Until recently, tomato (Solanum lycopersicum) was regarded as a model for plastid development for all fruits. However, recent studies of plastid morphology and development in apple (Malus x domestica) have revealed that not only are there extreme differences between apple and tomato, but there are also significant differences between apple cultivars as well throughout fruit development. The recent sequencing of the ‘Golden Delicious’ genome has provided a wealth of sequence information that has shed some light on why such a stark difference exists; the apple genome is predicted to contain nearly 5,000 unique plastid-targeted proteins which are not represented in the genomes of Arabidopsis, grape, peach, pear, poplar, or tomato. These unique genes represent over 40% of the total plastid proteome in apples, which is currently predicted to have 10,000 proteins. Because these proteins have not been documented in plastids before now, each may have functions that are entirely new to plant biology. Of the 5,000 unique genes, we have narrowed our focus to genes which may function in crosstalk between the plastid and nucleus or in modulation of photosynthesis. The gene set was enriched with GO terms for electron transfer, chromatin-binding, transcription, translation, and proteins containing transmembrane domains; members of these categories are undergoing expression analysis, localization, and photosynthesis phenotyping experiments. This work will shed light on the plastid morphodevelopment in non-model fruit systems such as apple, and may unveil proteins with novel functions that can be utilized in the future for development of improved varieties.
