Text | China Science Daily reporter Li Chen Intern Yang Chen
The fragrance of rice flowers says that it is a good year. From the rapid protrusion of the stamens to the cracking of the anthers, to the fall of pollen on the stigma, the 30-minute process of rice spikelets blooming is decisive for rice yield.
Regulating the flowering time of rice has always been a difficult problem for breeders to overcome.
Recently, Nature Communications published the latest results of the Shen Rongxin/Wang Haiyang team of South China Agricultural University online. In this study, the transcription factor OsMYB8, a key regulator of the difference in flowering time in indica-japonica rice, was identified.
The reviewers of the paper believe that this study is an important breakthrough in the field of flowering research, which is of great significance to solve the problem of indica-japonica rice flowering, which will help improve the yield of indica-japonica rice hybrid seed production and promote the further development of indica-japonica hybrid rice.
The "hard bones" that have plagued me for a long time
Shen Rongxin, an associate professor at South China Agricultural University, told China Science Daily that most of the hybrid rice in the mainland is a hybrid variety within the indica rice subspecies, and due to the close genetic relationship between the hybrid parents, its yield potential has reached the limit, which is one of the main reasons for the stagnation of rice production in the mainland in the past 20 years.
"Indica and japonica are two subspecies of Asian cultivated rice that produce significant differentiation during the domestication process, and the interspecific hybrids have a strong heterosis, and the yield is expected to be about 15% to 30% higher than the currently widely cultivated Indica/Indica hybrid rice. Shen Rongxin said that when configuring hybrids, the pollen should leave the indica rice as the male parent and land on the stigma of the japonica rice as the female parent to complete pollination.
In hybrid rice production, hybrid seeds can only be produced efficiently when the flowering time of the spikelets of both parents meets. Therefore, flowering time is a key trait to improve the efficiency of hybrid rice seed production. The efficiency of hybrid seed production is an important factor that determines whether the hybrid combination can be widely used.
However, in most hybrid combinations, especially indica-japonica strong heterogeneous combinations, the average flowering time of the female japonica rice is usually later than that of the male indica rice, that is, the flowering time is not met. "This time difference greatly reduces the pollination efficiency in the process of hybrid seed production, resulting in low yield and high cost of hybrid seed production, and the high price of hybrid seeds, which seriously affects the utilization of strong heterosis among indica-japonica subspecies. Wang Haiyang, a professor at South China Agricultural University, said.
The phenomenon of asynchronous parental flowering is not new. In fact, in the practice of agricultural production, breeders have long paid attention to the fact that the flowering time of the parents is one of the prerequisites for ensuring the yield of hybrid seed production.
However, the flowering time of rice is a complex quantitative trait controlled by multiple genes, and it is difficult to investigate the flowering time trait and is easily affected by external environmental factors, so it is difficult to study in depth. Wang Haiyang told China Science Daily that so far, the genes that regulate the differences in indica and japonica rice at flowering have not been cloned, and the genetic basis and molecular mechanism of the differences in indica japonica rice at flowering are unclear.
Slurry absorbs water: find the breaking point
"Theoretically, a stigma only needs one grain of pollen to fertilize. Heng Yueqin, an associate professor at South China Agricultural University, said that the process of flowering and pollination is affected by a variety of internal and external factors, and in order to ensure successful pollination, plants generally have only one flower column per flower, but will produce a large amount of pollen to ensure successful breeding.
Rice florets generally bloom in the early morning and before the high temperature in the afternoon, and can quickly respond to temperature changes and change the flowering time. In the process of indica-japonica hybrid seed production, if the indica rice as the male parent blooms 1 hour earlier than the japonica rice as the female parent, then the female parent has missed the best pollination period of the male parent when it is ready to receive pollen, thus reducing the seed production yield.
However, the duration of rice spikelets from opening to closing is short, and it is difficult to investigate the flowering of multiple rice plants at one time. "It is difficult to clone genes related to flowering time through traditional forward genetics. Shen Rongxin said.
They found a breakthrough based on previous research.
The flower flower of rice is composed of inner and outer barley, stamens, pistils and a pair of pulp pieces located at the base of the inner and outer barley. It was found that before the opening of rice spikelets, the pulp flakes were smaller and the inner and outer barleys were hooked together, and as the flowering time approached, the pulp gradually absorbed water and expanded, pushing open the inner and outer barleys, and after the opening of the pulp sheets, the pulp lost water and shrank, and the inner and outer barneys closed again.
"The earlier the slurry absorbs water and expands to its maximum volume, the earlier the rice will flower. Therefore, Shen Rongxin said, they focused their research on the key structure of the pulp flakes and the key process that affects the water absorption and expansion of the pulp flakes.
However, the most difficult part is the investigation of the flowering traits and the sampling of the slurry.
These two "arduous tasks" were handed over to Gou Yajun, a doctoral student at South China Agricultural University. She first used visual inspection to investigate the approximate blooming time of the field material in advance, selected the appropriate rice ears one afternoon in advance, and rushed to the field the next day before the first flowering, surveyed the number of flowers every 30 minutes and lightly marked them with a marker, and did not end the survey until the flowers were finished that day. The researchers also cut suitable rice ears in the field and put them in a liquid nitrogen tank, and after returning to the laboratory, they judged the opening time of the spikelets according to the location of the spikelets and the location of the anthers, and separated the appropriate slurry pieces for preservation for follow-up research.
It provides new ideas for hybrid rice breeding
Through a comparative analysis of the slurry transcriptome of early flowering indica rice and late flowering japonica rice, the researchers finally locked the key regulatory gene on the transcription factor OsMYB8. The expression level of OsMYB8 was lower in the slurry on the day before flowering, and gradually increased with the approach of flowering time, reaching the peak 1 hour before flowering, and the expression level of OsMYB8 was higher in indica rice slap than that of japonica rice.
According to Shen Rongxin, there is a natural variation on the indica-japonica OsMYB8 promoter, which leads to the high expression of OsMYB8 in indica rice slurry, which in turn promotes the expression of the downstream factor OsJAR1. The OsMYB8-OsJAR1 molecular module promotes the opening of spikelets and earlier flowering by affecting the content of active jasmonic acids in pulp sheets, which ultimately leads to the failure of indica japonica rice flowering.
In the next step, the team focused their research on the germplasm cultivation of early-flowering japonica rice.
"Shortening the flowering time interval of indica-japonica rice is of great significance for improving the outcrossing rate. Shen Rongxin said that previous studies have found that if the flowering time of japonica rice is advanced by 20 minutes, the yield of hybrid seed production can be significantly improved. OsMYB8 is expected to be used to breed japonica rice sterile lines at early flowering and improve the yield of indica-japonica hybrid seed production. The gene editing promoter can also be used to increase the expression level of the allele of japonica rice parents and create early-flowering japonica rice germplasm.
In addition, with global warming, extreme heat weather induced by climate change is becoming more and more frequent, more intense and longer, which greatly threatens global agricultural production.
"Some varieties with late flowering time are highly susceptible to high temperature stress during flowering, resulting in pollen sterility and reduced yields. Wang Haiyang said that cultivating early-flowering rice varieties at high temperatures is an effective strategy to improve the resistance of rice to high temperatures during the flowering period.
He believed that the introduction of the OsMYB8 allele into japonica rice with high expression level, or the directional editing of the OsMYB8 promoter in japonica rice into indica type through gene editing technology to increase the expression of OsMYB8 in japonica rice, could make japonica rice flower time earlier, so as to avoid the decline of pollen fertility caused by high temperature.
Related Paper Information:
https://doi.org/10.1038/s41467-024-46579-z