Abstract
Rice reproductive development is sensitive to high temperature and soil nitrogen supply, both of which are predicted to be increased threats to rice crop yield. Rice spikelet development is a critical process that determines yield, yet little is known about the transcriptional regulation of rice spikelet development in response to the combination of heat stress and low nitrogen availability. Here, we profiled gene expression of rice spikelet development during meiosis under heat stress and different nitrogen levels using RNA-seq. We subjected plants to four treatments: 1) NN: normal nitrogen level (165 kg ha-1) with normal temperature (30°C); 2) HH: high nitrogen level (264 kg ha-1) with high temperature (37°C); 3) NH: normal nitrogen level and high temperature; and 4) HN: high nitrogen level and normal temperature. The de novo transcriptome assembly resulted in 52,250,482 clean reads aligned with 76,103 unigenes, which were then used to compare differentially expressed genes (DEGs) in the different treatments. Comparing gene expression in samples with the same nitrogen levels but different temperatures, we identified 70 temperature-responsive DEGs in normal nitrogen levels (NN vs NH) and 135 DEGs in high nitrogen levels (HN vs HH), with 27 overlapping DEGs. We identified 17 and seven nitrogen-responsive DEGs by comparing changes in nitrogen levels in lower temperature (NN vs HN) and higher temperature (NH vs HH), with one common DEG. The temperature-responsive genes were principally associated with cytochrome, heat shock protein, peroxidase, and ubiquitin, while the nitrogen-responsive genes were mainly involved in glutamine synthetase, amino acid transporter, pollen development, and plant hormone. Rice spikelet fertility was significantly reduced under high temperature, but less reduced under high-nitrogen treatment. In the high temperature treatments, we observed downregulation of genes involved in spikelet development, such as pollen tube growth, pollen maturation, especially sporopollenin biosynthetic process, and pollen exine formation. Moreover, we observed higher expression levels of the co-expressed DEGs in HN vs HH compared to NN vs NH. These included the six downregulated genes (one pollen maturation and five pollen exine formation genes), as well as the four upregulated DEGs in response to heat. This suggests that high-nitrogen treatment may enhance the gene expression levels to mitigate aspects of heat-stress. The spikelet genes identified in this study may play important roles in response to the combined effects of high temperature and high nitrogen, and may serve as candidates for crop improvement.
Highlights
More than half of the world’s population relies on rice (Oryza sativa L.) as a staple food
We obtained over 10 million clean reads from the samples of four digital gene expression profiling libraries (NH, nitrogen and temperature levels (NN), HH, HN), and found many upregulated and downregulated differentially expressed genes (DEGs) among the libraries
The loss-of-function mutant of rice cytosolic HSP70 gene (OsctHSP70-1) did not show a clear defective phenotype under high temperature, and this is likely because of the existence of another gene family member that is closely clustered with OsctHSP70-1, which showed compensatory increase in expression [52]
Summary
More than half of the world’s population relies on rice (Oryza sativa L.) as a staple food. The human population is expected to increase by 50% by the end of the century, creating to an elevated need for rice production meet the dietary needs of the global population [1]. There was a boost in rice production over the last decades through the elevated application of nitrogen fertilizers. The global use of nitrogen fertilizers augments plant productivity, as a consequence of the fact that most of the high-yield rice varieties have high demands of nitrogen. Decreasing levels of soil nitrogen caused by increased farming continues to be a challenge that influences rice yield. Another challenge is the increasing global temperatures caused by increased atmospheric CO2 concentration. There is a need to better understand how nitrogen and temperature affect rice growth and development
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