A natural mutant kumquat fruit with abnormal oil glands shows lower volatile production, better fruit flavor and higher resistance to postharvest disease

Oil glands of citrus peel are known sites for synthesis and deposition of volatile organic compounds (VOCs). However, it is unclear how oil glands and VOCs affect fruit flavor and postharvest disease resistance in vivo. Here, we report characterization of a natural mutant 'Huapi' (HP) kumquat (Fortunella crassifolia Swingle) compared with its wild progenitor 'Rongan' (RA). HP fruit decreased in both size and number of oil glands from the onset of fruit development when compared to RA. The total VOCs concentration and the number of individual VOCs in mature HP fruit were lower than that in RA. Limonene, alpha-pinene and beta-myrcene concentrations showed the greatest reduction (30-, 21- and 20-fold). Sensory evaluation showed that flavor of HP fruit was more acceptable to consumers compared to that of RA. Sweeter, less acidic features of HP fruit were likely due to higher soluble sugars content and lower organic acid content, respectively. Changes in HP fruit VOCs likely contribute to its less pungent and better aroma features. HP fruit displayed stronger resistance to the fungal pathogens Penicillium italicum and Geotrichum candidum compared to RA in in vivo inoculation experiments. Transcriptomic analysis and qPCR validation indicated that FcERF096, FcCHI, FcCXE, FcPi21L1, FcPi21L2, FcPIP and FcXYLL potentially involve in enhanced disease resistance of HP fruit. Taken together, these results reveal the potential of oil glands and terpenes in affecting fruit sensory attributes and postharvest disease resistance in planta in kumquat, providing new insights into the technologies improving these important traits in citrus.