Self-incompatibility Related to Seedless Fruit Production in <i>Citrus</i> 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.

The influence of the pollination compatibility type on the pistil S-RNase expression in European pear (Pyrus communis)

“…Based on these findings, the authors proposed a positive feedback mechanism in which compatible pollen tubes simultaneously degrade the S-RNase protein in their cytoplasm and send out a signal to reduce S-RNase expression, hence reinforcing the compatible reaction (Liu et al, 2009). A decreased S-RNase expression after crosspollination and higher expression after self-pollination was also found in lemon, a member of the Rutaceae family which was recently also found to harbor a similar type of S-RNase-dependent GSI (Honsho, 2023). Expression analysis of the S3-RNase in lemon styles showed ten times higher expression after self-pollination compared to cross-pollination 20 h after pollination (Li et al, 2022).…”

The origin and the genetic regulation of the self-compatibility mechanism in clementine (Citrus clementina Hort. ex Tan.)

“…ex Tan.) is the most representative SI species within the mandarin group ( Ngo et al., 2001 ; Chao, 2005 ; Distefano et al., 2009a ; Honsho, 2023 ). Clementine was selected from a mandarin chance seedling and now represents the principal mandarin varietal group cultivated in the Mediterranean basin.…”

“…As a result of spontaneous bud sport mutations from the original ‘Comune’ clementine, many different clementine varieties have been selected. In addition, several cultivars have been originated by crossing the SI clementine with other varieties or hybrids ( Montalt et al., 2021 ; Honsho, 2023 ). Some reports described cross-incompatibility in sibling cultivars obtained from crosses using clementine as the female ( Kitajima et al., 2001 ) or male ( Yamamoto et al., 2006 ) parent.…”

“…To date, eighteen S-RNases , seventeen SI and one SC, have been reported in pummelo ( S 1 -S 9 , S 16 ), mandarin ( S 10 -S 11 ), sweet orange ( C . sinensis (L.) Osbeck; S m ) and in other citrus species ( S 12 -S 15 , S 17 ) ( Honsho, 2023 ). The citrus S-RNases are characterized by the same general structure as those found in Plantaginaceae , Solanaceae , and Rosaceae , all showing five conserved domains (C1–C5) with three histidine residues and 5 hypervariable domains (HV1–HV5) with a single intron within the HV1 ( Liang et al., 2020 ; Honsho et al., 2021 ).…”

“…ex Tan. ; SI) ( Honsho, 2023 ); while the Chinese cultivars ‘Guiyou No. 1’ and ‘Zigui Shatian’ are SC mutants of ‘Shatian’ pummelo (SI; Honsho, 2023 ).…”