Plant-parasitic nematodes (PPNs) are responsible for the reduction of 10% of the world crop production (McCarter, 2009). Among these parasites are the root-knot nematodes (RKNs, Meloidogyne spp.) that are widespread throughout the world and account for a large percentage of the US$ 100 billion annual loss attributed to PPNs (Ralmi et al., 2016). They present more than one reproduction mode, which contributes to making the control of these nematodes difficult (Moens et al., 2009; Adams et al., 2009). In that genus, we can find Meloidogyne incognita (Kofoid & White) Chitwood, which is a cosmopolitan and polyphagous nematode, that parasites monocots, dicots, herbaceous and woody plants (Hunt and Handoo, 2009).

Nematode management is generally based upon chemical treatments. However, most chemical nematicides are non-specific, notoriously toxic, and pose a threat to the soil ecosystem, groundwater, and human. Therefore, the use of chemical nematicides has been increasingly restricted. For example, methyl bromide and aldicarb (2-methyl-2-(methylthio)propanal-O-(N-methylcarbamoyl) oxime), previously used as nematicides, are now prohibited in several countries, making it more difficult to establish a viable crop production in some situations. In order to overcome these problems, some new nematicides have been launched in the market in recent years. This is the case, for example, of fluensulfone (Castillo et al., 2018), thioxazafen (Slomczynska et al., 2015), and fluzaindolizine (Lahm et al., 2017). However, the demand for new products for the control of PPNs is still high. We can take as an example fluensulfone, which in Brazil has been marketed under the name of Nimitz™, and is an expensive nematicide. Furthermore, in preliminary tests, it was observed in experiments with Meloidogyne exigua Goeldi in Hevea brasiliensis L. that fluensulfone was inefficient in terms of productivity per hectare and per tree (Souza, 2018). In another study with soybean (Glycine max (L.) Merr.), fluensulfone was classified as ineffective for the control of Heterodera glycines Ichinohe (Teodoro and Santos, 2017). Working with Globodera pallida (Stone) Behrens, the authors concluded that fluensulfone was less efficient than oxamyl or fostiazate (Norshie et al., 2016). In other work with Pratylenchus penetrans (Cobb) Filipjev & Schuurmans Stekhoven and Tylenchorhynchus spp., for tobacco production, fluensulfone was inefficient to reduce the number of nematodes in the soil or to increase plant growth and yield (Saude et al., 2016). Regarding toxicity, fluensulfone, in general, seems less problematic than older nematicides, according to the World Health Organization report (Dewhurst and Tasheva, 2013). However, in experiments with mice, fluensulfone increased the incidence of female lung tumors. In addition, in experiments conducted in our research group, phytotoxicity has been observed in some experiments with fluensulfone (unpublished results). Therefore, there is still much to be improved to meet the demand for new, less toxic, and more efficient nematicides, at affordable costs.

As a consequence, studies focused on the development of plant defense strategies based on natural products have increased (Chitwood, 2002; Abad and Williamson, 2010; WHO, 2000; Rich et al., 2004). In this regard, one alternative is the screening of secondary metabolites occurring in plants, as their metabolites have proved to be active against PPNs (Ntalli et al., 2010a, Ntalli et al., 2010b; Echeverrigaray et al., 2010; Thoden et al., 2009; Shakil et al., 2008; Sultana et al., 2010). Probably, these metabolites with nematicidal properties have arisen over a very long period of co-evolution between parasitic species and plants (Taba et al., 2008). For example, several substances of plant origin such as triglycerides, sesquiterpenes, alkaloids, steroids, diterpenes, and flavonoids, have presented nematicidal activity (Chitwood, 2002).

Several nematicides act through the inhibition of acetylcholinesterase (AChE), which is an enzyme that hydrolyzes choline esters. Acetylcholinesterase has very high catalytic activity, in 1 s each AChE molecule can degrade 25 000 acetylcholine (ACh) molecules, close to the limit allowed by substrate diffusion (Quinn, 1987). AChE is found in many types of conducting tissues: nerve and muscle, central and peripheral tissues, motor and sensory fibers, and cholinergic and noncholinergic fibers (Massoulié et al., 1993), existing in multiple molecular forms, with similar catalytic properties, but different oligomeric assembly and mode of fixation to the cell surface. This hydrolase is expressed in a wide range of eukaryotes, including nematodes (Nelmes et al., 1973; Johnson et al., 1982; Marks and Elliot, 1975; Pree et al., 1987). Several substances from natural sources, specially produced by plants, can inhibit AChE (Ranjan and Kumari, 2017; Patel et al., 2018).

As part of a research program focused on the bioprospection of Brazilian plant diversity from Cerrado and Atlantic Forest, in this work, the extracts of eleven Brazilian plant species were tested against M. incognita for nematostatic activity, aiming to contribute to the development of new products to control RKNs. Among the tested species Pterogyne nitens Tulasne (Fabaceae) showed to be the most active plant. Popularly named as “amendoim-bravo”, “amendoinzeiro”, “balsamo”, “cocal”, and “yvi-raro'“, P. nitens is the sole member of the genus and is used as ornamental plant due to its beauty and the odor of its flowers, fruits, and leaves. It is found in non-protected South America areas and it is at risk of extinction belonging to the list of species recommended for conservation genetics in Brazil (Lorenzi, 1998). Compounds isolated from the most active fraction were then tested against M. incognita. Such metabolites were also employed in in vitro and in silico studies with the electric eel (Electrophorus electricus L.) AChE.