ABSTRACTCUCUMBER (CUCUMIS SATIVUS L.) FRUIT DEVELOPMENT: FACTORS INFLUENCING FRUIT SIZE, SHAPE, AND RESISTANCE TO PHYTOPHTHORA CAPSICIByMarivi ColleFruit size and shape are important quality traits in cucumber (Cucumis sativus L.) influencing market class and value, however the underlying mechanism driving variation is not known. Two sequenced cucumber cultivars representing extremes in fruit size and shape, ‘Gy14’ (pickling type) and ‘9930,’ (Chinese long, CL) (long, narrow fruit) and their F2 and RIL progeny were evaluated for ovule number, ovary length and diameter, fruit length and diameter, cell number and cell size from 7 days pre-anthesis (dpa) to 20 days post-pollination (dpp). Size and shape differences were influenced by numerous independent factors acting both pre-anthesis and post-pollination including the timing and orientation of cell division and cell expansion. Factors controlling fruit length were largely determined pre-anthesis, while factors regulating diameter were largely determined post anthesis. Expression of select marker genes and homologs of known fruit size genes were compared between CL and two pickling cucumber cultivars across fruit development, and were located with respect to fruit size QTL in cucumber. Several fruit growth related genes clustered in fruit size QTL in chromosomes 3 and 6. A cucumber homolog of Arabidopsis ATHB-2, a gene that controls direction of cell expansion, showed elevated expression earlier in CL relative to Gy14, correlating with longer cells in the longitudinal section. ATHB-2, which maps to major fruit size QTL FS3.1, had a deletion within the GAGA regulatory element in the 5’ noncoding region of the CL allele. Fruit development also influences susceptibility to infection by Phytophthora capsici, a major constraint in cucumber production. Our prior work showed that cucumber fruit (cv. Vlaspik) exhibit age-related resistance (ARR) to P. capsici. Young fruits are highly susceptible, but as they reach the end of exponential growth (~10-12dpp), they become resistant. Screening of 8dpp and 16dpp fruit from 21 cucumber cultivars showed genetic variation in ARR expression to P. capsici. Crosses between ARR+ cultivars and Gy14 (ARR-) and their F1 and F2 progeny were used to examine inheritance of ARR in cucumber. F1 fruits showed intermediate values between the parents. F2 progeny showed a bimodal distribution suggesting one or more dominant factors regulating ARR. Our previous studies indicated that cucumber fruit surface was associated with ARR, suggesting possible physical or chemical components of resistance. Cucumber peels from 8dpp and 16dpp Vlaspik (ARR+) fruit were sequentially extracted with water and methanol, and a microtiter plate assay was developed to evaluate the antimicrobial activity of peel extracts against P. capsici by both visual growth and fluorescence assay. Greater inhibition of P. capsici growth was observed in wells treated with methanolic extracts from 16dpp fruit than 8dpp fruit. The aqueous extracts did not inhibit P. capsici growth. Finally, in an effort to identify a source of resistance that would be expressed in very young fruit, a streamlined detached fruit method for high throughput screening was developed to test the U.S. cucumber Plant Introduction (PI) collection. A total of 1076 PI accessions, from 54 geographic locations, and the susceptible commercial cultivar, Vlaspik, were grown in the field over two seasons. Very young fruit (~4dpp) were tested for resistance to P. capsici. A set of 29 potentially resistant PIs was retested in the field. Three accessions, PI109483, PI178884 and PI214049, and their selfed progeny showed consistent, low disease ratings and may be considered useful for resistance breeding.
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