Self-incompatibility Related to Seedless Fruit Production in Citrus Plants

Abstract

Self-incompatibility in Citrus species is an important trait related to fruit set and seed formation. In particular, self-incompatible citrus varieties combined with sufficient parthenocarpy produce seedless fruits. The characteristics of self-incompatibility have been studied for many years, and essential traits, such as pollen tube elongation behavior and self-incompatibility genotypes, have been characterized. Recently, it has been shown that the genetic mechanism of self-incompatibility in citrus varieties is S-RNase-based gametophytic self-incompatibility. To date, 18 S-RNases (17 self-incompatible alleles and 1 self-compatible allele) have been identified. The DNA markers for S-RNases can enable the early identification of self-incompatibility/compatibility status. The expression of self-compatibility in Citrus species is ascribed to the presence of the self-compatibility S_m allele, which is a defective S-RNase, and to the suppression of S-RNase expression. Polyploidization induces self-compatibility in Citrus species: Citrus tamurana ‘Hyuganatsu’ is substantially self-incompatible; however, its bud mutation, ‘Nishiuchi Konatsu’, is self-compatible. ‘Nishiuchi Konatsu’ is diploid; however, it forms unreduced pollen, which causes the breakdown of self-incompatible reaction when self-pollinated because of a competitive interaction within the same individual. In addition, after fertilization by unreduced pollen, seed formation is also inhibited by triploid block caused by interploid hybridization between diploid pollen and haploid egg cells. Therefore, ‘Nishiuchi Konatsu’ shows self-compatibility regardless of the self-incompatibility haplotype and produces fruits with few seeds. The seedlessness trait could be beneficial for citrus breeding in the future; however, the genetic mechanisms involved in the expression of this trait remain unclear. This review focuses on the recent advances in the genetics of self-incompatibility in citrus plants, implicating the mechanisms involved in self-incompatibility and their applications for achieving the desired trait of seedlessness in citrus fruits.