The significance of the biosynthesis of plant hormones in Toxoplasma gondii

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Recently, we found that Toxoplasma gondii produces a plant hormone, abscisic acid, and that it is essential for the infection of this parasite. So, we studied the effect of other plant hormones cytokinins on Toxoplasma gondii. We found that genes encoding the enzymes for the trans-zeatin biosynthesis in plants are present in T. gondii genome. Consistently, we detected intermediates for two possible trans-zeatin-biosynthesis pathways by MS analysis. A natural cytokinin, trans-zeatin, accelerated the proliferation of T. gondii. In contrast, a synthetic cytokinin, thidiazuron, inhibited the parasite proliferation in culture. This inhibition was also observed in laboratory animals. Since cytokinins regulate the cell cycle progression by enhancing the expression of D-type cyclin in plant, we carried out flowcytometric analysis to observe the effect of cytokinins on the cell cycle progression of T. gondii. Trans-zeatin hastened the cell cycle progression from G1 to S phase while thidiazuron caused the halt there. Quantitative-PCR(qPCR) analysis revealed that trans-zeatin upregulated the expression of one cyclin like as higher plant system, whereas thidiazuron downregulated its expression. These results suggest that this cyclin plays a crucial role in controlling the cell cycle progression in T. gondii and concomitantly its proliferation. Since cytokinins are also known to regulate chloroplast development in plants, we examined whether cytokinins affect apicoplast, which is a homologous organella to chloroplast in T. gondii. Immunofluorescence microscopy and qPCR analysis revealed trans-zeatin increased the number of apicoplasts but thidiazuron extinguished apicoplast. These result suggest that cytokinins are very important molecules to regulate the cell cycle of the parasite and division of apicoplast.