The nucleotide sequence of a cDNA prepared from poly(A)+ RNA from Lycopersicon esculentum fruit codes for a protein, M(r) 20,812, with features representative of the protein core of arabinogalactan proteins. The deduced amino acid sequence resembles that of peptides of arabinogalactan proteins isolated from carrot and rose and is most similar to the sequence of tryptic peptides from Lolium multiflorum (Gleeson et al., Biochem J 264 (1989) 857-862). The similar sequences include a number of Ala-Pro repeats, a feature considered distinctive of arabinogalactan proteins. The amino acid composition is similar to that of the peptide core of the Lolium multiflorum arabinogalactan protein; alanine, serine and proline account for 57% of the polypeptide. The mRNA corresponding to the cDNA sequence was detected in roots, leaves and fruit. The levels of mRNA are reduced in older leaves, in fruit that have commenced ripening and in leaves and fruit that have been wounded.

Arabinogalactan proteins (AGPs) represent a major class of plant hydroxyproline-rich glycoproteins (HRGPs) and are components of cell walls and plasma membranes. AGPs are thought to play roles in cell differentiation, development, and cell-cell interactions. Using a synthetic DNA oligonucleotide based upon an amino acid sequence motif common to AGPs from Lolium, rose, and carrot (i.e., Hyp-Ala-Hyp-Ala-Hyp), we have isolated and sequenced the first AGP gene from a partial Sau3A tomato genomic library packaged in bacteriophage charon 35. The deduced 215 amino acid protein contains 20% Ala, 22% Pro, 10% Gly, and 11% Ser and consists of two Pro-Ala-Pro-Ala-Pro pentapeptide repeats and 16 Ala-Pro dipeptide repeats, consistent with known AGP amino acid compositions and sequences. Comparison of the genomic sequence to a reverse transcribed PCR product and tomato cDNA confirmed the AGP gene is expressed and contains one large intervening sequence. RNA blot hybridization analysis in tomato indicates this AGP gene is strongly expressed in stem and flower, moderately expressed in root and green fruit, and weakly expressed in leaves and red fruit as a 980 nucleotide transcript. Five-day-old seedlings also express this transcript; however, this expression is not regulated by light. More significantly, a gradient of AGP gene expression is observed in tomato stems, ranging from high levels of expression in young internodes to low levels of expression in old internodes. Wounding serves to down-regulate expression in young and old internodes. Heat shock also affects AGP gene expression in stems by transiently down-regulating mRNA levels.