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 parthenocarpy with self-incompatibility or male sterility. Traits related to seedlessness, such as parthenocarpy, can be 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.
Aliza Vardi, Ilan Levin, and Nir Carmi
Yosef Burger, Uzi Saar, Nurit Katzir, Harry S. Paris, Yelena Yeselson, Ilan Levin, and Arthur A. Schaffer
Fruit sweetness is the major determinant of fruit quality in melons (Cucumis melo L.) and reflects the concentration of the three major soluble sugars, sucrose, glucose, and fructose, present in the fruit flesh. Of these three sugars, sucrose is the prime factor accounting for both the genetic and the environmental variability observed in sugar content of C. melo fruit. Faqqous (subsp. melo var. flexuosus), a cultivar having a low sucrose and total sugar content, was crossed with Noy Yizre'el (subsp. melo var. reticulatus), a cultivar having a high sucrose and total sugar content. F1 plants had a sucrose content averaging slightly higher than that of the low-sucrose parent, indicating that low sucrose content is nearly completely dominant. Segregation in the F2 and backcross progenies indicated that high sucrose accumulation in melon fruit flesh is conferred by a single recessive gene herein designated suc. When the high-sucrose parent was crossed with the moderate-sucrose landrace known as Persia 202 (subsp. melo var. reticulatus), the segregation in the filial and backcross progenies suggested that additional genetic factors affect the amount of sucrose accumulation.
Marina Petreikov, Lena Yeselson, Shmuel Shen, Ilan Levin, Arthur A. Schaffer, Ari Efrati, and Moshe Bar
Soluble sugar accumulation is a major determinant of tomato (Solanum lycopersicum) fruit quality. One strategy of increasing sugar levels in the mature fruit is via the increase of the transient starch pool in the immature fruit, which is subsequently degraded to contribute to its soluble sugar levels. ADP-glucose pyrophosphorylase [AGPase (E.C. 220.127.116.11)] is a limiting enzyme in starch synthesis and we therefore developed introgression lines of cultivated tomato harboring the wild species (Solanum habrochaites) allele for the regulatory large subunit (L1H) of this heterotetrameric enzyme. Comparison of numerous near-isogenic lines of tomato segregating for the L1 allele, during multiple seasons, showed that the wild species allele led to an increase in fruit total soluble solids concentration (TSS) without a concomitant decrease in fruit size. Rather, in practically all lines studied, fruit size increased together with TSS, leading to an even larger increase in TSS × yield. A comparative developmental study of fruit carbohydrates, starch, and sugars between genotypes showed that the wild species allele led to increases in fruit size, carbohydrate concentration, and carbohydrate content of the whole fruit unit. This was related to a large increase in the transient starch reservoir that, upon degradation, accounted for the subsequent increase in soluble sugars. These results are evidence that modifying fruit sink carbohydrate metabolism via a single rate-limiting enzymatic step can increase the net import of photoassimilate into the fruit.