Breakdown of Self-Incompatibility in Citrus by Temperature Stress, Bud Pollination and Polyploidization

Abstract

Self-incompatibility (SI) is present in around half of all species of flowering plants. SI limits endogamy and contributes to increased genetic diversity. SI is a very important trait in citrus because, when coupled with parthenocarpy, it allows seedless fruit production. Otherwise, SI is an impediment to genetic studies and breeding programs. Temperature stress, bud pollination and polyploidization can induce the breakdown of the SI mechanism in several species. In this work, we investigated how the SI mechanism can be broken down in two self-incompatible diploid citrus genotypes: ‘Fortune’ mandarin and ‘Clemenules’ clementine. The influence of temperature stress on the SI mechanism was assessed in self-pollinated flowers of ‘Fortune’ mandarins subjected to 2 temperature regimes (10 °C and 30 °C), whereas the bud pollination effect was investigated in the same genotype and in ‘Clemenules’ clementines cultivated under field conditions. The tetraploid ‘Clemenules’ clementine cultivated under field conditions was used to study if tetraploidization can bypass the SI reaction. Histological observations of pollen tube growth and seed production in self-pollinated flowers were used to evaluate the breakdown of SI, while the genetic analysis with SSR and SNP markers confirmed that all recovered plants were zygotic and had been originated by selfing. Our results confirm that the SI reaction can be surpassed by temperature stress, bud pollination and tetraploidy. To our knowledge, this is the first report in citrus in which the SI reaction breakdown by these three different strategies is demonstrated by molecular markers.

Observed autogamy in Baobab (Adansonia digitata L.)

“…The inability of the dichogamous owers to develop fruits could be attributed to the high selfincompatibility reported for baobab by Venter et al (2017). Self-incompatibility is known in 50% of angiosperms (Montalt et al, 2022). The nding suggests that both self-compatibility and incompatibility are present in baobab trees, therefore self-pollination and cross-pollination are both possible.…”

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

“…Thus, the incompatibility reaction appears to occur particularly late, just before anthesis. Moreover, tetraploidized self-incompatible citrus plants emerged to become self-compatible (Montalt et al, 2022;Yamashita et al, 1990). More on this subject is mentioned in a later section.…”

“…Some practical methods to break SI have been reported. Bud pollination (Distefano et al, 2009b;Montalt et al, 2022;Wakana et al, 2004), repeated pollination of immature buds (Yamashita, 1980(Yamashita, , 1981, and hightemperature treatment (Aloisi et al, 2020) can artificially break the SI in Citrus species. Wakana et al (2004) reported that the most appropriate time for producing self-fertilized seeds by the bud pollination of selfincompatible Citrus species is when flowers are half of their total length.…”

“…Because citrus SI is considered to be an S-RNase-based GSI with a non-selfrecognition system, polyploidization is considered to be significant for SI breakdown. Based on the same mechanism, when a diploid (2x) pollen from a tetraploidized (4x) plant pollinates the original diploid plant, the self-incompatible reaction is avoided and fertilization occurs (Montalt et al, 2022;Yamashita et al, 1990). Tetraploid 'Hyuganatsu' and 'Clemenules' Clementine are self-compatible (Montalt et al, 2022;Yamashita et al, 1990).…”

“…Based on the same mechanism, when a diploid (2x) pollen from a tetraploidized (4x) plant pollinates the original diploid plant, the self-incompatible reaction is avoided and fertilization occurs (Montalt et al, 2022;Yamashita et al, 1990). Tetraploid 'Hyuganatsu' and 'Clemenules' Clementine are self-compatible (Montalt et al, 2022;Yamashita et al, 1990). It was reported that diploid pollen grains from tetraploidized 'Hyuganatsu', when used to pollinate the original diploid self-incompatible 'Hyuganatsu', can grow pollen tubes by avoiding SI reaction in the pistil, leading to successful fertilization with x egg cells in the ovary.…”