Plant home care

Plant home care

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Plant home care is important for the quality of crops, and many studies have been conducted on this topic. The quality of crops is greatly affected by soil contamination with pesticides in general, especially harmful insecticides. It has been reported that an insecticide, imidacloprid (IMI), affects a pest-control mechanism of a parasitoid, *Diaeretiella rapae* (Hymenoptera: Braconidae), by inhibiting an acetylcholine receptor (AChR) in the parasitoid larva ([@iew075-B23], [@iew075-B27]). In other cases, phytochemicals such as oxalic acid and glycyrrhizin inhibit parasitoid larval growth and development ([@iew075-B30], [@iew075-B20], [@iew075-B1], [@iew075-B25], [@iew075-B26]). In contrast, we previously reported that the amino acid tryptophan (TRP) enhances the parasitism success rate of *D. rapae* on IMI-treated corn. This enhanced parasitism success is considered to be due to a growth stimulation of the parasitoid larva by TRP ([@iew075-B9]). We found that TRP has a synergistic effect on parasitism success and on the body size of the parasitoid. This research provides further insight into parasitism biology. To investigate TRP synthesis from plant endogenous TRP biosynthesis and secretion in parasitized larvae, we investigated a putative TRP transporter (PAT) gene in *D. rapae*. Based on sequence homology and sequence analysis, we cloned and expressed the *DR-PAT* gene. We examined the ability of the transporter to form hetero-oligomers. This analysis determined that hetero-oligomerization occurred in the trans-Golgi network. We then analyzed the effect of silencing the *DR-PAT* gene on larval development by microinjecting IMI into *DR-PAT*-siRNA-treated third-instar *D. rapae* larvae. We found that larval growth and development were significantly reduced by siRNA-*DR-PAT*. A combination of TRP and IMI increased parasitism success in larvae injected with *DR-PAT*-siRNA. These results were also confirmed by a *DR-PAT*-siRNA injection experiment. Taken together, our results suggest that endogenous TRP biosynthesis, secretion, and transfer in larvae are critical for larval parasitism success.

TRP plays a central role in the central nervous system (CNS) of both humans and other animals ([@iew075-B27]). TRP deficiency is involved in mood, anxiety, and behavioral problems in humans ([@iew075-B6]). Interestingly, a genetic linkage study demonstrated that the *DR-PAT* gene is closely linked to the *Larva migrans, a disease*-*inducing agent*, gene in the lepidopteran *Helicoverpa armigera* ([@iew075-B8]). Interestingly, *DR-PAT*-siRNA was shown to be highly lethal to larvae of *H. armigera*, even in the presence of a carrier, a small amount of siRNA in water was sufficient to cause death in this species. Thus, the siRNA might have been actively transferred through the hemolymph into the larval nervous system.

In *T. molitor*, the larval nervous system, including the central and peripheral nervous systems, is well developed and plays critical roles in the formation and development of the integument, feeding, and reproduction ([@iew075-B18], [@iew075-B17], [@iew075-B16]). Larval development in *T. molitor* starts at the 1st instar, after hatching. In this developmental stage, two larval brains develop as bilateral symmetrical structures called the first and second brains ([Fig. 1](#iew075-F1){ref-type="fig"}). *T. molitor* larval brains play critical roles in olfaction, gustation, and feeding ([@iew075-B18], [@iew075-B17]). There is evidence that TRP is involved in the neurogenic development of the central nervous system in *T. molitor* larvae ([@iew075-B14], [@iew075-B16]).

Here, we first identified *TRP* in the nervous system of *T. molitor*. A BLAST search of the nucleotide sequence of *TRP* obtained in this study revealed that the open reading frame contains a putative signal peptide of 27 amino acids. This is a typical feature of *Drosophila TRP* genes ([@iew075-B37], [@iew075-B35]). Phylogenetic analysis of *T. molitor* TRP and the four TRP genes of other insects indicated that *T. molitor* TRP clusters within the *D. melanogaster* TRP group ([Fig. 2](#iew075-F2){ref-type="fig"}). The *T. molitor* TRP protein contains the six predicted transmembrane domains typical of TRP proteins ([Fig. 3](#iew075-F3){ref-type="fig"}). Two cysteine residues, C1150 and C1160, are highly conserved in this group.

A previous study indicated that *T. molitor* brain neuroblasts undergo two additional rounds of symmetrical divisions in the 2nd and 3rd instar larval brain ([@iew075-B22]). In contrast, neuroblasts undergo only one more round of asymmetric divisions after the 3rd instar larval brain. By contrast, we found that *T. molitor* TRP is expressed in early- and late-born neuroblasts ([Fig. 4](#iew075-F4){ref-type="fig"}). This finding is consistent with our previous observation that the early- and late-born neuroblasts in the *T. molitor* brain produce *T. molitor* TRP transcripts with a similar abundance, even though the number of late-born neuroblasts is greater than the number of early-born neuroblasts ([@iew075-B37]). Because *T. molitor* TRP is primarily expressed in late-born neuroblasts, we believe that TRP is involved in neural differentiation during metamorphosis. The neuroblast of *T. molitor* undergoes a single round of symmetrical divisions before differentiation in the 2nd larval instar brain. We speculate that this may be the reason why the number of early-born neuroblasts is fewer than the number of late-born neuroblasts ([@iew075-B22]).

In addition to the above mentioned, we found that *T. molitor* TRP also regulates apoptosis during metamorphosis. However, we did not identify the signal pathways that regulate *T. molitor* TRP-mediated cell death. *D. melanogaster* *Bric-a-brac/Tramtrack/Broad* (*btb*) was shown to function in the nervous system of *Drosophila* ([@iew075-B40], [@iew075-B41]). However, the role of *btb* in metamorphosis has not been reported. As one of the major regulators of neural development, *Trk* is highly expressed in early