Gene expression profiling of peach fruit during stone development

The availability of seedless grapes and watermelons has energized these fruit markets and resulted in increased consumption. Seedless stone fruits including peaches, plums, and cherries would undoubtedly have similar positive impacts on these industries. However, this would require the elimination of not only the seed, but also the stone, a hard woody tissue layer surrounding the seed. Towards this end, we have begun physiological and functional genomic studies to understand stone formation. Stones appear to be a type of specialized wood that are likely the result of a lignification process. To understand which pathways and genes are involved in stone formation we conducted gene expression profiling studies. RNA was extracted from fruit samples collected at 7 time points during early fruit development. A later stage fruit collected one month after stone hardening was used as a reference. Indirectly labeled cDNAs were hybridized to 2 microarray platforms: 1) A 5K long oligoarray based on the Peach AROS oligo set 1.0 (Operon) and 2) A 15K long oligoarray derived from Apple ESTs (HortResearch, NZ). Peach gene expression profiles were largely consistent between the 2 array platforms. Results showed that the phenylpropanoid pathway and a number of lignin polymerizing enzymes were significantly induced during stone hardening. These findings were consistent with lignin staining data. The flavonoid pathway also showed a similar induction pattern. Collectively, these data indicate that stone formation is controlled by up-regulation of lignin pathway genes during early fruit development.