Ls. Zheng et al., DIFFERENTIAL EXPRESSION OF THE 2 SUBUNITS OF TOMATO POLYGALACTURONASEISOENZYME-1 IN WILD-TYPE AND RIN TOMATO FRUIT, Plant physiology, 105(4), 1994, pp. 1189-1195
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.