Symbiotic Root Nodules of the Actinorhizal PlantDatisca glomerata Express Rubisco Activase mRNA1

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Abstract
N2-fixing symbiotic root nodules of the actinorhizal host Datisca glomerata express Dgrca (D. glomerata Rubisco activase) mRNA, a transcript usually associated with photosynthetic organs or tissues. In northern blots a mature, 1700-nucleotide Dgrca mRNA was detected in green plant organs (leaves, flowers, and developing fruits) and in nodules but was not detected in roots. A second message of 3000 nucleotides was observed only in nodules. Both size classes of transcripts were polyadenylated. The larger transcript was 2- to 5-fold more abundant than the mature mRNA; it was hybridized to an intronic probe, indicating that a stable, incompletely spliced transcript was accumulating. Treatment with light on excised nodules did not alter the relative abundance of the two species. In in situ hybridizations the Dgrca message was expressed intensely in the nuclei of infected cells. The Dgrca transcripts also accumulated at lower levels in uninfected cortical cells adjacent to the periderm and the vascular cylinder. mRNA encoding the large subunit of Rubisco (DgrbcL) was abundant in mature infected cells and in the amyloplast-rich sheath of uninfected cortical cells lying between the infected cells and nodule periderm. The proteins Rubisco activase, Rubisco, and the 33-kD O2-evolving complex subunit did not accumulate to detectable levels, indicating that a functional photosynthetic apparatus was not prevalent in nodule tissue. Signals or factors required for the transcription of Dgrca appeared to be present in nodules, but efficient splicing and translation of the message were not observed in Frankia-infected tissue where transcript accumulation was highest.