Whole genome scanning of a Mediterranean basin hotspot collection provides new insights into olive tree biodiversity and biology

Variations in Fruit Ploidy Level and Cell Size between Small- and Large-Fruited Olive Cultivars during Fruit Ontogeny

Olive (Olea europaea L.) is one of the major oil fruit tree crops worldwide. However, the mechanisms underlying olive fruit growth remain poorly understood. Here, we examine questions regarding the interaction of endoreduplication, cell division, and cell expansion with olive fruit growth in relation to the final fruit size by measuring fruit diameter, pericarp thickness, cell area, and ploidy level during fruit ontogeny in three olive cultivars with different fruit sizes. The results demonstrate that differences in the fruit size are related to the maximum growth rate between olive cultivars during early fruit growth, about 50 days post-anthesis (DPA). Differences in fruit weight between olive cultivars were found from 35 DPA, while the distinctive fruit shape became detectable from 21 DPA, even though the increase in pericarp thickness became detectable from 7 DPA in the three cultivars. During early fruit growth, intense mitotic activity appeared during the first 21 DPA in the fruit, whereas the highest cell expansion rates occurred from 28 to 42 DPA during this phase, suggesting that olive fruit cell number is determined from 28 DPA in the three cultivars. Moreover, olive fruit of the large-fruited cultivars was enlarged due to relatively higher cell division and expansion rates compared with the small-fruited cultivar. The ploidy level of olive fruit pericarp between early and late growth was different, but similar among olive cultivars, revealing that ploidy levels are not associated with cell size, in terms of different 8C levels during olive fruit growth. In the three olive cultivars, the maximum endoreduplication level (8C) occurred just before strong cell expansion during early fruit growth in fruit pericarp, whereas the cell expansion during late fruit growth occurred without preceding endoreduplication. We conclude that the basis for fruit size differences between olive cultivars is determined mainly by different cell division and expansion rates during the early fruit growth phase. These data provide new findings on the contribution of fruit ploidy and cell size to fruit size in olive and ultimately on the control of olive fruit development.

Wild and cultivated olive tree genetic diversity in Greece: a diverse resource in danger of erosion

The genetic relationships between Greek wild olive tree populations and cultivars were investigated. A total of 219 wild genotypes and 67 cultivar genotypes were analyzed by employing 10 SSR markers. Data evidenced that the wild populations exhibited high levels of genetic diversity and exclusively host 40% of the total number of alleles detected. Inbreeding was observed within populations, probably as a consequence of their fragmented spatial distribution. The genetic differentiation between cultivars and wild individuals, as well as within wild populations, was low. Nevertheless, three gene pools of wild trees were detected, corresponding to the geographical areas of Northeastern Greece, Peloponnese-Crete and Epirus. Most cultivars clustered in a separate group, while the rest of them formed a heterogenous group with membership coefficients akin to the three wild olive clusters. Regarding the history of olive cultivation in Greece, bidirectional gene flow was detected between populations of Peloponnese-Crete and the gene pool that composes some of Greece's most important cultivars, such as "Koroneiki" and "Mastoidis", which is inferred as an indication of a minor domestication event in the area. A strategy for the protection of Greek-oriented olive genetic resources is proposed, along with suggestions for the utilization of the genetically diverse wild resources with regard to the introgression of traits of agronomical interest to cultivars.

Hormonal Content and Gene Expression during Olive Fruit Growth and Ripening

The cultivated olive (Olea europaea L. subsp. europaea var. europaea) is one of the most valuable fruit trees worldwide. However, the hormonal mechanisms underlying the fruit growth and ripening in olives remain largely uncharacterized. In this study, we investigated the physiological and hormonal changes, by ultra-high performance liquid chromatography-mass spectrometry (UHPLC-MS), as well as the expression patterns of hormone-related genes, using quantitative real-time PCR (qRT-PCR) analysis, during fruit growth and ripening in two olive cultivars, 'Arbequina' and 'Picual', with contrasting fruit size and shape as well as fruit ripening duration. Hormonal profiling revealed that olive fruit growth involves a lowering of auxin (IAA), cytokinin (CKs), and jasmonic acid (JA) levels as well as a rise in salicylic acid (SA) levels from the endocarp lignification to the onset of fruit ripening in both cultivars. During olive fruit ripening, both abscisic acid (ABA) and anthocyanin levels rose, while JA levels fell, and SA levels showed no significant changes in either cultivar. By contrast, differential accumulation patterns of gibberellins (GAs) were found between the two cultivars during olive fruit growth and ripening. GA1 was not detected at either stage of fruit development in 'Arbequina', revealing a specific association between the GA1 and 'Picual', the cultivar with large sized, elongated, and fast-ripening fruit. Moreover, ABA may play a central role in regulating olive fruit ripening through transcriptional regulation of key ABA metabolism genes, whereas the IAA, CK, and GA levels and/or responsiveness differ between olive cultivars during olive fruit ripening. Taken together, the results indicate that the relative absence or presence of endogenous GA1 is associated with differences in fruit morphology and size as well as in the ripening duration in olives. Such detailed knowledge may be of help to design new strategies for effective manipulation of olive fruit size as well as ripening duration.