High temperature treatment induced production of unreduced 2n pollen in Camellia oleifera

Unreduced gametes through chromosome doubling play a major role in the process of plant polyploidization. Our previous work confirmed that Camellia oleifera can produce natural 2n pollen, and it is possible to induce the 2n pollen formation by high temperature treatment. This study focused on the optimization of the 2n pollen induction technique and the mechanisms of high temperature-induced 2n pollen formation in C. oleifera. We found that the optimal protocol for inducing 2n pollen via high temperature was to perform 45 °C with 4 h at the prophase I stage of the pollen mother cells (PMCs). Meanwhile, high temperature significantly decreased the yield and fertility of 2n pollen. Through the observation of meiosis, abnormal chromosome and cytological behaviour was discovered under high-temperature treatment, and we confirmed that the formation of 2n pollen is caused by abnormal cell plate. Based on weighted gene co-expression network analysis, fifteen hub genes related to cell cycle control were identified. After male flower buds were exposed to heat shock, polygalacturonase gene (CoPGX3) was significantly upregulated. We inferred that high temperature causes the CoPGX3 gene to be overexpressed and that CoPGX3 is redistributed into the cytosol where it degrades cytoplasmic pectin, which leads to an abnormal cell plate. Furthermore, abnormal cytokinesis resulted in the formation of dyads and triads, and PMCs divided to produce 2n pollen. Our findings provide new insights into the mechanism of 2n pollen induced by high temperature in a woody plant and lay a foundation for further ploidy breeding of C. oleifera.