Establishing a Physiogenomics-Based Understanding of Cold-Induced System 2 Ethylene in Pear

Development of genomics-guided physiological solutions in biological systems lacking significant genomic resources is challenging.  Utilization of well-defined physiological models combined with high resolution temporal analysis of global gene expression can address these challenges. European pear (Pyrus communis), a climacteric fruit, requires varying chilling requirements to induce autocatalytic ethylene production, depending on variety.  D’Anjou genotype requires 60-day chilling at 0-5°C for the fruit to gain competency for ripening and System 2 ethylene production, while Bartlett require only 15 days.  This well-defined phenotype makes pear as an ideal model for investigation of the molecular underpinnings of System 2 ethylene induction. Fruit of each variety were harvested at maturity, and then conditioned at 3 storage temperatures, with or without ethylene exposure. Peel samples were obtained at regular intervals during conditioning for quantitative real-time PCR analysis of 85 candidate genes controlling ripening onset representing multiple signaling and phytohormone pathways.  Preliminary qPCR analysis of fruit during conditioning and ripening has identified candidate genes involved in regulation of the System 2 ethylene transition. Some of these gene expression observations agree with known behavior in other climacteric systems, while others are unique. As a complementary effort that enables identification of unique genes in pear, a 72 X draft genome has been assembled comprising data from PacBio, Illumina HiSeq, and Roche 454 Titanium FLX platforms.  Ongoing pooled BACseq approach will further enhance the quality of the draft genome in subsequent versions to enable accurate gene identification.