Symbiotic root nodules of the actinorhizal plant Datisca glomerata expressRubisco activase mRNA

N-2-fixing symbiotic root nodules of the actinorhizal host Datisca glomerat a express Dgrca (D. glomerata Rubisco activase) mRNA, a transcript usually associated with photosynthetic organs or tissues. In northern blots a matur e, 1700-nucleotide Dgrca mRNA was detected in green plant organs (leaves, f lowers, and developing fruits) and in nodules but was not detected in roots . A second message of 3000 nucleotides was observed only in nodules. Both s ize classes of transcripts were polyadenylated. The larger transcript was 2 - to 5-fold more abundant than the mature mRNA; it was hybridized to an int ronic probe, indicating that a stable, incompletely spliced transcript was accumulating. Treatment with light on excised nodules did not alter the rel ative abundance of the two species. In in situ hybridizations the Dgrca mes sage was expressed intensely in the nuclei of infected cells. The Dgrca tra nscripts also accumulated at lower levels in uninfected cortical cells adja cent to the periderm and the vascular cylinder. mRNA encoding the large sub unit of Rubisco (DgrbcL) was abundant in mature infected cells and in the a myloplast-rich sheath of uninfected cortical cells lying between the infect ed cells and nodule periderm. The proteins Rubisco activase, Rubisco, and t he 33-kD O-2-evolving complex subunit did not accumulate to detectable leve ls, indicating that a functional photosynthetic apparatus was not prevalent in nodule tissue. Signals or factors required for the transcription of Dgr ca appeared to be present in nodules, but efficient splicing and translatio n of the message were not observed in Frankia-infected tissue where transcr ipt accumulation was highest.