Among Citrus species, lemon is one of the most susceptible to mal secco disease, a tracheomycosis caused by the mitosporic fungus Plenodomus tracheiphilus, which induces chlorosis followed by leaf drop and progressive desiccation of twigs and branches. Severe infection can cause the death of the plant. Since no effective control strategies are available to efficiently control the pathogen spread, host tolerance is the most desirable goal in the struggle against mal secco disease. To date, both traditional breeding programs and biotechnological techniques were not efficient in developing novel varieties coupling tolerance to mal secco with optimal fruit quality. Furthermore, the genetic basis of host resistance has not been fully deciphered yet, hampering the set-up of marker-assisted selection (MAS) schemes. This paper provides an overview of the biotechnological approaches adopted so far for the selection of mal secco tolerant lemon varieties and emphasizes the promising contribution of marker-trait association analysis techniques for both unraveling the genetic determinism of the resistance to mal secco and detecting molecular markers that can be readily used for MAS. Such an approach has already proved its efficiency in several crops and could represent a valuable tool to select novel lemon varieties coupling superior fruit quality traits and resistance to mal secco.

Background: Anthocyanin pigmentation characterizes a number of tissues of Citrus and its relatives. The gain and loss of pigmentation is intriguing and is inherited variously among species. Methods: Citrus germplasm was used to investigate the anthocyanin pigmentation of tissues never before considered, including stamen, style and stigma, and of young leaves, petals, rind and flesh of 28 genotypes belonging to 14 species. Citrus genotypes encompassed citron, lemon, sweet orange, lime, and Citrus relatives included Microcitrus, Murraya, and Severinia. A relative qRT-PCR analysis was carried out on the structural and regulatory genes: phenylalanine ammonia-lyase (PAL), chalcone synthase (CHS), chalcone isomerase (CHI), flavanone 3′-hydroxylase (F3H), dihydroflavonol 4-reductase (DFR), anthocyanidin synthase (ANS), uridine diphosphate glucose flavonoid glucosyl-transferase (UFGT), glutathione S-transferase (GST), Ruby and Noemi. Image analysis and a genomic approach were employed to evaluate how the red pigmentation is inherited among tissues and species. Results: Pigmentation of young leaves and petals is specific to citron and its hybrids. Ruby controls the pigmentation of petals, but not of leaves. The red color of the rind and flesh is a trait that particularly characterizes a diversity of sweet oranges, citron hybrids and Citrus relatives. Color expression depends on external factors and also on developmental stage. The coloration of stamen and style is citron-specific, while a red stigma is exclusive to Moro orange and its hybrids. Conclusion: It is hypothesized that there is a relationship among Citrus species and genes controlling anthocyanin pigmentation.

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.