Magnesium oxide induces immunity against Fusarium wilt by triggering the jasmonic acid signaling pathway in tomato
Introduction
Tomato (Solanum lycopersicum L., Solanaceae) is the second most important fruit or vegetable crop following potato (Solanum tuberosum L.). Currently, the global production of tomato is approximately 100 million tons of fresh fruit per 3.7 ha (Food and Agriculture Organization (FAO), 2020). Fusarium wilt, caused by Fusarium oxysporum f. sp. lycopersici (Sacc.) W. C. Snyder and H. N. Hans (FOL) is a detrimental soil-borne fungal disease that occurs worldwide and can reduce tomato crop production under both greenhouse and field conditions (Sartaj et al., 2011; Harikrushana et al., 2014). Fusarium oxysporum f. sp. lycopersici infects infects tomato plants via their roots, colonizes the xylem vessels, and causes symptoms, such as wilting, yellowing, and necrosis of the aerial plant parts. Fusarium wilt in tomatoes is mainly managed through chemical fumigation of the soil and cultivation of resistant cultivars. Although the application of soil fumigants, such as methyl bromide, is an effective method of disease management, the use of such pesticides can adversely affect the environment and human health (Sivan and Chet, 1993; Xie et al., 2015). While breeding Fusarium wilt-resistant tomato cultivars is both a cost-effective and environmentally friendly approach, new FOL strains continue to emerge, counteracting the resistance of such cultivars. These limitations in Fusarium wilt control have led researchers to focus on various alternatives. Among these, biocontrol using endophytic microorganisms has gained considerable attention as an ecofriendly and cost-effective approach to stimulate plant innate immunity by inducing systemic resistance and promoting plant growth for sustainable crop production (Aime et al., 2013; Jogaiah et al., 2013; Pieterse et al., 2014; Abdelrahman et al., 2016). However, the degree of resistance induced by these biological agents is highly variable across seasons, crops, or fields (de Lamo and Takken, 2020).
Stimulating plant innate immunity through natural and synthetic elicitors may also be a promising alternative to conventional Fusarium wilt control methods (Jogaiah et al., 2018). Although many natural and synthetic elicitors that stimulate plant innate immunity are available (Dewen et al., 2017; Zhou and Wang, 2018), the ones that can induce plant innate immunity against soil-borne pathogens, including FOL, are scarce. Recently, metal monoxide nanoparticles (NPs) have garnered much attention as plant immune inducers (Imada et al., 2016; Rastogi et al., 2017; Achari and Kowshik, 2018). Elmer et al. (2018) reported that foliar spray application of metal oxide (e.g., cupric oxide) NPs protected watermelon against Fusarium oxysporum f. sp. niveum. In these plants, polyphenol oxidase and pathogenesis-related protein 1, which are involved in plant immunity against many pathogens, were strongly upregulated. Similarly, drenching roots with calcined magnesium oxide (MgO) NPs (diameter, 20–200 nm; mean diameter, 100 nm) induced immunity against the soil pathogen Ralstonia solanacearum in both tomato (Imada et al., 2016) and Arabidopsis thaliana (Ota et al., 2019). Magnesium oxide NPs have several advantages, such as low cost, non-toxicity, high biocompatibility, great stability under harsh processing conditions, relevant biomedical applications, and strong antimicrobial properties without photo-activation (Akram et al., 2018).
In a previous study, we demonstrated that reactive oxygen species (ROS) and the salicylic acid (SA) signaling pathway induced by MgO were essential for MgO-induced immunity against bacterial wilt caused by R. solanacearum in A. thaliana (Ota et al., 2019). However, the efficiency of MgO in controlling fungal plant diseases, such as Fusarium wilt in tomatoes, remains to be examined. Since both R. solanacearum and FOL infect tomato plants via their roots and colonize the xylem vessels, we speculated that Fusarium wilt could also be suppressed by MgO. In this context, the aims of the present study were to investigate the effects of MgO on tomato plant immunity against FOL and to explore the possible mechanisms regulating this immunity.
