Biosynthesis and beneficial effects of microbial gibberellins on crops for sustainable agriculture.

Soil microbes promote plant growth through several mechanisms such as secretion of chemical compounds including plant growth hormones. Among the phytohormones, auxins, ethylene, cytokinins, abscisic acid and gibberellins are the best understood compounds. Gibberellins were first isolated in 1935 from the fungus Gibberella fujikuroi and are synthesized by several soil microbes. The effect of gibberellins on plant growth and development has been studied, as has the biosynthesis pathways, enzymes, genes and their regulation. This review revisits the history of gibberellin research highlighting microbial gibberellins and their effects on plant health with an emphasis on the early discoveries and current advances that can find vital applications in agricultural practices.

Auxins of microbial origin and their use in agriculture.

To maintain the world population demand, a sustainable agriculture is needed. Since current global vision is more friendly with the environment, eco-friendly alternatives are desirable. In this sense, plant growth-promoting rhizobacteria could be the choice for the management of soil-borne diseases of crop plants. These rhizobacteria secrete chemical compounds which act as phytohormones. Indole-3-acetic acid (IAA) is the most common plant hormone of the auxin class which regulates various processes of plant growth. IAA compound, in which structure can be found a carboxylic acid attached through a methylene group to the C-3 position of an indole ring, is produced both by plants and microorganisms. Plant growth-promoting rhizobacteria and fungi secrete IAA to promote the plant growth. In this review, IAA production and mechanisms of action by bacteria and fungi along with the metabolic pathways evolved in the IAA secretion and commercial prospects are revised.Key points• Many microorganisms produce auxins which help the plant growth promotion.• These auxins improve the plant growth by several mechanisms.• The auxins are produced through different mechanisms.

Antimicrobial secondary metabolites from agriculturally important bacteria as next-generation pesticides.

The whole organisms can be packaged as biopesticides, but secondary metabolites secreted by microorganisms can also have a wide range of biological activities that either protect the plant against pests and pathogens or act as plant growth promotors which can be beneficial for the agricultural crops. In this review, we have compiled information about the most important secondary metabolites of three important bacterial genera currently used in agriculture pest and disease management.

Antimicrobial secondary metabolites from agriculturally important fungi as next biocontrol agents.

Synthetic chemical pesticides have been used for many years to increase the yield of agricultural crops. However, in the future, this approach is likely to be limited due to negative impacts on human health and the environment. Therefore, studies of the secondary metabolites produced by agriculturally important microorganisms have an important role in improving the quality of the crops entering the human food chain. In this review, we have compiled information about the most important secondary metabolites of fungal species currently used in agriculture pest and disease management.

Biofilm formation and regulation of salicylic acid-inducible genes expression in Arabidopsis by Algerian indigenous bacteria from wheat and potatoes rhizospheres in semi-arid Sétif region.

45 bacterial isolates from potatoes and wheat rhizospheres near Sétif (Algeria) pre-selected for their antagonistic activity against three fungal plant pathogens, two necrotrophic Fusarium solani var. coeruleum and Phytophtora infestans, and a systemic F. oxysporum f. sp. albedinis. Molecular typing of the isolates showed abundance of Bacillus compared to Pseudomonas. Some of the tested strains have shown very high biofilm formation. Among the 24 Gram-positive bacilli screened for four cyclic lipopeptides genes, some isolates harbor two or more genes, while others have a single gene or have none. Four selected isolates were able to regulate the expression of six defense-related genes in Arabidopsis and produce salicylic acid. Upon the features assessed in this study, strain B. amyloliquefaciens A16 was selected for a subsequent use as seed treatment and biocontrol agent in semi-arid region fields. This strain showed important biofilm formation, regulation of Arabidopsis defenses, and harbored three cLPs genes.

Algerian propolis extracts: Chemical composition, bactericidal activity and in vitro effects on gilthead seabream innate immune responses.

