An alternative biocontrol agent of soil-borne phytopathogens: A new antifungal compound produced by a plant growth promoting bacterium isolated from North Algeria

Alcaligenes faecalis Juj3 alleviates Plasmodiophora brassicae stress to cabbage via promoting growth and inducing resistance

Clubroot is a devastating disease threatening global cruciferous vegetable production caused by Plasmodiophora brassicae (Pb). We have evaluated the positive effects of the Alcaligenes faecalis Juj3 on cabbage growth promotion and Pb stress alleviation through pot and field experiments. The Juj3 strain was isolated from a healthy cabbage rhizosphere with growth-promoting characteristics and was identified as A. faecalis based on morphological traits and phylogeny. Seed germination assays revealed that Juj3 inoculation enhances cabbage bud shoot and root growth. In pot experiments, inoculation with Juj3 fermentation powder at cabbage sowing dates significantly improved the seedling biomass. Combining seed treatments with root irrigation after transplanting considerably reduced the clubroot disease index and resulted in appreciable biocontrol efficacy (83.7%). Gene expression analyses of cabbage after Juj3 inoculation showed that PR2 and EIN3 expression were significantly up-regulated. Physiologically, Juj3 inoculation enhanced cabbage chlorophyll content and root activity in a normal environment. Irrespective of whether plants were under normal environment or Pb stresses, Juj3 improved photosynthesis. Field trial analyses revealed that Juj3 exhibits satisfactory biocontrol efficacy in cabbage (51.4%) and Chinese cabbage (37.7%). Moreover, Juj3 could also enhance cabbage and Chinese cabbage biomass to improve the yield quality. These findings pave the way for future use of A. faecalis as biocontrol agents for clubroot and reveal the great potential of the rhizobacterium for plant growth-promoting applications in agriculture and horticulture.

Bacterial Lipodepsipeptides and Some of Their Derivatives and Cyclic Dipeptides as Potential Agents for Biocontrol of Pathogenic Bacteria and Fungi of Agrarian Plants

Biotic stresses (fungi, bacteria, insects, weeds, etc.) are some of the most important causes of the decrease in the quality and quantity of crops that could become an emergency due to a noteworthy increase in the world population. Thus, to overcome these problems, massive use of chemical pesticides has been carried out with heavy consequences for environmental pollution and food safety. An eco-friendly alternative can be using natural compound-based biopesticides with high efficacy and selectivity. Some bacterial lipodepsipeptides (tolaasins I, II, A, D, and E and WLIP together with hexacetyl- and tetrahydro-tolaasin I and WLIP methyl ester) and cyclic dipeptides (cyclo(l-Pro-l-Tyr), cyclo(d-Pro-l-Tyr), cyclo(l-Pro-l-Val), and cyclo(l-Pro-l-Leu)) were assayed against several pathogenic bacteria and fungi of important agrarian plants. Lipodepsipeptides showed strong growth inhibition of all microorganisms tested in the range of 0.1-0.8 μg/mL, while cyclodipeptides, despite preserving this ability, showed a noteworthily reduced antimicrobial activity being active only in the range of 15-900 μg/mL. Among the lipodepsipeptides and cyclic dipeptides assayed, tolaasin d and cyclo(l-Pro-l-Tyr) (also named maculosin-1) appeared to be the most toxic compounds. Some structure-activity relationships of lipodepsipeptides were also discussed along with their practical application as biopesticides in agriculture.

Plant Growth Promotion Function of Bacillus sp. Strains Isolated from Salt-Pan Rhizosphere and Their Biocontrol Potential against Macrophomina phaseolina

In recent decades, intensive crop management has involved excessive use of pesticides or fertilizers, compromising environmental integrity and public health. Accordingly, there has been worldwide pressure to find an eco-friendly and safe strategy to ensure agricultural productivity. Among alternative approaches, Plant Growth-Promoting (PGP) rhizobacteria are receiving increasing attention as suitable biocontrol agents against agricultural pests. In the present study, 22 spore-forming bacteria were selected among a salt-pan rhizobacteria collection for their PGP traits and their antagonistic activity against the plant pathogen fungus Macrophomina phaseolina. Based on the higher antifungal activity, strain RHFS10, identified as Bacillus vallismortis, was further examined and cell-free supernatant assays, column purification, and tandem mass spectrometry were employed to purify and preliminarily identify the antifungal metabolites. Interestingly, the minimum inhibitory concentration assessed for the fractions active against M. phaseolina was 10 times lower and more stable than the one estimated for the commercial fungicide pentachloronitrobenzene. These results suggest the use of B. vallismortis strain RHFS10 as a potential plant growth-promoting rhizobacteria as an alternative to chemical pesticides to efficiently control the phytopathogenic fungus M. phaseolina.

The Bactericidal Activity of Protein Extracts from Loranthus europaeus Berries: A Natural Resource of Bioactive Compounds

Loranthus europaeus is a well-known and important medicinal plant, with a long history of traditional medicine use. Several studies showed that it contains many bioactive compounds with a wide range of pharmacological effects. In light of these past researches, L. europaeus were chosen to consider its potential antimicrobial action. To this aim, different protocols were performed to selectively extract protein compounds, from L. europaeus yellow fruits, and evaluate the antimicrobial activity against four phytopathogenic fungi (Aspergillus niger, Alternaria spp., Penicillium spp., Botritis cinereus) and a number of foodborne bacterial pathogens (Listeria monocytogenes, Staphylococcus aureus strains, Salmonella Typhimurium and Escherichia coli) by using serial dilutions and colony formation assays. Results evidenced no antifungal activity but a notable bactericidal efficiency of a crude protein extract against two foodborne pathogens, with minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values between 0.2 and 0.5 mg/mL, being S. aureus strains the most susceptible bacteria. Moreover, a strong bactericidal activity against S. aureus M7 was observed by two partially purified protein fractions of about 600 and 60 kDa molecular mass in native conditions. Therefore, these plant protein extracts could be used as natural alternative preventives to control food poisoning diseases and preserve foodstuff avoiding health hazards of chemically antimicrobial applications.