Biological control of the root-knot nematode (Meloidogyne incognita) on eggplants with various chitinase-producing Streptomyces strains

Actinobacteria are ubiquitous Gram-positive filamentous bacteria that are well-known for their ability to control plant diseases through mechanisms such as the production of antibacterial and antifungal antibiotics, induction of plant resistance, parasitism, and even simple competition for resources. For their potential as nematode biocontrol agents, streptomycete actinobacteria have received the greatest attention. The current study aimed to isolate chitinase-producing streptomycete actinobacteria from eggplant rhizospheres with the potential to be used as biocontrol agents for the root-knot nematode Meloidogyne incognita. Twenty-four different strains of streptomycete actinobacteria were isolated and tested for chitinase production. Only five high chitinase-producing isolates were chosen for further experiments and were identified as ST01; Streptomyces levis, ST07; S. enissocaesilis, ST10; S. plicatus, ST15; S. rochei, and ST16; S. sparsus. The isolates were identified using cultural, morphological, biochemical, physiological and chemotaxonomical characteristics as well as16S rRNA gene sequencing. The phylogenetic analyses indicated that the actinobacteria isolates in this study belonged to the genus Streptomyces. The in vitro tests compared the culture filtrates of five isolates at three different concentrations (5, 10, and 20%) for their ability to reduce nematode numbers and the percentage of eggs hatching of M. incognita. When exposed to higher concentrations (20%) for longer periods of time (up to 72 h), second stage juveniles (J2) of M. incognita had a higher mortality rate. The Streptomyces strains, ST15, ST10, ST16, ST01, and ST07, significantly (P < 0.05) reduced egg hatching by 86.8, 85.4, 78.9, 77.8, and 68.1%, respectively. In addition, these five isolates were tested under greenhouse conditions to control M. incognita on eggplants. In both the presence and absence of M. incognita, the treatment of strain ST15 was the most effective and significantly (P < 0.05) increased the length, fresh weight, and dry weight of eggplant roots and shoots, total chlorophyll and carotenoids contents compared to the control and to the other strains (ST10, ST16, ST01, and ST07). Strain ST15 significantly (P < 0.05) reduced the number of galls, number of eggs, number of egg masses and the population of M. incognita in soil compared to the control and to the other strains. Malonaldehyde, hydrogen peroxide concentration, and the antioxidant enzyme (superoxide dismutase, peroxidase, and catalase) activity were all significantly (P < 0.05) decreased in eggplant tissues after treatment with strain ST15, compared to infected control. Based on our research, we have concluded that chitinase-producing Streptomyces strains, and specifically St15, have the potential to be used as significant biocontrol agents against the root-knot nematode M. incognita and as promotor of eggplant performance.