Transcriptomic Analysis of Cucumber Fruit Peel and Relationship to Resistance to Phytophthora capsici

Cucumber production is subject to severe losses due to fruit rot caused by Phytophthora capsici.  Very young cucumber fruit are highly susceptible, but develop resistance approximately 10-12 days post pollination (dpp).  This age-related resistance is manifested in the peel as indicated by peel section tests, pathogen growth analysis, and peel extract assays.  Transcriptomic analysis was performed to identify genes that are differentially expressed in peels from resistant- vs. susceptible-age fruit.  The peel fraction was enriched for transcripts associated with fruit surface associated functions including photosynthesis, production of cuticle and extracellular compounds, and response to abiotic and biotic stresses.  Transcriptomic changes within the peel from 8dpp to 16dpp reflected changing functions of the fruit surface from photosynthetic activity to cuticle production and defense.  A set of 105 transcripts met the combined criteria of top 5% for expression in the peel and increase in 16dpp peel relative to 8dpp peel (8-800 fold enriched in 16dpp vs. 8 dpp peel).  Of those, >40% were potentially associated with pathogen defense including homologs of genes that have been associated with basal, microbial associated molecular pattern (MAMP)-triggered, or effector-triggered, defense pathways.  Notably, this group included homologs of several genes that confer resistance to penetration of oomycete pathogens in Arabidopsis and barley as well as genes associated with elicitor response, programmed cell death, R-gene mediated hypersensitive response, defense signaling, and phytoalexin production.  Collectively these observations suggest a concerted change in gene expression toward increased defense and may provide insight into mechanisms conferring resistance to P. capsici.