Studies of the initial interactions of Striga asiatica with the non-host plant species Tagetes erecta (marigold) established that parasite penetration through the root is arrested most frequently in the cortex. The arrest of parasite ingress is associated with browning and necrosis of root cortical cells flanking the invading endophyte and with increased intracellular wall appositions on the root cell walls directly adjacent to the plant-parasite interface. Using a polymerase chain reaction-based differential cDNA amplification strategy followed by 5'-RACE, we have identified several gene products whose expression is induced in marigold roots during attempted parasitism by Striga. Among these was a 917 bp cDNA encoding a 221 amino acid protein with significant homology to proteins encoded by disease resistance genes from other plant species, including N, RPP5, L6 and M. This cDNA was subsequently used to isolate a nuclear gene, designated NRSA-1, for non-host resistance to Striga asiatica. NRSA-1 is a member of a small gene family in marigold consisting of two to four members. RNA gel blot analysis showed that NRSA-1 transcripts accumulate to high levels in roots near the site of Striga invasion within 120 h after parasite attachment, and appear at lower levels throughout the rest of the plant under Striga parasitism. NRSA-1 expression is rapidly induced by treatment with jasmonic acid (JA), but not by mechanical wounding, treatment with salicylic acid, paraquat or ABA. A possible role for NRSA-1 in the non-host resistance mechanism is discussed.
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