Host-root exudates increase gene expression of asparagine synthetase in the roots of a hemiparasitic plant Triphysaria versicolor (Scrophulariaceae)

Triphysaria is a facultative root parasite in the Scrophulariaceae family. Similar to other related parasites, the development of the parasitic life c ycle is initiated by molecular signals released from appropriate host roots . Using a differential display, we isolated cDNAs preferentially abundant i n T, versicolor roots exposed to Trifolium repens (white clover) root exuda tes in vitro. Sequence analysis indicated that one of the differentially ex pressed cDNAs had significant homology to the nitrogen-assimilating enzyme, asparagine synthetase (AS). T. versicolor AS cDNA clones were isolated and placed into three distinct classes on the basis of nucleotide sequence var iations. All three classes encoded identical AS proteins. AS was expressed in both roots and shoots of in-vitro-cultured T, versicolor. Steady-state l evels of AS mRNA increased in T. versicolor roots several-fold when seedlin gs were exposed to exudate obtained from hydroponically grown Arabidopsis t haliana roots. Therefore, AS transcript levels increased in response to exu dates from two different hosts (Trifolium and Arabidopsis). The T. versicol or AS message levels increased to a similar magnitude when seedlings were i ncubated in the dark. Interestingly, AS levels were unaffected by treatment with the Striga haustoria inducer 2,6-dimethoxybenzoquinone. The potential role of AS in root parasitism is discussed. (C) 1998 Elsevier Science B.V. All rights reserved.