Induction of Ltp (lipid transfer protein) and Pal (phenylalanine ammonia-lyase) gene expression in rice roots colonized by the arbuscular mycorrhizal fungus Glomus mosseae

The expression of a lipid transfer protein (LTP) gene is regulated in Oryza sativa roots in response to colonization by the mycorrhizal fungus Glomus mosseae. Transcript levels increased when the fungus forms appressoria and penetrates the root epidermis and decreased at the onset of the intercellular colonization of the root cortex. The analysis of histochemical GUS staining in transgenic rice plants carrying the Ltp/Gus construct confirm the induction of Ltp gene associated with fungal appressoria formation and penetration area. The induction of Ltp gene expression coincided in time with a transient increase in the expression of a phenylalanine ammonia-lyase (Pal) gene and a transient accumulation of salicylic acid (SA) in the mycorrhizal roots. The expression of Ltp and Pal was induced in rice roots after treatment with SA and Pseudomonas syringae indicating that both genes could be implicated in the plant defence response. The exogenous application of SA to rice interacting with the mycorrhizal fungus did not affect appressoria formation but, instead, resulted in a transient delay of root mycorrhization. Nevertheless, although Ltp maintained a prolonged SA-induced expression level, mycorrhizal formation could still proceed.