It has been reported that MgO NPs possess antibacterial (He et al., 2016; Imada et al., 2016; Cai et al., 2018; Abdel-Aziz et al., 2020), antifungal (Parizi et al., 2014; Koka et al., 2019; Chen et al., 2020; Kong et al., 2020), and antiprotozoal (Hussein et al., 2018) properties. Thus, we also examined the antifungal activity of MgO against FOL in the present study.
Section snippets
MgO preparation
The MgO (UCM250; Ube Material Industries, Yamaguchi, Japan) was produced by calcining Mg (OH) 2 at 700–800 °C. Water-dispersible MgO powder (KF-37) was prepared by Sankei Chemical Co. Ltd. (Tokyo, Japan).
Plant material
Tomato plants (‘Momotaro’; Takii Seed, Kyoto, Japan) were grown in small pots (diameter, 9 cm; height, 7.6 cm) containing a vermiculite and perlite (1:1) mixture at 25 °C under a 12-h photoperiod with a photon flux density of 100 μmol m−2 s−1 for 30 days. The 30-day-old tomato plants were
Antifungal effects of MgO on FOL
The MgO treatment showed no fungistatic or fungicidal activity against FOL at a concentration of 100 ppm, but showed strong fungicidal activity at a concentration of 1000 ppm (Fig. S1).
Detection of H2O2 in tomato plants
Reactive oxygen species generation is pivotal in MgO-induced immunity against the bacterial vascular pathogen R. solanacearum (Ota et al., 2019). Thus, we examined changes in H2O2 accumulation in the roots of MgO-treated tomato plants using DAB staining. We observed ROS generation in the root tips and xylem
Discussion
Abdel-Aziz et al. (2020) showed that MgO NPs completely inhibited the mycelial growth of FOL at a concentration of 15.36 μg mL−1 as well as severely altered hyphal morphology and significantly damaged fungal membrane integrity. Moreover, Parizi et al. (2014) showed that FOL growth was inhibited by approximately 30 % when cultured in a liquid medium containing 2 % MgO NPs. In contrast, in the present study, MgO did not inhibit the mycelial growth of FOL at a concentration of 100 ppm, although it
Conclusions
To the best of our knowledge, the present study is the first to reveal that pretreatment of tomato roots with the safe and ecofriendly compound MgO induced long-term immunity against FOL. JA signaling is essential for MgO-induced immunity against FOL. The master regulator of JA signaling, MYC2, and MTFs were rapidly upregulated in tomato plants treated with MgO. Moreover, the late wound-responsive gene TD, regulated by MYC2, was rapidly upregulated earlier than its upstream genes, including MYC2
Funding
This work was supported by a Grant-in-Aid for Scientific Research (C) [grant number 18K05647] from the Japanese Society for Promotion of Sciences (JSPS).
CRediT authorship contribution statement
Isamu Fujikawa: Investigation, Writing - original draft, Visualization. Yushi Takehara: Investigation, Visualization. Makiko Ota: Investigation, Visualization. Kiyoshi Imada: Investigation, Visualization. Kazunori Sasaki: Investigation, Visualization. Hiroshi Kajihara: Investigation, Visualization. Shoji Sakai: Project administration. Sudisha Jogaiah: Project administration, Writing - original draft, Writing - review & editing. Shin-ichi Ito: Supervision, Conceptualization, Methodology, Writing
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 authors thank Dr. Takashi Tsuge, Nagoya University, Japan, for kindly providing Fusarium oxysporum CK3-1 and Dr. Philipp Franken, Leibniz-Institute of Vegetables and Ornamental Crops, Germany, for kindly providing the seeds of tomato hormone mutants. The authors also thank Ube Materials Co. Ltd. and Sankei Chemical Co. Ltd. for kindly providing the MgO samples. We would like to thank Editage (www.editage.com) for English language editing.
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