Histidine Domain-Arabinogalactan Proteins (HD-AGPs) in the Solanaceae

Arabinogalactan Proteins (AGPs) are a large family of hydroxyproline-rich glycoproteins found in the extracellular matrix of diverse cell types in plants. These glycoproteins are proposed to play a role in cell wall development, cell-cell interactions, and pattern formation. The goal of this research was to compare Histidine-Domain Arabinogalactan Proteins (HD-AGPs) in the Solanaceae and to examine the patterns of expression in vegetative and reproductive tissue. HD-AGP cDNA clones were isolated from pistils of Petunia inflata (PifPRP1), P. axillaris parodii (Pa2PRP1), and P. exserta (PePRP1). HD-AGPs from the two-interbreeding species, P. axillaris parodii and P. exserta, are 99.9% identical at the nucleotide level and identical at the amino acid level. HD-AGPs from P. inflata (PifPRP1) and P. axillaris parodii (Pa2PRP1), two reproductively isolated species, show that PifPRP1 is 98.5% identical to Pa2PRP1, with a 12 bp deletion in the hypervariable domain that is specific to the P. axillaris complex. PCR using genomic DNA yielded a single amplicon from each of these species, but two bands from Petunia hybrida and from Nicotiana tabacum. Comparison of intron sequence yielded some interesting insights about the evolutionary trajectory of sister species in these clades. Immunoblots using P. axillaris axillaris revealed discrete glycoforms of HD-AGPs in vegetative tissues contrasting with the tremendous size heterogeneity displayed by HD-AGPs expressed in the pistil. Etiolated seedlings accumulate higher levels of the protein, including significant amounts of an unglycosylated isoform. These findings suggest an important role for HD-AGPs in rapid elongation of cell types in both reproductive and vegetative tissues.