Characterization of endophytic Streptomyces strains from roots of cocoyam (Xanthosoma sagittifolium L. Schott) in the South West Region of Cameroon, their in vitro plant growth promoting abilities and biocontrol efficacy against Pythium myriotylum
Introduction
Cocoyam (Xanthosoma sagittifolium L. Schott) is a staple food in the tropics and subtropics and is one of the six most important root and tuber crops worldwide. Africa, as a continent, produces more than 71% of the world's cocoyam. In Cameroon, it is the second most consumed tuber crop after cassava. Its edible tubers and young leaves contain carbohydrates and essential amounts of proteins, vitamins, and minerals (Sefa-Dedeh and Agyir-Sackey, 2004) and it provides cash income for farmers (Onyenweaku and Okye, 2007). Despite its importance, Cocoyam Root Rot Disease (CRRD), caused by Pythium myriotylum, is a major disease that limits the growth and productivity of this crop in Cameroon (Nzietchueng, 1983). Although the root rot disease and the causal pathogen have been well characterized (Boudjeko et al., 2005; Boudjeko et al., 2006; Nyochembeng et al., 2007), knowledge of their reliable control measures is still poor. Additionally, no resistant cocoyam variety has yet been developed. A sustainable complementary approach would be to increase the biological input of nutrients by the exploitation of microorganisms which are largely untapped natural resources for plant growth-promotion (Thomas and Soly, 2009). This environment-friendly alternative can help to complement current strategies for integrated management of the disease.
Field observations indicate that andosols (FAO soil classification system) of Mount Cameroon area are naturally suppressive to P. myriotylum on cocoyam. Adiobo et al. (2007) suggested that their high mineral content may contribute to suppressiveness through enhanced plant growth. Otherwise, this suppressiveness was significantly reduced following pasteurization and treatment with fungicide and bactericide (Adiobo et al., 2007). Based on this knowledge, research has been initiated to evaluate the use of these microorganisms to control CRRD. Fluorescent Pseudomonas strains (CMR5c and CMR12a) antagonistic to P. myriotylum were isolated from healthy cocoyam rhizosphere and these strains were identified as novel and promising biocontrol agents of this pathogen on cocoyam (Perneel et al., 2007; Ma et al., 2016). In vivo pretreatment of cocoyam plants with strains of Trichoderma asperellum (strains PR10 and PR11) isolated from Cameroon reduced P. myriotylum infection by 50% (Mbarga et al., 2012). Actinobacteria, one of the more numerous microbial groups in Mount Cameroon suppressive andosols, are common inhabitants of plant rhizosphere and plant tissues (Doumbou et al., 2001; Goudjal et al., 2013). The presence of active actinobacteria in cocoyam roots could explain why cocoyams are grown continuously for long periods without root rot disease in this particular region.
Endophytic microorganisms are defined as microbial species that invade internal plant tissues without causing disease (Ryan et al., 2008; Matsumoto and Takahashi, 2017). Endophytic actinobacteria are found residing in the living tissues (leaves, branches, roots) of various agronomic crops and woody plants. Assessments of their communities have demonstrated that both Streptomyces and non-Streptomyces species are normally present in plant tissues (Cao et al., 2004; Shutsrirung et al., 2013). Although many species of actinobacteria, particularly those belonging to the genus Streptomyces inhibit many fungal pathogens and are considered as efficient biocontrol agents, their metabolic products can influence plant growth (Mouafo-Tchinda et al., 2016; Awla et al., 2017; Djuidje-Kouomou et al., 2019).
In this study, Actinobacteria strains were isolated from cocoyam roots and their antagonistic activity was screened against P. myriotylum and other plant pathogens. Selected isolates were identified and evaluated for hydrolytic enzyme production and plant growth-promotion properties on tissue culture-derived cocoyam plantlets. Further characterization of agro-morphological and biochemical analyses was performed to demonstrate their effect on cocoyam root growth.
Section snippets
Microorganisms and culture conditions
Fungal phytopathogens such as Pythium myriotylum, Phythophtora megakarya, Fusarium oxysporum, and Colletotrichum gloeosporioides were obtained from the Institute of Agricultural Research for Development (IRAD-Cameroon). The strains were revived on Potato Dextrose Agar (PDA) at an incubation of 26 °C.
Pathogenicity Test
To establish Koch's postulates, Pythium myriotylum, Phythophtora megakarya, Fusarium oxysporum and Colletotrichum gloeosporioides were subjected to pathogenicity test as described by
Isolation and antifungal activity
A total of 171 actinobacteria were isolated from healthy white cocoyam roots. Each isolate was tested for its capacity to inhibit the growth of plant pathogenic oomycetes and fungi. Isolates PERM1, PERM2, PERM4, and PERM5 showed more than 50% of inhibition against P. myriotylum. They also showed stronger antagonistic activities against P. megakarya, F. oxysporum, and C. gloeosporioides (Table 1).
Cultural, morphological, physiological and biochemical characters of selected isolates
The selected isolates PERM1, PERM2, PERM4 and PERM5 grew very well on different ISP media except on
Discussion
This study presents the identification, in vitro antagonistic activities, and plant growth promoting properties of potentially useful actinobacteria for the biological control of P. myriotylum. These strains were isolated from cocoyam roots from localities of the South West region of Cameroon. This is an agricultural region recognized for its importance as a producer of cocoyams. In this region, cocoyam can be grown continuously for a long time without root rot disease if the plot is not
Conclusion
Among the 171 actinobacteria isolated from cocoyam root in this particular region of Mount Cameroon, four isolates PERM1, PERM2, PERM4 and PERM5, are considered as active isolates due to their stronger antagonistic activities against P. myriotylium and other fungi. All the antagonistic endophytic actinobacteria investigated in this study demonstrated the ability to produce protease, chitinase, lipase and β-1,3-glucanase. Two of the four antagonistic actinobacteria (PERM2 and PERM4) showed
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The fellow Djuidje K.P.F is grateful to the Organization for Women in Science for the Developing World (OWSD) for providing financial support to realize this work. The authors are also grateful to Dr Sylvain Lerat for his excellent technical assistance.
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