Cell cycle genes as markers to study the ontogeny of nematode feeding sites in plant roots

Abstract

Research on the ontogeny of feeding sites induced by sedentary plant-endoparasitic nematodes has already been in focus for many decades. Nematodes induce root cells to de-differentiate into large multinucleate and cytoplasm-dense feeding cells. Arabidopsis thaliana was chosen as a model host to study cell cycle progression in nematode feeding sites induced by the root-knot nernatode, Meloidogyne incognita, and the cyst nematode Heterodera schachtii. Cell cycle markers were used to monitor how nematode feeding sites develop and what the initial signals are that trigger certain root cells to evolve into giant cells embedded in a gall, or into a syncytium. Tritiated thymidine incorporation experiments were applied to detect DNA synthesis, which was used as a marker for the S phase of the cell cycle. Extending our analysis, the expression pattern of two cyclin-dependent kinases and two mitotic cyclins were examined. The CYCB1;1 gene was used as a marker for the G2 phase and CDKB1;1 arid CYCA2;1 for the S and G2 phases, whereas the CDKA;1 gene was used as a marker for all cell cycle phases. Nematode-infected seedlings were also treated with two cell cycle inhibitors (hydroxyurea and oryzalin) to investigate the relevance of DNA synthesis and mitosis on the development of these feeding sites. A strong correlation was observed between initiation of feeding cells by both root-knot and cyst nematodes, and the induction of DNA synthesis and the expression of particular cell cycle genes. Our results show that endoreplication and mitotic cycles play a role during the root cell dedifferentiation process caused by nematode infection.