Bt Research 2024, Vol.15, No.3, 110-117 http://microbescipublisher.com/index.php/bt 112 3.3 The role of salicylic acid and jasmonic acid pathways in systemic resistance The salicylic acid (SA) and jasmonic acid (JA) pathways are critical components of plant defense mechanisms, and Bacillus spp. have been found to interact with these pathways to enhance systemic resistance against Meloidogyne spp. For instance, B. amyloliquefaciens QST713 and B. firmus I-1582 have been shown to upregulate genes involved in the initial stages of the JA synthesis pathway, suggesting the stimulation of an intermediate molecule, likely OPDA, rather than JA itself in the short-term systemic response (Gattoni et al., 2023). Additionally, these Bacillus spp. stimulated a SA-responsive defense-related gene after one week, indicating the involvement of SA in long-term systemic defense (Gattoni et al., 2023). B. firmus I-1582 also primed SA and JA-related genes in tomato plants at different times after nematode inoculation (Ghahremani et al., 2020). These interactions with the SA and JA pathways underscore the complex role of Bacillus spp. in plant defense against nematode infections. Obviously, Bacillus spp. exhibit a multifaceted approach to managing Meloidogyne spp. through direct antagonism and the induction of systemic resistance, with the SA and JA pathways playing a significant role in the latter. These mechanisms highlight the potential of Bacillus spp. as biocontrol agents in integrated pest management strategies against root-knot nematodes. 4 Efficacy of Bacillus spp. in ControllingMeloidogyne spp. in Agricultural Settings 4.1 Case study 1: control of Meloidogyne incognita in cotton using Bacillus spp. Meloidogyne incognita represents a significant threat to cotton production, particularly in the south-eastern United States, where it is considered the most economically damaging pathogen. The nematode's wide host range, extensive geographical distribution, and the severe damage symptoms it causes, coupled with its complex biology and life cycle, make it a formidable pest to manage (Davis and Kemerait, 2021). Recent advances in integrated pest management (IPM) have highlighted the potential of Bacillus spp. as biological control agents against M. incognita. Studies evaluating the efficacy of Bacillus spp. reveal that certain strains, such as B. amyloliquefaciens QST713 and B. firmus I-1582, can manage nematode populations effectively. These strains have been shown to exhibit both direct antagonistic capabilities and systemic activity against M.incognita (Gattoni et al., 2023). The direct antagonistic effect of B. firmus I-1582, for instance, has been demonstrated through in vitro assays, where extracted metabolites from the bacterium significantly increased the mortality rate of M. incognita's second-stage juveniles. This indicates a potential for these metabolites to be used in nematode management strategies (Gattoni et al., 2023). Systemic resistance is another critical aspect of the biological control exerted by Bacillus spp. Split root assays have shown that both B. amyloliquefaciens QST713 and B. firmus I-1582 can induce systemic resistance in cotton plants, leading to a decrease in nematode population density. Interestingly, the systemic activity observed was associated with the upregulation of genes involved in the jasmonic acid (JA) synthesis pathway, suggesting that an intermediate molecule, likely OPDA, is stimulated by the bacteria rather than JA itself in the short-term response. Furthermore, after one week, a salicylic acid (SA)-responsive defense-related gene was upregulated, indicating that SA also plays a role in the long-term systemic defense response(Gattoni et al., 2023). The ability of these Bacillus spp. to colonize cotton roots effectively and maintain their population over time is crucial for their success as biological control agents. Quantitative PCR (qPCR) assays have confirmed that B. amyloliquefaciens QST713 and B. firmus I-1582 can successfully colonize cotton roots, with their concentration remaining stable over a 24-day period (Gattoni et al., 2023). This case demonstrates that the integration of Bacillus spp. into IPM strategies offers a promising avenue for the control of M. incognita in cotton. The dual action of direct antagonism and induced systemic resistance provided by strains such as B. amyloliquefaciens QST713 and B. firmus I-1582 represents a sustainable and effective approach to managing this pervasive nematode pest (Davis and Kemerait, 2021; Gattoni et al., 2023).
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