Robust in vitro assay system for quantitative analysis of parasitic root-knot nematode infestation using Lotus japonicus.

Root-knot nematodes are sedentary endoparasites that induce permanent infestation sites inside the roots of a broad range of crop plants. The development of effective control strategies require understanding the root-knot nematode parasitic process, however, the key molecular determinants for host manipulation during infestation remain elusive. One limiting factor has been the lack of a standardized conventional method for quantitative measurement of host parasitism by root-knot nematodes, particularly one that enables efficient downstream analyses and is free from other biological sources of variability. We report here a robust, highly reproducible system for quantitative analysis of all stages of root-knot nematode infestation using the legume Lotus japonicus as the plant host. This system provides a high quality nematode inoculum that maintains consistency in juvenile age and viability even between independently prepared populations. An optimized root transformation protocol was also developed for L. japonicus to facilitate downstream molecular studies in conjunction with the quantitative assay. Hairy root transformation efficiencies up to 91% were achieved. Root-knot nematodes formed egg masses at the root surface of both intact plants and transgenic hairy root cultures within eight weeks, confirming the assay conditions support an efficient completion of the infestation cycle. The in vitro assay system described here is compatible with other plant hosts and will benefit agricultural biotechnology research as it now enables specific high-throughput screening of nematode resistance traits together with subsequent mechanistic elucidation of the causative factors.