Analyses of phosphoenolpyruvate carboxylase gene structure and expression in alfalfa nodules.

Phosphoenolpyruvate carboxylase (PEPC) plays a crucial role in the assimilation of CO2 during symbiotic N2 fixation in legume root nodules. In this study, an alfalfa PEPC gene (PEPC-7), whose transcripts are found at elevated levels in nodules relative to either leaves or roots, has been isolated and characterized. The intron/exon structure of this gene is identical to that of most other plant PEPC genes except for the presence of an additional intron in the 5' untranslated region. In situ RNA hybridization studies showed that PEPC transcripts were present in the nodule meristem, the infection zone, the nitrogen-fixing zone, and the parenchyma. PEPC transcripts were also found in vascular tissue of roots and nodules and in the pulvinus of petioles. In transgenic alfalfa, a chimeric reporter gene was expressed in these same regions except that little expression was found in the nodule meristem. Analyses of promoter deletions suggest that the region between -634 and -536 is of particular importance in directing transcriptional activity to the infected zone of nodules. Within this region is a mirror repeat sequence that is potentially capable of forming an H-DNA structure. These results indicate that PEPC-7 has a central role in nitrogen-fixing nodules and that regulation of transcription is an important determinant of its activity.

Identification of an Arabidopsis locus required for resistance to turnip crinkle virus.

Inoculation of turnip crinkle virus (TCV) into a (TCV)-resistant line of Arabidopsis thaliana, Di-17, results in the development of a hypersensitive response (HR) on the inoculated leaves. In contrast, an HR does not occur when leaves of the TCV-susceptible Di-3 line or the susceptible ecotypes Columbia (Col-0), or Landsberg erecta (Ler) are inoculated. Genetic analysis of progeny from crosses between Di-17 and either Di-3, Col-0 or Ler demonstrates that the development of an HR is regulated by a single dominant nuclear locus, herein designated HRT. Using progeny from a Di-17 x Col-0 cross, HRT was mapped to chromosome 5, where it is tightly linked to the DFR locus. We also demonstrate that a variety of resistance-associated phenomena, including the TCV-induced accumulation of salicylic acid, camalexin and autofluorescent cell-wall material, correlate with the HR, suggesting the possibility that HRT is required for their activation.