Induction of Seedlessness in Citrus: From Classical Techniques to Emerging Biotechnological Approaches

By A. Vardi, Ilan Levin, Nir Carmi. 2008

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ADDITIONAL INDEX WORDS. citrus, cytoplasmic male sterility, seedless, parthenocarpy, mutation breeding, ploidy ABSTRACT. Seedlessness can be obtained through parthenocarpy (i.e., fruit formation without fertilization or embryo abortion). In practice, the actual reduction in seed number in parthenocarpic plants is often exaggerated by coupling parthenocarpywithself-incompatibilityormalesterility.Traitsrelatedtoseedlessness, suchasparthenocarpy,canbe introduced into genetic accessions through conventional cross-breeding. However, conventional breeding in Citrus L. species faces several limitations. First, these species have long juvenile (nonflowering) periods during which a relatively thick canopy develops, which limits the size of seedling populations that can be maintained for further evaluation. Second, they have a narrow genetic base, which limits the availability of alternative alleles that could be introgressed into other lines for the formation of a particular phenotype such as parthenocarpy. Third, breeding efforts are limited by the lack of knowledge of the mode of inheritance of specific characteristics. Fourth, and last, breeding efforts are limited by the polygenic nature of many important traits. Despite these limitations, conventional breeding in fruit trees has yielded improved cultivars and will most likely continue to be a very important strategy. However, emerging biotechnological approaches should be continuously evaluated for their potential for expediting such breeding efforts. The objective of this review is to present, evaluate, and discuss conventional and emerging biotechnological approaches for the induction and maintenance of seedlessness in a variety of crops. Particular attention will be paid to citrus crops, including the presentation and discussion of some preliminary data on the genetic inheritance of parthenocarpy. Citrus is a common term for a genus (Citrus) of flowering plants in the family Rutaceae, documented as early as the ChineseBook of History, dated 1000 BCE. Citrus fruit are valued for their vivid colors; strong fragrances; sharp, delicious tastes; and high nutritional value. Today, citrus ranks first in world fruit production (Food and Agriculture Organization of the United Nations (FAO), 2005). It seems likely that the center of origin of commercially important citrus species lies in southeast Asia; many of these species probably originated in China. From the proposed dis- persal history of ancestral taxa, it seems that although orange (Citrus sinensis (L.) Osbeck.), lemon (Citrus limon (L.) Burm. f.), mandarin (Citrus reticulata Blanco), pummelo (Citrus grandis Osbeck), and grapefruit (Citrus paradisi Macf.) are now distributed worldwide, they share a relatively narrow genetic base. All were initially selected by humans and later propagated through apomictic embryos, cuttings, or grafting. Nearly all the variation in the polyembryonic cultivars appears to be the result of apomixis and somatic mutations. For centuries, some zygotic seedlings have been selected and propagated, such as the 'Clementine' mandarin and grapefruit. Seeded grape (Vitis vinifera L.), citrus, and watermelon (Citrullus lanatus Thunb.) were long accepted by consumers. However, during the past few decades, consumer interest in seedless fruit has increased, and seedlessness has become an important characteristic for fresh market fruit, including citrus. Parthenocarpy

Subfields: Molecular Biology, Plant Science

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