Molecular evidence for induction of phenylalanine ammonia-lyase during Puccinia graminis infection and elicitation in wheat

Abstract

Degenerate primers corresponding to the conserved sequences in phenylalanine ammonia-lyase (PAL) genes cloned from rice and parsley were used to amplify segments of the PAL genes from wheat (Triticum aestivum). More than 76% base sequence identity with the homologous genes confirmed that a 437 base pairs (bp) fragment was derived from the coding region of PAL. Using this fragment as probe, two PAL genes linked in tandem, Wpalt1 and Wpalt2, were isolated from a wheat genomic library. Northern blot and RNA dot blot analyses using a probe for Wpalt1 demonstrated that Wpalt1 expression was organ specific. Wpalt1 expression was high in roots and moderate in stems, but no detectable expression was found in leaves. Infection of leaves with Puccinia graminis f. sp. tritici (Pgt) induced, within 4 to 8 d, a high level of Wpalt1 transcripts in the Pgt resistant near-isogenic Prelude Sr6 line, while a delayed induction was observed in the Pgt susceptible isogenic Prelude sr6 line. In cell suspension cultures, treatment with chitin oligomers or an elicitor derived from P. graminis germ tube walls activated Wpalt1 gene expression. The Pgt-derived elicitor activated PAL expression earlier than did chitin oligomers, whereas chitin oligomers appeared to be more potent inducers of PAL expression than was the Pgt-derived elicitor.