Physiogenomics of cold-induced System-2 ethylene induction in climacteric fruit

The precise genetic trigger underlying the transition from basal System 1, to autocatalytic System 2 ethylene production in some climacteric fruits remains poorly understood.  Recent work in tomato has shown this transition to be a highly complex, and regulated phenomenon featuring coordinated interaction of multiple phytohormone, cold-signaling, and additional pathways at many points of ethylene perception, signaling and biosynthetic pathways.  Multiple points of regulation of ethylene-related genes have been reported at the transcriptional and post-translational level, but an integration of these findings into a post-harvest developmental model of System 2 ethylene production that coincides with ripening is still needed.  With different genotypes exhibiting varying degrees of a chilling requirement to induce climacteric ripening, pear (Pyrus communis) serves as an ideal model for the investigation of the molecular underpinnings of System 2 ethylene induction.  Anjou pears require 60-day chilling at 0-5°C for the fruit to gain competency for ripening and System 2 ethylene production, while the Bartlett variety requires only 15 days.  Preliminary quantitative gene expression analysis of young and mature post-harvest pears has identified a candidate membrane-integral protein (MIP) which may have roles in auxin-influx and regulation of the System 2 ethylene induction.  To better understand the role that MIP gene and other previously reported genes involved in ripening-related interacting pathways may have during System 2 ethylene onset and the ripening trigger, we established a post-harvest conditioning and ripening model for Anjou and Bartlett fruit harvested at maturity. Fruit firmness was determined at regular intervals during acquisition of ripening competency in 3 storage temperatures, and 2 treatments of exogenous ethylene.  Fruit peel and core samples were also obtained at these intervals for quantitative real-time PCR analysis of 165 candidate genes controlling ripening onset from 9 signaling pathways.  As expected, increased expression of ethylene perception and biosynthetic genes throughout post-harvest storage was observed, while expression of the APETLA2a (AP2a) transcriptional repressor decreased.  Initial conclusions and further directions are also presented.