Clubroot disease caused by soilborne pathogen Plasmodiophora brassicae Woron is an emerging threat to the Brassicaceae family crop production worldwide, including China (Wei et al., 2021). P. brassicae has a broad host range that infects more than 300 species of cruciferous plants and is widely distributed in more than 60 countries, resulting in 10–15% yield reduction losses around the globe (Ren et al., 2016). Incidence of clubroot disease is reported all over China with an average yield loss are range between 20–30%. However, the regions of Chongqing, Hubei, Sichuan, Hunan, Yunnan, and Zhejiang are severely affected by this disease and under serious threat of yield reduction (Chai et al., 2014, Hu et al., 2021).
P. brassicae have a very complex life cycle consisting of different zoosporic stages, the formation of plasmodia inside host cells, and resting spores (Kageyama and Asano, 2009). Infected soil acts as a primary source of infection, and pathogen survives in the soil for a longer time (up to 20 years) in the form of resting spores (HWANG et al., 2012). During the growing season, the zoospores germinate from the resting spores enter the host plants through wounds and root hairs, which cause cell swelling and results in "gall or club" formation on the roots, multiply in the xylem vessels that inhibit the water and nutrients uptake ability from the soil (Peng et al., 2015). Infected plants generally show symptoms of yellowing, stunting growth, and wilting, leading to the death of the whole plant (Peng et al., 2015). Once the pathogen completes the life cycle within a host plant, millions of zoospores are released from galls to rhizosphere soil and spread from plant-plant, within the field, and field-filed through irrigation water, mechanical operations, or water erosion (Chai et al., 2014).
The prevention and control of cruciferous clubroot have become a major concern due to the wide host range and longtime persistence in the soil as a lethal infection. To date, the important methods to control this disease are the application of fungicides such as benzimidazoles, chlorothalonil, cyclophosphamide, and quintozene (Chai et al., 2014), resistant cultivars (Diederichsen et al., 2009), crop rotation (YANG et al., 2020), and liming (Murakami et al., 2002). Many studies have reported that the excessive use of agrochemicals develops resistance in the pathogen, is environmentally unfriendly, and have human health concerns (Botero et al., 2019, Hu et al., 2021), and crop lose their resistance against the pathogen due to rapid mutation ability and strong pathogenicity of P. brassicae (Strelkov et al., 2018). It is suggested that methods have limitations and only alleviate the incidence of clubroot disease but cannot completely control it. So, there is an urgent need to develop durable, efficient, and environmentally friendly control measures to mitigate this devastating disease.
Biological control through potent endophytes and rhizobacteria provides an effective and environmentally friendly alternative control measure to mitigate soilborne diseases (Ahmed et al., 2022). It is reported that biocontrol agents (BCAs) suppress soilborne diseases through the mechanism of antibiosis, niche exclusion, nutrient acquisition, induction of resistance, plant growth promotion, and production of antimicrobial compounds (Xu et al., 2011, Mendes et al., 2013, Nguvo and Gao, 2019). Bacillus subtilis XF-1 can produce antimicrobial compounds and significantly suppress the incidence of clubroot disease up to 76.92% by improving the soil microbial diversity (Liu et al., 2018). The application of Streptomyces alfalfae XY25T improves soil health, regulates rhizospheric bacterial and fungal communities, enhances plant growth, and mitigates clubroot disease up to 69.4% (Hu et al., 2021). B. subtilis QST713 can reduce clubroot incidence in canola crops up to 86% through induction of host resistance and antibiosis (Lahlali et al., 2013).
The plant rhizosphere acts as a hot spot habit for the diversity of microorganisms and is considered one of the most complex ecosystems on the earth (Raaijmakers et al., 2009). Many studies are reported that soil health and rhizospheric microbial diversity plays an important role in maintaining plant health (Daval et al., 2020, Cai et al., 2021), and an imbalance of rhizospheric microbial diversity results in the development of soilborne diseases (Wei et al., 2019, Zhang et al., 2020). Improper use of agrochemicals such as fertilizers, pesticides, herbicides, and root exudates leads to changes in the soil microenvironment, which results in the imbalance of soil micro-ecological environment, affect soil health, and also influence the survival and growth of the pathogen in the soil (Sudini et al., 2011, Xue et al., 2018). Therefore, healthy soil and balanced microbial diversity are considered key factors for healthy crop production and disease suppression (Janvier et al., 2007, Zhang et al., 2020).
In our previous study, we evaluated the biocontrol effect of inter-genus (Bacillus cereus BT-23 + Lysobacter antibioticus 13 − 6) and intra-genus (L. capsici ZST1-2 + L. antibioticus 13 − 6) bacterial co-culture to suppress clubroot disease of Chinese cabbage in greenhouse experiment through the metabolomic approach (Wei et al., 2021). Results showed that inter-genus bacterial co-culture produced more secondary metabolites and significantly suppressed clubroot disease incidence than intra-genus bacterial co-culture. However, their potential effect on rhizospheric bacterial community diversity is still unknown. In this study, we enhance our knowledge to decipher the impact of these biocontrol agents on soil health and bacterial community diversity as single, inter-/intra-genus co-culture, and microbial consortia. This study aimed to provide theoretical and experimental knowledge to explore the relationship between rhizospheric bacterial community diversity and P. brassicae to mitigate clubroot by engineering the rhizosphere microbiome.