Propolis has been used as a medicinal agent for centuries. The chemical composition of four propolis samples collected from four locations of the Sétif region, Algeria, using gas chromatography-mass spectrometry was determined. More than 20 compounds and from 30 to 35 compounds were identified in the aqueous and ethanolic extracts, respectively. Furthermore, the antimicrobial activity of the propolis extracts against two marine pathogenic bacteria was evaluated. Finally, the in vitro effects of propolis on gilthead seabream (Sparus aurata L.) leucocyte activities were measured. The bactericidal activity of ethanolic extracts was very high against Shewanella putrefaciens, average against Photobacterium damselae and very low against Vibrio harveyi. The lowest bactericidal activity was always that found for the aqueous extracts. When the viability of gilthead seabream head-kidney leucocytes was measured after 30 min' incubation with the different extracts, both the ethanolic and aqueous extracts of one of the propolis samples (from Babor) and the aqueous extract of another (from Ain-Abbassa) provoked a significant decrease in cell viability when used at concentrations of 100 and 200 µg ml-1. Furthermore, significant inhibitory effects were recorded on leucocyte respiratory burst activity when isolated leucocytes where preincubated with the extracts. This effect was dose-dependent in all cases except when extracts from a third propolis sample (from Boutaleb) were used. Our findings suggest that some of Algerian propolis extracts have bactericidal activity against important bacterial pathogens in seabream and significantly modulate in vitro leucocyte activities, confirming their potential as a source of new natural biocides and/or immunomodulators in aquaculture practice.

Inhibitory Effect of Rhizobia Isolated from Several Leguminous against Phytopathogenic Fungi

Aims: This study was carried out to evaluate the antagonistic effect of ten isolated characterized Rhizobium sp. and three referenced strains against aggressive phytopathogenic fungi Fusarium spp., Aspergillus spp., Penicillium spp., Alternaria spp., Humicola spp. and Cladosporium spp. isolated from infected and wilted plants in vitro and in vivo. Study Design:  First, we have ten strains of rhizobia isolated from leguminous and characterized, after that,  some strains of fungi were isolated from infected  and wilt plants,  such as Fusarium, Aspergillus, Penicillium,  Alternaria, Humicola and Cladosporium. Finally, The investigation of the potential of the isolated rhizobia and three referenced strains was evaluated in dual culture, in pots experiments and on seeds. Place and Duration of Study: The study was carried out at the Department of Microbiology, Faculty of Nature and Life Sciences, Laboratory of Applied Microbiology, between April and November 2016. Methodology: Rhizobia isolates were identified upon their phenotypic traits as: the morphology of the colony, of the physiological characters (growth temperature, salt tolerance, resistance to antibiotics, metabolism of carbon source, generation time…) and also the production of extracellular metabolites as siderophores and proteases. Fungal isolates were identified by their macroscopic and microscopic characters. The antagonistic effect of rhizobia against phytopathogenic fungi was examined in vitro by: confrontation in Petri dishes and inoculation of seeds; and in vivo by inoculation of plants. Results: The isolated rhizobia were resistant to penicillin and amoxicillin with generation time ranged from 1.9 to 6.4 h, they were able to grow at temperatures from 4°C to 37°C, tolerate salt concentration (0.5 –2%) and produce siderophores and proteases. The results demonstrated the effectiveness of the rhizobia isolates and the referenced strains against the fungi isolates in vitro and in vivo. In dual culture, all rhizobia isolates inhibited the mycelial growth of the pathogens. The best disease control was obtained with isolate F3 from faba bean which inhibited the fungal growth with more than 70%. The highest inhibition growth was obtained against Alternaria spp.2, Penicillium spp.2, Cladosporium spp. and Humicola spp. with an inhibition rate ranging from 90 to 96%. In pot experiments, Rhizobia isolates from chickpea, lentil and faba bean reduced significantly all disease incidence  with more than 75%; where the best fight was observed in lentil plants, while in faba bean no wilted plants were observed. Seeds inoculated with rhizobia and fungi in Petri dishes showed that isolates from faba bean and lentil were the most effective in reducing fungi disease incidence. Conclusion: Rhizobia have a high potentiality to inhibit the growth of tested pathogens and could be fielded within an integrated disease management package.

Isolation and Selection of Indigenous Bacterial Strains with Suppression Properties from the Rhizospheres of Potato and Wheat.

Soil-borne fungal and bacterial root pathogens can cause serious losses to agricultural crops. The enhancement of disease suppressive properties of soils will limit disease development, thus, being of great importance for sustainable agriculture as well as organic farming systems. The aim of the present study is to isolate, identify and to select indigenous bacterial strains with antifungal activity from the potato and wheat rhizospheres. 111 bacterial strains were obtained in the preliminary screening, from the antagonism test plates, 50 from potato and 61 from wheat. About 55% were Gram+ and about 46% were Gram-. Fourteen bacterial strains from potato revealed an antagonistic activity in vitro against the phytopathogenic fungi, Phytophthora infestans, Fusarium oxysporum f. sp. albedinis and Fusarium solani var. coeruleum with a percentage of inhibition varying from 0 to 92.30%. Twenty four bacterial strains from wheat had an antagonistic activity in vitro against the studied fungi with a range from 0 to 87%. This shows a promising beginning for detecting suppressive soils in Sétif aria.