Abstract
BackgroundRice is highly sensitive to temperature fluctuations. Recently, the frequent occurrence of high temperature stress has heavily influenced rice production. Proper heading date in specific environmental conditions could ensure high grain yield. Rice heading greatly depends on the accurate measurement of environmental changes, particularly in day length and temperature. In contrary to the detailed understanding of the photoperiod pathway, little has been known about how temperature regulates the genetic control of rice heading.ResultsNear isogenic lines that were segregated for qHd1, were developed from a cross between indica rice varieties Zhenshan 97 (ZS97) and Milyang 46 (MY46). Using a five sowing-date experiment in the paddy field, we observed the involvement of qHd1 in temperature responses. With the gradual increase of temperature from Trial I to V, heading date of MY46 homozygotes continued to decrease for about 5 d per trial from 76 to 58 d, while that of ZS97 homozygotes was promoted at the same rate from Trial I to III and then stabilized at 69 d. This thermal response was confirmed in a temperature-gradient experiment conducted in the phytotron. It is also found that tolerance of the ZS97 allele to heading acceleration at high temperature was associated with higher grain weight that resulted in higher grain yield. Then, by qRT-PCR and RNA-seq, we found the pathway OsMADS51-Ehd1-RFT1/Hd3a underlying the qHd1-mediated floral response to temperature. By sequence comparison, OsMADS51 for qHd1 displayed a 9.5-kb insertion in the 1st intron of the ZS97 allele compared to the MY46 allele. Furthermore, this large insertion is commonly found in major early-season indica rice varieties, but not in the middle- and late-season ones, which corresponds to the requirement for high-temperature tolerance during the heading and grain-filling stages of early-season rice.ConclusionsBeneficial alleles at qHd1 confer tolerance to high temperatures at the heading and grain-filling stages, playing a significant role in the eco-geographical adaptation of early-season indica rice during modern breeding. These results, together with the underlying OsMADS51-Ehd1-RFT1/Hd3a floral pathway, provide valuable information for better understanding the molecular mechanism of temperature responsive regulation of heading date and yield traits in rice.
Highlights
Rice is highly sensitive to temperature fluctuations
Effect of qHd1 on the tolerance to high temperatures shown in the multiple sowing-date experiment Photoperiod and temperature conditions With the sowing date changing at a specific rice cropping area, the available photo-temperature resources and the development progress of a rice variety varied greatly
The largest fold changes were found in the first or second sampling time, including 6.2 fold of OsMADS51 in 30 days after sowing (DAS), 4.6 fold of Early heading date 1 (Ehd1) in 35 DAS, 11.3 fold of Rice flowering locus T1 (RFT1) in 30 DAS, and 3.7 fold of Heading date 3a (Hd3a) in 30 DAS. These results suggest that the 9.5-kb sequence structure variation in OsMADS51 could underlie a qHd1-mediated heading response to temperature, by regulating the Ehd1-RFT1/ Hd3a expression
Summary
Rice is highly sensitive to temperature fluctuations. Recently, the frequent occurrence of high temperature stress has heavily influenced rice production. The influence of climate changes on rice productivity has become increasingly concerned, such as more frequent high temperature stress accompanied with global warming [1]. In rice production and breeding practice, heading date and yield traits are generally considered as key indicators for the response to temperature variation; for instance, early heading and yield loss have been commonly observed at high temperatures. Flower initiation is the indication of transition from vegetative to reproductive stage, which largely determines the heading date variation among different rice genotypes and is governed by thermal sensitivity besides the basic vegetative growth and photoperiod sensitivity [5,6,7]. Rice is highly sensitive to temperature changes, at the heading and grain-filling stages. Understanding how temperature plays a role in regulating rice heading would be essential to mitigate the impacts of climate changes, when temperature appears to be rising
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