Transcriptomic and Metabolomic Analysis of Age Related Resistance of Cucumber Fruit to Phytophthora capsici

Cucumber (Cucumis sativus L.) fruit are susceptible to infection by the oomycete pathogen, Phytophthora capsici.  Earlier studies have shown that some cultivars exhibit an age-related resistance (ARR) wherein young fruit are highly susceptible, but develop resistance at approximately 12-16 days post pollination (dpp), and that ARR is associated with the fruit peel. Comparison of fruit peel transcriptomes of an ARR expressing (ARR+) cultivar, ‘Vlaspik’, and an ARR- line, ‘Gy14’, at 8 and 16dpp identified 65 genes uniquely upregulated in the resistant 16 day old ‘Vlaspik’ peels.  This group of genes was highly enriched for secondary metabolite synthesis, especially flavonoid biosynthesis. Eleven of the 65 genes were annotated as flavonoid associated genes; genes associated with terpene synthesis also were specifically upregulated in the resistant tissue.   As flavonoids and terpenes have been implicated in disease resistance in several plant pathogen systems, metabolomic fingerprinting was performed to identify compounds potentially associated with cucumber fruit ARR. Methanol extracts of peels from three AAR+ and three ARR- cultivars were analyzed by UPLC-QToF-MS. Principal component analysis and subsequent component interpretation identified 277 compounds uniquely abundant in 'Vlaspik' ARR+ 16dpp peels relative to both susceptible 8dpp ‘Vlaspik’ and ’Gy14’, as well as the susceptible 16dpp ’Gy14’peels. Relative mass defect (RMD) filtering indicated that the most abundant compounds in this group fell within the RMD ranges that are consistent with polyphenols and terpenes. Evaluation of these compounds across the six cultivars identified 5-10 polyphenolic compounds uniquely abundant in the extracts of ARR+ cultivars at the resistant age.