Among Citrus species, the sweet orange (Citrus sinensis (L.) Osbeck) is the most important in terms of production volumes and cultivated areas. Oranges are particularly appreciated for the organoleptic characteristics and the high nutraceutical value of the fruits (thanks especially to their high content of antioxidants). Recent advances in citrus genetic and genomic resources, such as the release of the reference genomes of several sweet orange cultivars, have contributed to (i) understanding the diversification of C. sinensis and its relation with other citrus species, (ii) assessing the molecular mechanisms underlying traits of interest, (iii) identifying and characterizing the candidate genes responsible for important phenotypic traits, and (iv) developing biotechnological methods to incorporate these traits into different citrus genotypes. It has been clarified that all the genetic diversity within the sweet orange species was derived from subsequent mutations starting from a single ancestor and was derived from complex cycles of hybridization and backcrossing between the mandarin (Citrus reticulata Blanco) and the pummelo (Citrus maxima (Burm.) Merr.). This paper provides an overview of the varietal panorama together with a description of the main driving forces in present and future sweet orange breeding. In fact, for the sweet orange, as well as for other citrus species, the release of novel varieties with improved characteristics is being pursued thanks to the employment of conventional and/or innovative (molecular-based) methods. The state of the art methods together with the innovations in genomics and biotechnological tools leading to the so-called new plant breeding technologies were also reviewed and discussed.

In Citrus, flower induction represents the transition from vegetative to reproductive growth. The regulation of flower induction is mainly triggered by exposure to low temperatures and water-deficit stress, which activates the signaling cascade leading to an increased expression of the citrus orthologs of the FLOWERING LOCUS T (CiFT). In this study, the relationship between rootstock and flower induction under Mediterranean field conditions was investigated by monitoring the expression levels of the floral promoter CiFT2 in leaves of the pigmented sweet orange “Tarocco Scirè” grafted onto “C35” citrange and “Swingle” citrumelo rootstocks. The latter two are known to confer, respectively, high and low yield efficiency to the scion. In both rootstock/scion combinations, CiFT2 showed a seasonal expression with a peak during the inductive period in January triggered by cold temperature. The “Tarocco Scirè”/”C35” citrange combination showed the highest expression levels for CiFT2; this increased expression was correlated with yield and a higher number of flowers in the following spring, suggesting a significant effect of rootstocks on flower induction mediated by the overexpression of the CiFT2 gene.

The pear (genus Pyrus) is one of the most ancient and widely cultivated tree fruit crops in temperate climates. The Mount Etna area claims a large number of pear varieties differentiated due to a long history of cultivation and environmental variability, making this area particularly suitable for genetic studies. Ninety-five pear individuals were genotyped using the simple sequence repeat (SSR) methodology interrogating both the nuclear (nDNA) and chloroplast DNA (cpDNA) to combine an investigation of maternal inheritance of chloroplast SSRs (cpSSRs) with the high informativity of nuclear SSRs (nSSRs). The germplasm was selected ad hoc to include wild genotypes, local varieties, and national and international cultivated varieties. The objectives of this study were as follows: (i) estimate the level of differentiation within local varieties; (ii) elucidate the phylogenetic relationships between the cultivated genotypes and wild accessions; and (iii) estimate the potential genetic flow and the relationship among the germplasms in our analysis. Eight nSSRs detected a total of 136 alleles with an average minor allelic frequency and observed heterozygosity of 0.29 and 0.65, respectively, whereas cpSSRs allowed identification of eight haplotypes (S4 Table). These results shed light on the genetic relatedness between Italian varieties and wild genotypes. Among the wild species, compared with P. amygdaliformis, few P. pyraster genotypes exhibited higher genetic similarity to local pear varieties. Our analysis revealed the presence of genetic stratification with a ‘wild’ subpopulation characterizing the genetic makeup of wild species and the international cultivated varieties exhibiting the predominance of the ‘cultivated’ subpopulation.

Background: Floral sterility and nucellar embryony are peculiar traits of several Citrus species and represent an obstacle to traditional breeding. Morphological sterility mainly results in pistil abortion and anther atrophy, while polyembryony is due to a mechanism known as sporophytic apomixis, which consists of the presence of embryos in the seed generated from the nucellar (maternal) tissue alongside the zygotic embryo (sexual origin). Considering the growing interest in lemon [Citrus limon (L.) Burm. f.] breeding, and the lack of information on floral sterility and the polyembryony trait among different lemon cultivars, a morphological and molecular characterization of these traits of interest was performed on forty Sicilian and international lemon cultivars available in the citrus germplasm collection of Catania University (Italy). Methods: Eight traits related to the reproductive biology were assessed on the selected lemon cultivars, namely: pistil abortion and anther atrophy, number of seeds per fruit, number of embryos per seed, percentage of seeds showing polyembryony, germination, percentage of seeds resulting in more than one plantlet, and average seed weight. Moreover, seedlings recovered after the germination assay were genotyped with SNP and SSR markers for ascertaining their nucellar or zygotic origin. In addition, PCR analysis were performed to assess the allele combination of the miniature inverted-repeat transposable element (MITE) insertion in CitRKD1, a gene associated with the occurrence of apomixis in citrus. Results: All traits showed high variability among the accessions analyzed. As for polyembryony, lemon ‘Adamopoulos’ scored the highest percentage of polyembryonic seeds (67.6%), whilst lemon ‘Lunario’ showed the lowest value (8.7%). Conclusions: Insights on the level of polyembryony within lemon varieties will represent a valuable tool for breeders for the set-up of novel mating schemes. In fact, when a polyembryonic female parent is used in cross breeding, the selection of the zygotic individual is hampered by the presence of a nucellar one.