Citrus is a genus with gametophytic self-incompatibility, which prevents fertilization by self-pollination. Polyamines (PAs), as a class of active small molecules, widely participate in various cellular activities. To investigate the effects of the changes in PA contents [putrescine, spermidine (SPD), and spermine] on pollen tube growth in self-incompatible (SI) plants, we cultured pollen of Citrus maxima ‘Banpeiyu’ and C. tamurana ‘Hyuganatsu’ in a Citrus mature pollen culture system and detected the PA content changes in pollen by high-performance liquid chromatography (HPLC) analysis during germination and after treatment with crude protein extracts of compatible or incompatible styles. We profiled the PA content changes during the germination course in pollen of ‘Banpeiyu’ and ‘Hyuganatsu’ by HPLC, and identified increases of free and perchloric-acid-insoluble SPD contents in pollen tubes in the incompatible treatment. Exogenous SPD treatment obviously increased the SPD contents in the pollen tube, inhibited pollen tube elongation, and resulted in some abnormal morphological alterations, such as tip inflation, tube content leakage and tip twist, which were observed in both ‘Banpeiyu’ and ‘Hyuganatsu’ pollen tubes exposed to incompatible treatment. This finding implied that SPD might participate in the SI response in Citrus.

Self-incompatibility (SI) is an important mechanism in higher plants that promotes outcrossing and prevents self-fertilization. ‘Banpeiyu’ (Citrus maxima) and ‘Hyuganatsu’ (Citrus tamurana), two of the Citrus cultivars distributed in Kyusyu, Japan, show gametophytic SI. In this study, we used the Citrus mature pollen culture system and stylar crude protein extracts to simulate compatible (C) and SI responses in ‘Banpeiyu’ pollen tubes. We analyzed the protein changes in pollen tubes with the C- and SI-like treatments by nano-liquid chromatography–mass spectrometry (nano-LC-MS); 14 and 27 proteins were identified in C- and SI-like treatments, respectively. We picked up some candidate genes that were particularly prevalent in SI-like treatment and analyzed their expression level changes during C- and SI-like treatments in ‘Banpeiyu’ and ‘Hyuganatsu’ pollen tubes. The expression levels of copper/zinc superoxide dismutase (Cu/Zn SOD), manganese SOD (Mn SOD), catalase (CAT), and cysteine protease (CYP) increased after SI-like treatment. We used a fluorescent probe to visualize reactive oxygen species (ROS) level changes in ‘Banpeiyu’ and ‘Hyuganatsu’ pollen tubes after C- and SI-like treatments and found that 2-hour SI-like treatment induced ROS levels to increase in the pollen tubes of both cultivars. These results suggest that an ROS increase could be one of the key phenomena in the SI response of Citrus and that gene expression changes were responses to ROS generation.

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