm and inhibition rate higher than 90%. The IC50 values of mono-rhamnolipid were lower than 5 mg/L against the tested bacterium and fungus, whereas the IC50 values of di-rhamnolipid were ranged from 10 mg/L to 20 mg/L. Mono-rhamnolipid stimulated the tested strains to generate higher level of intracellular ROS. Mono-rhamnolipid exhibited better antimicrobial activity to the potential agricultural pathogens, such as Alternaria alternata, Pantoea agglomerans and Cladosporium sp. The mono-rhamnolipid crude extract of strain P. aeruginosa SGΔrhlC can replace the separated mono-rhamnolipid. After 50 times dilution, the fermentation broth of the mono-rhamnolipid producing strain SGΔrhlC exhibited equal antimicrobial effect to mono-rhamnolipid (200 mg/L). Prospects of mono-rhamnolipid were also discussed for antimicrobial applications in agriculture.

Conclusions

This work discovered that mono-rhamnolipid was superior to di-rhamnolipid on antimicrobial activity for agricultural applications. Mono-rhamnolipid is an excellent candidate for agricultural biocontrol. The knockout strain P. aeruginosa SGΔrhlC is an excellent microbial cell factory for high producing mono-rhamnolipid. Its mono-rhamnolipid crude extract and its diluted fermentation broth are cost-effective antimicrobial agents. This work provided new insights to develop green and efficient antimicrobial agents for agricultural applications.

Plant Beneficial Bacteria as Bioprotectants against Wheat and Barley Diseases

Wheat and barley are the main cereal crops cultivated worldwide and serve as staple food for a third of the world's population. However, due to enormous biotic stresses, the annual production has significantly reduced by 30-70%. Recently, the accelerated use of beneficial bacteria in the control of wheat and barley pathogens has gained prominence. In this review, we synthesized information about beneficial bacteria with demonstrated protection capacity against major barley and wheat pathogens including Fusarium graminearum, Zymoseptoria tritici and Pyrenophora teres. By summarizing the general insights into molecular factors involved in plant-pathogen interactions, we show to an extent, the means by which beneficial bacteria are implicated in plant defense against wheat and barley diseases. On wheat, many Bacillus strains predominantly reduced the disease incidence of F. graminearum and Z. tritici. In contrast, on barley, the efficacy of a few Pseudomonas, Bacillus and Paraburkholderia spp. has been established against P. teres. Although several modes of action were described for these strains, we have highlighted the role of Bacillus and Pseudomonas secondary metabolites in mediating direct antagonism and induced resistance against these pathogens. Furthermore, we advance a need to ascertain the mode of action of beneficial bacteria/molecules to enhance a solution-based crop protection strategy. Moreover, an apparent disjoint exists between numerous experiments that have demonstrated disease-suppressive effects and the translation of these successes to commercial products and applications. Clearly, the field of cereal disease protection leaves a lot to be explored and uncovered.