NRSA-1: a resistance gene homolog expressed in roots of non-host plants following parasitism by Striga asiatica (witchweed)

Studies of the initial interactions of Striga asiatica with the non-host pl ant 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 ampli fication strategy followed by 5'-RACE, we have identified several gene prod ucts whose expression is induced in marigold roots during attempted parasit ism by Striga. Among these was a 917 bp cDNA encoding a 221 amino acid prot ein with significant homology to proteins encoded by disease resistance gen es from other plant species, including N, RPP5, L6 and M. This cDNA was sub sequently used to isolate a nuclear gene, designated NRSA-1 for non-host re sistance to Striga asiatica. NRSA-1 is a member of a small gene family in m arigold consisting of two to four members. RNA gel blot analysis showed tha t NRSA-1 transcripts accumulate to high levels in roots near the site of St riga invasion within 120 h after parasite attachment, and appear at lower l evels throughout the rest of the plant under Striga parasitism. NRSA-1 expr ession is rapidly induced by treatment with jasmonic acid (JA), but not by mechanical wounding, treatment with salicylic acid, paraquat or ABA. A poss ible role for NRSA-1 in the nonhost resistance mechanism is discussed.