DIFFERENTIAL EXPRESSION OF THE 2 SUBUNITS OF TOMATO POLYGALACTURONASEISOENZYME-1 IN WILD-TYPE AND RIN TOMATO FRUIT

The beta subunit of tomato (Lycopersicon esculentum Mill.) fruit polyg alacturonase 1 is a cell wall glycoprotein that binds to and apparentl y regulates the catalytic PG2 polypeptide in vivo. beta Subunit and po lygalacturonase 2 (PG2) expression have been investigated in both wild -type and ripening inhibitor (rin) mutant fruit. During fruit developm ent and ripening, beta subunit expression was unrelated to expression of the catalytic PG2 protein. In wild-type fruit, beta subunit mRNA an d protein were first detected early in development and increased to ma ximal levels before PG2 mRNA and protein were detected. At the onset o f ripening beta subunit mRNA decreased dramatically, but beta subunit protein levels remained stable. In rin fruit, which fail to ripen, bet a subunit expression was similar to that in wild type, although PG2 mR NA and protein were not detected. These data suggest that beta subunit expression is ethylene independent and regulated primarily by develop mental cues. This conclusion is supported by results from ethylene-tre ated immature (20 days after pollination) wild-type and rin fruit in w hich no significant differences were observed in beta subunit expressi on patterns in response to ethylene treatment. Surprisingly, RNA blot analysis indicated that catalytic PG2 mRNA was induced in immature rin fruit after 3 d of exogenous ethylene treatment. In addition, beta su bunit mRNA and protein were also detected at lower levels in root, lea f, and flower tissues of both genotypes, suggesting a broader function al role for the protein.