Transcriptional changes in wheat leaf nuclei during the early and intermediate stages of rust infection

Nuclei isolated from wheat (Triticum aestivum L., var. Zenith) susceptible to race 126-ANZ-5,6,7,11 of the rust fungus (Puccinia graminis f.sp. tritici) are transcriptionally active in vitro. A comparison of RNA synthesis in nuclei isolated from uninfected and rust infected leaves has revealed that there is a significant increase in the synthesis of some high molecular weight species of RNA at 3 and 5 days after inoculation. On the basis of the effect of α-amanitin on nuclear transcription at 20 mM and 240 mM (NH4)2SO4 and hybridization of nuclear transcripts to cloned wheat 28S, 18S and 5S rRNA genes, we conclude that this increase in RNA synthesis is predominantly due to the activity of RNA polymerase II, the enzyme responsible for the synthesis of mRNA precursors. The possibility that the observed changes in transcription are due to differential degradation of newly synthesized RNA is unlikely in view of the finding that RNase inhibitor has no detectable effect on the size of RNA synthesized in the Percoll gradient-purified nuclei. These results suggest that alterations in host nuclear transcription due to changes in RNA polymerase II activity occur in wheat leaves during the early stages of rust infection.