"In the fragrance of rice flowers, we speak of a bountiful year." From the rapid extension of stamens, the bursting of anthers, to the pollen dropping onto the stigma, the blooming process of rice flowers typically takes less than 30 minutes but plays a crucial role in rice yield.
Regulating the flowering time of rice has always been a challenging task for breeders.
On March 13, the latest findings from the team led by Rongxin Shen and Haiyang Wang at South China Agricultural University were published online in Nature Communications. The study uncovered a key regulatory factor, the transcription factor OsMYB8, responsible for the timing difference in flowering between Indica and Japonica rice.
Rice flowering! See the image provided by the interviewee.
Reviewers of the paper believe this study is a significant breakthrough in the field of flowering research, with crucial implications for addressing the problem of asynchronous flowering in indica and japonica rice. It is expected to enhance hybrid seed production and further advance the development of hybrid indica-japonica rice, potentially increasing yields by 15-30%.
Tackling the Long-standing Challenge
Hybrid rice breeding stands as a monumental achievement in agricultural breeding history, contributing immensely to global food security. According to Dr. Rongxin Shen, co-corresponding author and associate professor at South China Agricultural University, most of China's hybrid rice varieties are intraspecific hybrids of indica rice, where the limited exploitation of yield potential due to the close genetic relationship of hybrid parents has been a major factor contributing to stagnant rice yields over the past two decades.
"Indica and japonica rice represent two distinct subspecies that emerged during the domestication process of Asian cultivated rice. Crosses between these subspecies exhibit strong hybrid vigor, with an expected yield increase of around 15-30% compared to widely planted indica/indica hybrids," Dr. Shen explained, emphasizing the need for pollen from the indica parent to reach and fertilize the pistil of the japonica parent during hybrid seed production.
In hybrid rice production, efficient production of hybrid seeds relies on synchronization of flowering between the two parents. Therefore, flowering time stands as a critical trait governing the efficiency of hybrid seed production, which in turn is a crucial factor determining the widespread adoption of hybrid combinations.
The interviewee provided an image.
In most hybrid combinations, especially in indica-japonica strong hybrid combinations, the average flowering time of the maternal indica rice is usually later than that of the paternal japonica rice, resulting in asynchronous flowering. "This time difference greatly reduces the pollination efficiency during hybrid seed production, leading to low yields, high costs, expensive hybrid seed prices, and seriously affecting the utilization of indica-japonica inter-subspecific hybrid advantages," said Professor Wang Haiyang, co-corresponding author of the paper and a professor at South China Agricultural University.
The phenomenon of asynchronous flowering between male and female parents of rice is not a new discovery. In fact, in agricultural production practice, breeders have long been aware that the synchronization of flowering between male and female parents is one of the prerequisites for ensuring hybrid seed production.
However, the flowering time of rice is a complex quantitative trait controlled by multiple genes, and the assessment of flowering time traits is difficult and easily influenced by external environmental factors, making it difficult to conduct in-depth research. Wang Haiyang told the "Chinese Science Bulletin" that so far, the genes regulating the differences in flowering time between indica and japonica rice have not been cloned, and the genetic basis and molecular mechanism of the differences in flowering time between indica and japonica rice are basically unclear.
It is against this background and based on previous research that in 2019, led by Wang Haiyang, a team consisting of Professor Liu Yaoguang, a member of the Chinese Academy of Engineering at South China Agricultural University, and Professor Chen Letian's team and Professor Zhuang Chuxiong's team, successfully applied for the National Natural Science Foundation of China's Innovative Research Group Project.
With the strong support of this project, the team led by Shen Rongxin/Wang Haiyang targeted the "tough nut" and began in-depth research on the cloning of genes responsible for asynchronous flowering between indica and japonica rice and their application in breeding.
Grain soaking: Finding a breakthrough
"In theory, a stigma only needs one pollen grain to complete fertilization," said Dr. Heng Yueqin, co-first author of the paper and associate professor at South China Agricultural University. The process of flowering and pollination is influenced by various internal and external factors. To ensure successful pollination, a plant generally has only one stigma per flower, but produces a large amount of pollen to ensure successful reproduction.
