MOLECULAR CHARACTERIZATION OF A PHLOEM-SPECIFIC GENE ENCODING THE FILAMENT PROTEIN, PHLOEM PROTEIN-1 (PP1), FROM CUCURBITA-MAXIMA

Sieve elements in the phloem of most angiosperms contain proteinaceous filaments and aggregates called P-protein. In the genus Cucurbita, th ese filaments are composed of two major proteins: PP1, the phloem fila ment protein, and PP2, the phloem lectin. The gene encoding the phloem filament protein in pumpkin (Cucurbita maxima I)uch.) has been isolat ed and characterized. Nucleotide sequence analysis of the reconstructe d gene gPP1 revealed a continuous 2430 bp protein coding sequence, wit h no introns, encoding an 809 amino acid polypeptide. The deduced poly peptide had characteristics of PP1 and contained a 15 amino acid seque nce determined by N-terminal peptide sequence analysis of PP1. The seq uence of PP1 was highly repetitive with four 200 amino acid sequence d omains containing structural motifs in common with cysteine proteinase inhibitors. Expression of the PP1 gene was detected in roots, hypocot yls, cotyledons, stems, and leaves of pumpkin plants. PP1 and its mRNA accumulated in pumpkin hypocotyls during the period of rapid hypocoty l elongation after which mRNA levels declined, while protein levels re mained elevated. PP1 was immunolocalized in slime plugs and P-protein bodies in sieve elements of the phloem. Occasionally, PP1 was detected in companion cells. PP1 mRNA was localized by in site hybridization i n companion cells at early stages of vascular differentiation. The dev elopmental accumulation and localization of PP1 and its mRNA parallele d the phloem lectin, further suggesting an interaction between these p hloem-specific proteins.