Arbuscular mycorrhizae maintain lemongrass citral levels and mitigate resistance despite root lesion nematode infection
Graphical abstract
Introduction
Cymbopogon citratus (DC.) Stapf (Poaceae), commonly known as lemongrass, is an aromatic plant whose essential oil is widely used as aroma and fragrance ingredient in the cosmetic industry (Zielinska et al., 2018). Citral, the major component of C. citratus essential oil, has antimicrobial and antioxidant activities (Mahajan et al., 2020) and is of great economic importance for its extensive pharmaceutical applications (Pereira et al., 2013).
Research has shown that environmental conditions may redirect metabolic pathways in plants, influencing essential oil production, both qualitatively and quantitatively (Lermen et al., 2015). Increased accumulation of secondary metabolites has been observed in medicinal and aromatic plants subjected to abiotic and biotic stresses (Mahajan et al., 2020). Another study found that secondary metabolite levels may decrease or remain unchanged under stress conditions (Selmar and Kleinwächter, 2013).
Plant-parasitic nematodes are limiting factors for the vegetative development of medicinal and aromatic plant species (Mendonça et al., 2017; Tiwari et al., 2017) and may alter the production of biologically active compounds (Gupta et al., 2017; Tiwari et al., 2017). Members of the genus Pratylenchus, commonly referred to as lesion nematodes, are among the most damaging pathogens in C. citratus production (Albuquerque et al., 2018; Gomes et al., 2007). The genus Pratylenchus comprises migratory endoparasites that migrate through the root cortex, thrusting their stylet into parenchymal cells and injecting hydrolytic enzymes to feed on cellular content. The results are cortical tissue degradation and reduced root growth accompanied by the formation of dark lesions and necrotic areas (Jones et al., 2013). Biological control is an environmentally friendly strategy to reduce the damage caused by phytonematodes, especially in plants grown for extraction of essential oils or other secondary metabolites.
Arbuscular mycorrhizal fungi (AMF) are important biological agents investigated for nematode control, with good results against Meloidogyne spp. (Khan et al., 2017; Sharma and Sharma, 2017). These fungi form mutually beneficial associations with host plant roots. AMF establish an extensive network of hyphae that help plants absorb water and nutrients from the soil (Campos, 2020) and in return are provided with carbohydrates and lipids by plant roots (Keymer et al., 2017). This relationship alters the composition of root exudates, influencing interactions with other organisms (Jung et al., 2012). In medicinal plants, AMF symbiosis may enhance essential oil production (Lermen et al., 2015; Urcoviche et al., 2015).
Another advantage of AMF inoculation is systemic resistance induction against nematodes. For instance, the AMF Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe was able to induce systemic resistance to Pratylenchus penetrans (Cobb) Filipjev & Schuurmans Stekhoven and Meloidogyne incognita (Kofoid & White) Chitwood in tomato (Vos et al., 2012). Glomus intraradices Schenck & Smith induced resistance to Pratylenchus coffeae (Zimmerman) Filipjev & Schuurmans Stekhoven and Rotylenchulus reniformis Linford & Oliveira in banana (Elsen et al., 2008) and against Xiphinema index Thorne & Allen in grapevine (Hao et al., 2012). Mycorrhizal-induced resistance may be associated with priming of jasmonic acid-dependent defenses and cell wall-mediated immunity or, less frequently, with priming of salicylic acid-dependent systemic acquired resistance (Cameron et al., 2013; Song et al., 2013).
It is important to highlight that AMF–plant–pathogen relationships are extremely complex, and the effects of biological agents may not always be positive (Alvarado-Herrejón et al., 2019; Brito et al., 2018; Frew et al., 2018). Factors such as inoculation time, plant species and cultivar, AMF order and genus, and nematode species can affect treatment efficiency (Gough et al., 2020). These are some of the reasons why biological control agents are not routinely used in agricultural practice (Salvioli and Bonfante, 2013). Further studies are still needed to elucidate the mechanisms of action and harness the full potential of biocontrol methods.
Despite the large number of studies on the use of AMF for nematode control and plant resistance induction, there is limited information available regarding the interactions between AMF, nematodes, resistance induction, and essential oil production. This study aimed to determine the penetration and reproduction of Pratylenchus brachyurus (Godfrey) Filipjev & Schuurmans Stekhoven on C. citratus roots, assess the effects of two AMF species on vegetative development and essential oil composition, and study the combined influence of these microorganisms on plant defense responses.
Section snippets
General experimental procedures
Experiments were conducted in a greenhouse (23°47′28.4″S 53°15′24.0″W, 379 m elevation) according to a completely randomized design. Experimental units consisted of plants grown in polystyrene pots containing 500 cm3 of substrate (soil and sand mixed at a 1:1 volumetric ratio and autoclaved for 2 h at 120 °C).
The applied soil texture was very clayey (11% silt, 15% sand and 74% clay), with 0.89% organic matter, 0.05 cmolc dm−3 potassium, 0.69 cmolc dm−3 magnesium, 0.93 mg dm−3 phosphorus,
Penetration of P. brachyurus into C. citratus
Host × Time exerted significant effects on the total number of P. brachyurus penetrating roots (Fig. 1). Regression analysis showed that the number of nematodes penetrating C. citratus was highest at 17 DAI, following a quadratic adjustment. In maize, nematode penetration increased linearly with time (Fig. 1), but significant differences in P. brachyurus number between maize and C. citratus roots were only observed at 22 DAI, with lower values in maize (Fig. 1).
Effect of AMF inoculation on P. brachyurus reproduction
C. etunicatum and R. clarus
Discussion
In this study, C. citratus was found to be susceptible to P. brachyurus, although less than maize, which is well-known for being a good host to nematodes (Fig. 1). Previous reports have shown evidence that C. citratus is susceptible to Pratylenchus spp. (Albuquerque et al., 2018; Gomes et al., 2007) and P. brachyurus (Jenkins, 1969), indicating the need for adequate management strategies.
AMF were not effective in reducing nematode population levels; on the contrary, AMF significantly increased
Conclusion
This study showed that coinoculation with AMF and P. brachyurus increased nematode reproduction on C. citratus, possibly affecting plant development. On the other hand, AMF maintained citral, the major compound of C. citratus essential oil, at levels similar to those of nematode-free plants and increased the activity of pathogenesis-related proteins such as POX, GLU, and PPO. Finally, the increase in the population of P. brachyurus observed in this experiment, as well as observations previously
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 the Brazilian National Council for Scientific and Technological Development (CNPq) for the master's scholarship granted to MTRS (grant no. 132466/2019–8) and the PhD productivity grant awarded to CRDA (grant no. 304070/2016–5). AC (grant no. 344568/2019-1), LKR (grant no. 344503/2019-01), and AM (grant no. 344530/2019-01) thank the Brazilian Federal Agency for Support and Evaluation of Graduate Education (CAPES) for the master's and PhD scholarships.
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