and Enhancing Growth of Gossypium hirsutum L

Use of Pseudomonas protegens to Control Root Rot Disease Caused by Boeremia exigua var. exigua in Industrial Chicory (Cichorium intybus var. sativum Bisch.)

Boeremia exigua var. exigua is a recurrent pathogen causing root rot in industrial chicory. Currently, there is no chemical or varietal control for this disease, and thus, management strategies need to be developed. This study determined the biocontrol effect of strains of Pseudomonas protegens bacteria with antimicrobial compounds on the fungus B. exigua var. exigua under in vitro, in vivo, and field conditions. In addition, root colonization by these bacteria was estimated by the phlD-specific PCR-based dilution end point assay. Eighteen isolates of Pseudomonas spp were evaluated, and the strains that showed the greatest in vitro inhibition of fungal mycelial growth (mm), Ca10A and ChB7, were selected. Inoculation with the strain ChB7 showed less severity (necrotic area) under in vivo conditions (root trials) compared with the control inoculated with the pathogen (p ≤ 0.05). The molecular analysis revealed that the root colonization of plants grown in pots was equal to or greater than 70%. Similar levels were observed in the field trials conducted at the Selva Negra and Canteras experimental stations (2015-2016 season), with values ranging from 85.7 to 70.5% and from 75.0 to 79.5%, respectively. Regarding yield (ton ha-1), values were higher in the treatments inoculated with strains Ca10A and ChB7 (p ≤ 0.05) at both experimental sites, while a lower incidence and severity of root rot were observed at Selva Negra. These results suggest that the Chilean strains of P. protegens are a promising tool for the control of root diseases in industrial chicory.

Isolation and Identification of Bacillus subtilis LY-1 and Its Antifungal and Growth-Promoting Effects

Peanut root rot, caused by Fusarium spp., is a devastating fungal disease. As part of a program to obtain a biocontrol agent to control peanut root rot in the field, a bacterial strain LY-1 capable of inhibiting the growth of the fungus in vitro was isolated from rhizosphere soil samples collected from wild mint by agar disk dilution and dual-culture assay. Strain LY-1 was identified as Bacillus subtilis based on morphological characteristics, 16S rDNA, and gyrA sequence analyses. The bacterial suspension and cell-free culture filtrate of LY-1 could significantly inhibit the growth of Fusarium oxysporum, Fusarium proliferatum and Fusarium solani, but volatile organic compounds from the cultures had only a weak effect on mycelial growth. The percentage inhibition of 20% concentration of the cell-free culture filtrate of LY-1 on conidium production of each of the three Fusarium species was greater than 72.38%, and the percentage inhibition by the culture filtration on the germination of conidia of the three species was at least 62.37%. The production of extracellular enzyme activity by LY-1 was studied in functional assays, showing protease, cellulase, amylase, chitinase, and β-1,3-glucanase activity, while LY-1 contained a gene encoding iturin, an antifungal lipopeptide. In addition, under pot culture in a greenhouse, culture filtrate of LY-1 significantly promoted the growth of peanut, increasing the fresh and dry mass of the plant by 30.77% and 27.27%, respectively, in comparison with the no-filtrate control. The culture filtrate of LY-1 increased the resistance of peanut plants to F. oxysporum, with the biocontrol efficiency reaching 44.71%. In conclusion, B. subtilis LY-1, a plant-growth-promoting rhizobacterium, was able to protect peanuts from Fusarium spp. infection.

Effect of Metarhizium anisopliae (MetA1) on growth enhancement and antioxidative defense mechanism against Rhizoctonia root rot in okra

Rhizoctonia solani is an important necrotrophic pathogenic fungus that causes okra root disease and results in severe yield reduction. Many biocontrol agents are being studied with the intent of improving plant growth and defense systems and reducing crop loss by preventing fungal infections. Recently, a member of the Hypocrealean family, Metarhizium anisopliae, has been reported for insect pathogenicity, endophytism, plant growth promotion, and antifungal potentialities. This research investigated the role of M. anisopliae (MetA1) in growth promotion and root disease suppression in okra. The antagonism against R. solani and the plant growth promotion traits of MetA1 were tested in vitro. The effects of endophytic MetA1 on promoting plant growth and disease suppression were assessed in planta. Dual culture and cell-free culture filtrate assays showed antagonistic activity against R. solani by MetA1. Some plant growth promotion traits, such as phosphate solubilization and catalase activity were also exhibited by MetA1. Seed primed with MetA1 increased the shoot, root, leaves, chlorophyll content, and biomass content compared to control okra plants. The plants challenged with R. solani showed the highest hydrogen peroxide (H2O2) and lipid peroxidation (MDA) contents in the leaves of okra. Whereas MetA1 applied plants showed a reduction of H2O2 and MDA by 5.21 and 14.96%, respectively, under pathogen-inoculated conditions by increasing antioxidant enzyme activities, including catalase (CAT), peroxidase (POD), glutathione S-transferase (GST), and ascorbate peroxidase (APX), by 30.11, 10.19, 5.62, and 5.06%, respectively. Moreover, MetA1 increased soluble sugars, carbohydrates, proline, and secondary metabolites, viz., phenol and flavonoid contents in okra resulting in a better osmotic adjustment of diseases infecting plants. MetA1 reduced disease incidence by 58.33% at 15 DAI compared to the R. solani inoculated plant. The results revealed that MetA1 improved plant growth, elevated the plant defense system, and suppressed root diseases caused by R. solani. Thus, MetA1 was found to be an effective candidate for the biological control program.

IAA and ACC deaminase producing-bacteria isolated from the rhizosphere of pineapple plants grown under different abiotic and biotic stresses

The roles of plant growth-promoting rhizobacteria in promoting plant growth and soil health, including alteration in plant metabolism and production of phytohormones such as indole-3-acetic acid (IAA) and the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, are indisputable. This study aimed to isolate and characterize beneficial bacteria isolated from the rhizosphere of pineapple from distinct stress-inducing habitats, including water excess-, herbicide-over-treated-, and pathogen-infected areas at PT Great Giant Foods located in Lampung, Indonesia. The isolated bacteria were screened based on IAA production and ACC deaminase activities. Six selected isolates produced IAA with concentrations of up to 36.93 mgL^-1. The highest value belongs to Bacillus sp. NCTB5I, followed by Brevundimonas sp. CHTB 2C (13.13 mgL^-1) and Pseudomonas sp. CHTB 5B (6.65 mgL^-1). All isolates were detected with ACC deaminase activities with Brevundimonas sp. CHTJ 5H consuming 88% of ACC over 24 h, the highest among all. Brevundimonas sp. CHTB 2C was detected with the highest ACC deaminase activity with the value of 13,370 nm α-ketobutyrate mg^-1h^-1. In another experiment, it was revealed that all selected isolates promote soybean growth. These bacteria are potential to be developed in the future as bioagents to promote plant growth, especially under stressful environmental conditions.