A novel lipoxygenase in pea roots. Its function in wounding and biotic stress

Five lipoxygenases (LOX) were known to be present at least in pea (Pisum sativum) roots. In this study a novel lipoxygenase gene (loxN2) was characterized. In order to assess the relationship between the expression of loxN2 gene and host response to cyst nematode infection, resistant and susceptible pea roots to Heterodera goettingiana were investigated. Northern blot analysis revealed that loxN2 gene expression was repressed in both susceptible and resistant infected pea roots, the latter showing the highest decrease 48h after nematode infection. LoxN2 product is a 93,787 Da protein which shares the least amino acid sequence homology among plant lipoxygenases identified to date. A novel feature of this gene product is the lacking of 32 aminoacids in one of the external loops of the C-domain.The cDNA was expressed in Pichia pastoris and characterised. Activity ratio was determined via the ratio of hydroperoxides which are formed from linoleic acid. RP-HPLC analysis showed that the purified LOX converts linoleic acid into 13-hydroperoxy linoleic acid and 9-hydroperoxy linoleic acid in a 6:4 ratio. In situ hybridization in root tissues revealed that loxN2 gene was expressed exclusively in parenchyma cells at the nematode invasion zone. This is the result might be expected if this gene takes part to the early response of the host to nematode attack. These findings suggest that the positional specificity and down-regulation of loxN2 in resistant roots may be crucial for its involvement in host defence during nematode infection.