Serotonin Regulates the Feeding and Reproductive Behaviors of Pratylenchus penetrans

Abstract

The success of all plant-parasitic nematodes is dependent on the completion of several complex behaviors. The lesion nematode Pratylenchus penetrans is an economically important parasite of a diverse range of plant hosts. Unlike the cyst and root-knot nematodes, P. penetrans moves both within and outside of the host roots and can feed from both locations. Adult females of P. penetrans require insemination by actively moving males for reproduction and can lay eggs both within and outside of the host roots. We do not have a complete understanding of the molecular basis for these behaviors. One candidate modulator of these behaviors is the neurotransmitter serotonin. Previous research demonstrated an effect of exogenously applied serotonin on the feeding and male mating behaviors of cyst and root-knot nematodes. However, there are no data on the role of exogenous serotonin on lesion nematodes. Similarly, there are no data on the presence and function of endogenous serotonin in any plant-parasitic nematode. Here, we establish that exogenous serotonin applied to P. penetrans regulates both feeding and sex-specific behaviors. Furthermore, using immunohistochemistry and pharmacological assays, our data suggest that P. penetrans utilizes endogenous serotonin to regulate both feeding and sex-specific behaviors.

Fig. 1

Serotonin and octopamine signaling affect stylet thrusting behavior in Pratylenchus penetrans. Adult male and female P. penetrans nematodes were exposed to either a 5% M9 buffer solution (white bars) or one of the following compounds: 5 mM serotonin, 2 mM fluoxetine, 10 mM imipramine, 10 mM octopamine, or 26 mM phentolamine (black bars). The number of stylet thrusts was counted for 1 min under a compound microscope; n > 30 per treatment; *** and ** indicate P < 0.001 and 0.01, respectively; error bars = standard error of the mean.

Fig. 2

Serotonin induces vulva contractions in Pratylenchus penetrans.20 to 30 adult female P. penetrans nematodes were exposed to either a control solution (white bars, 5% M9, with the exception of the 100 mM imipramine, which was dissolved in H₂O) or one of the following compounds: 5 mM serotonin, 2 mM fluoxetine, 10 mM imipramine, 100 mM imipramine,10 mM octopamine, or 26 mM phentolamine (black bars). The number of vulva contractions per min were counted for 1 min under a compound microscope. Data from each assay were analyzed using an unpaired t test; ***, **, and * indicate P < 0.001, 0.01, and 0.05, respectively; error bars = standard error of the mean.

Fig. 3

Antibody staining reveals endogenous serotonin in Pratylenchus penetrans. Fluorescence (left) and differential interference contrast overlay (right) images of antiserotonin staining. All images are arranged anterior to the left and dorsal aspect to the top. Arrowheads indicate serotonin immunoreactive cells. A, Anteriors of both sexes have two pairs of serotonin immunoreactive cells. Putative ADF homologs and one unidentified pair (question mark) were consistently and strongly immunoreactive. Unidentified neurons are anterior to the nerve ring whereas ADF are posterior to the nerve ring. The putative NSM homologs in the metacorpus are weakly and inconsistently immunoreactive. B, In the vulva region of adult females, the putative VC4 and VC5 neuron homologs are strongly immunoreactive. C. In the ventral nerve cord of the male, several putative CP neuron homologs are strongly immunoreactive.

Fig. 4

Schematic of antiserotonin immunoreactivity in Pratylenchus penetrans and Caenorhabditis elegans as determined by this study and previous research, respectively (Duerr et al. 1999; Loer and Kenyon 1993; Loer and Rivard 2007).