CHARACTERIZATION OF IN PLANTA INDUCED RUST GENES ISOLATED FROM A HAUSTORIUM-SPECIFIC CDNA LIBRARY

Rust fungi are plant parasites that depend on living host tissue for g rowth, For invasion of leaves, dikaryotic urediospores differentiate g erm tubes and infection structures that penetrate through stomata. Bio trophic growth occurs by intercellular mycelia that form haustoria wit hin host cells, A cDNA library was constructed from haustoria isolated from broad bean leaves infected by Uromyces fabae, Differential scree ning revealed that a high proportion (19%) of the haustorial cDNAs are specifically expressed in planta but are not expressed, or are much w eaker, in germlings or infection structures produced in vitro, A total of 31 different in planta-induced genes (PIGs) were identified, Some of the PIGs are highly expressed in haustoria. The PIGs are single or low copy number genes in the rust genome, A variety of developmentally regulated expression patterns of PIG mRNAs were observed, Sequence an alysis of PIG cDNAs revealed similarities to genes encoding proteins i nvolved in amino acid transport, thiamine biosynthesis, short-chain de hydrogenases, metallothioneins, cytochrome P-450 monooxygenases, and p eptidyl-prolyl isomerases.