Strong expression of phloem protein genes during rapid growth has been observed in other studies, including PP1 expression in green stage watermelon fruit. Specific expression of PP2. Exponential growth The group of genes with peak abundance at the 8 dpp, ex ponential growth stage, included cytoskeleton, cell wall, and water and carbohydrate transport genes. Tubulins, berwas observed in young pumpkin hypocotyls, peaking at 12 days after germination in concert with the period of peak growth and vascular dif ferentiation. In contrast, cucumber homologs of the ATPP2 B family had a nearly inverted pattern of tran script levels relative to PP2 A genes, peaking at 0 dpp, and dropping during exponential growth, suggesting pos sible functional divergence.
The period of rapid fruit enlargement was also asso ciated with marked changes in fruit surface, including an increase in cuticle thickness PR-957 as is typically observed dur ing rapid plant growth, and loss of the silica oxide powder based bloom. The homolog of the Cucurbita moschata silicon transporter showed age specific transcript abundance peaking at 8 dpp then dropping sharply, coinciding with the time of bloom loss from the middle of the fruit. Among the genes identified in other systems to be asso ciated with cuticle biosynthesis are the extracellular GDSL motif lipasehydrolase proteins and lipid transfer proteins, which have been implicated in lipid transport to extracel lular surfaces. The cucumber fruit transcriptome set included eleven GDSL motif lipasehydrolase protein family members that were represented by at least 30 ESTs, including five with more than 100 ESTs.
The majority showed peak levels at 8 or 1216 dpp, with virtually no measured reads until either 8 or 12 dpp. Twelve lipid transfer protein family members with greater than 30 ESTscontig also were observed in the transcriptome data set, including four with greater than 700 ESTs. As for the GDSL motif lipasehydrolase protein genes, the original source the majority of the lipid transfer proteins were most highly represented from 816 dpp. transcript levels of one gene peaked at 48 dpp. A homolog of the transcription factor gene SHINE1, which is associated with cuticle produc tion in Arabidopsis also exhibited peak transcript abundance at 8 dpp. Additionally, transcript levels of two cyctochrome P450 family members that have been associated with cutin biosynthesis. and two putative beta amyrin synthases, enzymes which have been associated with cuticular wax synthesis in tomato, also peaked at 8dpp. In contrast, two putative GDSL family members and one lipid transfer protein with moderate transcript levels were observed almost exclu sively at 0 dpp, suggesting possible floral, rather than fruit, expression.