Genomic and cDNA Sequences Encoding Histidine-Domain Arabinogalactan Proteins (HD-AGPs) in the Solanaceae

Arabinogalacatan proteins (AGPs) are found in diverse tissue types in flowering plants, but their precise role in growth and development is unclear. Our research focuses on a subfamily of AGPs, known as HD-AGPs, that are very abundant in the extracellular matrix of the pistil, the female reproductive structure in a flower. The first members of this subfamily (TTS-1 and TTS-2) were identified from tobacco pistils and shown to be important for pollen tube growth. Using immunoblots, we showed that these proteins are expressed not only reproductive tissues, but also in vegetative tissues such as seedlings, roots, and leaves, of tobacco. Furthermore, we found that orthologs of TTS-1/TTS-2 are found in other members of the Solanaceae (the plant family that tomatoes and potatoes belong to), including four Petunia species. We used PCR and RT-PCR to obtain genomic and cDNA sequences encoding HD-AGPs in P. hybrida, P. axillaris, P. exserta, and P. inflata. Comparison of the deduced amino acid sequences reveals distinct conserved and variable domains, including complete conservation of the eponymous histidine-domain, among these petunia HD-AGPs. The HD-AGPs from P. axillaris (PaaPRP1) and from P. exserta (PePRP1) are 99% identical at the nucleotide level, confirming that the two taxa are closely-related sister species. These two species have a 12-nucleotide deletion in the proline-rich hypervariable domain compared to the otherwise very similar cDNA sequence of the P. inflata (PiiPRP1) HD-AGP. The garden petunia, P. hybrida, is a synthetic species, and using HD-AGP-specific primers we PCR-amplified two fragments from its genomic DNA: one matched the P. axillaris-specific genomic fragment in size and the other matched the P. inflata genomic fragment. This pattern of genomic amplicons is consistent with the proposal that the two wild species are the progenitors of the garden petunia.