Molecular Plant Breeding 2024, Vol.15, No.4, 167-177 http://genbreedpublisher.com/index.php/mpb 171 4.1 Mechanisms of sterility in autotetraploids Many studies have found that during meiosis and subsequent pollen development in autotetraploid rice, a large number of genes are abnormally differentially expressed, leading to reduced pollen fertility. Studies indicated that the interaction between pollen sterility loci (Sa, Sb, and Sc) exerts an "epistasis-like" effect in F1 hybrids of autotetraploid rice, influencing pollen fertility through meiotic abnormalities that can result in partial abortion of male gametes and reduced seed set (He et al., 2011; Wu et al., 2015). Gene expression profiling identified meiosis-related genes, such as OsPUB73, and genes involved in photosystem I, whose downregulated expression severely impacted the coordination of the meiosis-related gene network, leading to reduced pollen fertility in the F1 hybrids (Wu et al., 2015). In the pollen mother cells of the autotetraploid rice Taichung 65-4x during meiosis, 786 DEGs (compared to its diploid progenitor) were detected (Wu et al., 2014), with 125 of these being downregulated, including genes involved in chromosome pairing and recombination, such as PAIR2 andOsDMC1B. Additionally, 75 meiosis-related DEGs were identified in the autotetraploid rice T449, including the important genes OsMTOPVIB for meiotic DNA double-strand breaks and OsMOF, which is involved in homologous chromosome pairing and synapsis, both of which were significantly downregulated (Chen et al., 2018). Differentially expressed miRNAs (DEM) and small RNAs have been identified as key players in the development of pollen and embryo sacs. Specific miRNAs, such as osa-miR1436_L+3_1ss5CT and osa-miR167h-3p, are associated with female meiosis, while osa-miR159a.1 and osa-MIR159a-p5 are related to male meiosis, indicating their role in sterility (Li et al., 2017). In the autotetraploid rice 02428-4x, important fertility-regulating genes such as OsMYB80, OsABCG15, PTC1, and CYP703A3 were found to be significantly downregulated (Li et al., 2018). The role of OsMND1 in PMeS autotetraploid rice lines was investigated. It was found that overexpression of OsMND1 improved pollen fertility and viability, early normal embryo development, and seed set rates in Balilla-4x. However, knocking down OsMND1 significantly hindered pollen and embryo development (Xiong et al., 2019). Furthermore, 941 differentially expressed proteins (DEPs) were identified in the meiotic anthers of newly synthesized autotetraploid rice, including 489 upregulated and 452 downregulated proteins, some of which may be related to the early degradation of the tapetum, ultimately affecting pollen fertility (Ku et al., 2022). 4.2 Environmental factors affecting fertility Environmental factors also influence the fertility of autotetraploid rice hybrids. It is well-known that factors such as temperature, humidity, and soil conditions can impact the expression of genes related to fertility and sterility (Yan et al., 2010). It is reported that seasons have a significant impact on both pollen and embryo sac fertility in both diploid and autotetraploid rice. This seasonal variation affects the overall seed setting rate, underscoring the importance of considering environmental conditions when evaluating fertility in autotetraploid rice varieties (Shahid et al., 2010). 4.3 Overcoming sterility barriers 4.3.1 Breeding strategies to enhance fertility Selection and screening of fertile lines are crucial steps in overcoming sterility barriers in autotetraploid rice hybrids. Hybridization techniques play a pivotal role in enhancing fertility in autotetraploid rice. Studies have shown that the development of three types of autotetraploid rice lines mentioned above (2.2), which exhibit high fertility and heterosis, can be achieved through rigorous selection processes. Cytogenetic studies have demonstrated that two hybrids of autotetraploid rice show extremely high pollen fertility, seed set and heterosis, further highlighting the importance of hybridization techniques in breeding strategies (Luan et al., 2007; He et al., 2011).
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