Abstract
BackgroundAutotetraploid rice hybrids have great potential to increase the production, but hybrid sterility is a major hindrance in the utilization of hybrid vigor in polyploid rice, which is mainly caused by pollen abortion. Our previous study showed that double pollen fertility neutral genes, Sa-n and Sb-n, can overcome hybrid sterility in autotetraploid rice. Here, we used an autotetraploid rice line harboring double neutral genes to develop hybrids by crossing with auto- and neo-tetraploid rice, and evaluated heterosis and its underlying molecular mechanism.ResultsAll autotetraploid rice hybrids, which harbored double pollen fertility neutral genes, Sa-n and Sb-n, displayed high seed setting and significant positive heterosis for yield and yield-related traits. Cytological observations revealed normal chromosome behaviors and higher frequency of bivalents in the hybrid than parents during meiosis. Transcriptome analysis revealed significantly higher expressions of important saccharides metabolism and starch synthase related genes, such as OsBEIIb and OsSSIIIa, in the grains of hybrid than parents. Furthermore, many meiosis-related and specific genes, including DPW and CYP703A3, displayed up-regulation in the hybrid compared to a parent with low seed setting. Many non-additive genes were detected in the hybrid, and GO term of carbohydrate metabolic process was significantly enriched in all the transcriptome tissues except flag leaf (three days after flowering). Moreover, many differentially expressed genes (DEGs) were identified in the yield-related quantitative trait loci (QTLs) regions as possible candidate genes.ConclusionOur results revealed that increase in the number of bivalents improved the seed setting of hybrid harboring double pollen fertility neutral genes. Many important genes, including meiosis-related and meiosis-specific genes and saccharides metabolism and starch synthase related genes, exhibited heterosis specific expression patterns in polyploid rice during different development stages. The functional analysis of important genes will provide valuable information for molecular mechanisms of heterosis in polyploid rice.
Highlights
Autotetraploid rice hybrids have great potential to increase the production, but hybrid sterility is a major hindrance in the utilization of hybrid vigor in polyploid rice, which is mainly caused by pollen abortion
Heterosis evaluation of hybrids generated by crossing of autotetraploid with neo-tetraploid rice Analysis of the agronomic traits of five hybrids, which were developed by crossing autotetraploid rice line (T449) with five neo-tetraploid rice lines, showed significant improvement in important yield-related traits, including number of filled grains per plant, yield per plant and seed setting
Evaluation of heterosis indicated that the values for mid-parent heterosis (MPH) were positive for all the traits except grain length and total grains per plant, and the highest MPH was found for grain yield per plant (170.89%)
Summary
Autotetraploid rice hybrids have great potential to increase the production, but hybrid sterility is a major hindrance in the utilization of hybrid vigor in polyploid rice, which is mainly caused by pollen abortion. Our previous study showed that double pollen fertility neutral genes, Sa-n and Sb-n, can overcome hybrid sterility in autotetraploid rice. The polyploid rice hybrids showed stronger biological advantage and yield potential compared with diploid rice hybrids, and has attracted the attention of many rice researchers (Shahid et al 2011; Wu et al 2013; Guo et al 2017). Our research team found that polyploidy enhanced pollen sterility loci interactions and increased chromosomal abnormalities in autotetraploid hybrid rice (Wu et al 2015), and revealed that intersubspecific diploid and autotetraploid hybrid rice sterility could be overcome by double neutral genes (Shahid et al 2013b; Wu et al 2017). Two photoperiod- and thermo-sensitive genic male sterile lines (PS006 and PS012) of polyploid rice showed stronger hybrid vigor and great potential for improving rice quality and productivity (Zhang et al 2017)
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