Rice panicles generally flower during the period after early morning and before noon when the temperature is high, and they can quickly respond to temperature changes to adjust flowering time. In the process of indica-japonica hybrid seed production, if the paternal japonica rice flowers 1 hour earlier than the maternal indica rice, then when the maternal plant is ready to accept pollen, it has already missed the optimal pollination period of the paternal plant, thereby reducing seed production.
The interviewee provided a photo.
However, the duration of rice flowering is short, making it difficult to investigate flowering traits across multiple rice plants simultaneously. "It's difficult to clone flowering-related genes through traditional forward genetics," said Rongxin Shen.
They found a breakthrough based on previous research.
Rice spikelets consist of lemma, palea, stamens, pistils, and a pair of lodicules located at the base of lemma and palea. Previous studies found that before rice spikelets open, the lodicules are small, and lemma and palea are hooked together; as the flowering time approaches, the lodicules gradually swell by absorbing water, pushing apart the lemma and palea, allowing the spikelet to open; after flowering, the lodicules lose water and shrink, and lemma and palea close again.
"The earlier the lodicules reach maximum swelling, the earlier the rice flowers," said Rongxin Shen. Therefore, they focused their research on the lodicules, a key structure in rice flowering, and the crucial process of lodicule swelling.
However, the most challenging tasks are investigating flowering traits and sampling lodicules.
These two "arduous tasks" were entrusted to co-first author of the paper, doctoral student Yajun Gou from South China Agricultural University. She first estimated the approximate flowering time of field materials using visual inspection, selected suitable panicles a day in advance, and then visited the field the next day before rice flowering began. She counted the number of flowering events every 30 minutes and lightly marked them with a marker until the spikelets finished flowering that day. "This work requires cooperation among several individuals to ensure that the counting can be completed in a short time," said Yajun Gou.
Sampling the lodicules also requires teamwork. Suitable panicles are cut from the field and placed in liquid nitrogen tanks. After returning to the laboratory, lodicules are isolated based on the position of spikelet emergence and the location of the stamen for subsequent studies.
Providing a New Perspective for Hybrid Rice Breeding
By comparing the transcriptomes of early-flowering indica rice and late-flowering japonica rice, they finally identified the key regulatory gene, OsMYB8. OsMYB8 expression in the lodicules is low one day before flowering, gradually increasing as flowering approaches, peaking one hour before flowering, and exhibiting earlier and higher expression in indica rice lodicules compared to japonica rice.
Rongxin Shen explained that a natural variation in the OsMYB8 promoter of indica and japonica rice results in higher OsMYB8 expression in indica rice lodicules, which promotes the expression of downstream factor OsJAR1. The OsMYB8-OsJAR1 molecular module promotes spikelet opening by affecting the content of active jasmonic acid compounds in the lodicules, resulting in earlier flowering, ultimately leading to prezygotic isolation between indica and japonica rice.
Next, the team will focus on breeding early-flowering japonica rice varieties.
"Shortening the flowering interval between indica and japonica rice is crucial for increasing outcrossing rates," said Rongxin Shen. Previous studies have shown that advancing the flowering time of japonica rice by 20 minutes significantly increases hybrid seed production. OsMYB8 is expected to be used to breed early-flowering japonica male sterile lines, thereby increasing hybrid seed production between indica and japonica rice. Alternatively, by gene editing the promoter, the expression level of alleles in japonica rice parents can be increased to create early-flowering japonica rice germplasm.
Furthermore, with global warming, extreme high-temperature events induced by climate change are becoming more frequent, intense, and prolonged, posing a significant threat to global agricultural production.
"Some late-flowering varieties are highly susceptible to heat stress during flowering, leading to pollen sterility and reduced yields," said Haiyang Wang. Breeding early-flowering rice varieties under high temperatures is an effective strategy for enhancing heat tolerance during the flowering period of rice.
He believes that introducing highly expressed alleles of indica rice OsMYB8 into japonica rice, or using gene editing technology to directionally edit the OsMYB8 promoter in japonica rice to the indica type, to increase OsMYB8 expression in japonica rice, can advance flowering in japonica rice, thereby mitigating the decrease in pollen fertility caused by high temperatures.
This study not only reveals the intrinsic mechanisms of rice flowering regulation but also provides new perspectives and methods for future hybrid rice breeding.
Related paper information: https://doi.org/10.1038/s41467-024-46579-z
