Breaking the isolation of distant reproduction: Crop breeding to achieve the freedom of “mate selection”

Bee pollination. Photo courtesy of Shandong Agricultural University

A hard-working bee collects nectar among the flowers, and its body is stained with pollen. When it rests on a large cabbage flower, pollen of the same species, allopollen, and even pollen of distant species may fall on the stigma.

How do plants find their own right pollen from different sources? How do they refuse their pollen and avoid “marriage of close relatives”? And how to isolate pollen from distant species and maintain their own genetic stability? How exactly do plants “choose mates”?

These have long been puzzles for scientists. On January 26, Beijing time, Nature published online the research results of the vegetable reproductive mechanism and breeding application team led by Duan Qiaohong, a professor at Shandong Agricultural University, to answer the above questions.

This study revealed the molecular mechanism of cruciferous vegetables such as Chinese cabbage by regulating the level of reactive oxygen species in the stigma to maintain interspecific reproductive isolation, and developed a breeding technology to break the reproductive isolation of distant hybrids, successfully obtained interspecific and intergenera distal hybrid embryos of Chinese cabbage, and opened up new ideas and new ways for distant hybrid breeding.

The reviewers of Nature believe that the paper expounds the original research results in this field, which is an important progress in the systematic understanding of the fertilization mechanism of cruciferous plants, and will promote a lot of discussion and further research in related fields.

The breeding bottleneck problem hangs in the balance

In nature, whether a group of organisms can be called a species depends largely on whether they can mate with other groups, that is, whether they have reproductive isolation. The existence of reproductive isolation allows species to ensure that genetic material is passed on from generation to generation and to achieve the reproduction of species.

At the same time, some plants have evolved mechanisms of self-inbreeding to prevent the loss of offspring diversity due to “inbreeding”, causing inbreeding decline and affecting the continuation of the population.

Cruciferous plants are one such type of plant with a complex way of “mate selection”. Duan Qiaohong, the corresponding author of the paper, explained to China Science News that the stigma of the pistils is the first barrier to prevent pollen from entering. On the one hand, stigmas inhibit self-pollen and promote cross-pollen growth to take advantage of hybrid advantages; On the other hand, stigmas inhibit distant pollen to maintain species genetic stability.

“The widespread phenomenon of ‘one-way incompatibility of distant hybrids’ in nature is also puzzling.” Duan Qiaohong said that the so-called distant hybrid unidirectional incompatibility means that the pistils of self-inbred incompatible plants inhibit pollen from self-inbred affinity plants, while the pistils of self-inbred incompatible plants allow pollen from self-inbred incompatible plants to grow on their stigmas.

Moreover, how does stigma inhibit congenital self-pollen through inbred incompatibility? How do stigmas of inbred incompatible plants inhibit distant pollen? Since the stigmas of self-inbred affinity plants allow distant pollen to grow, how can they avoid mixing with other species? These questions are not only fundamental questions that have been unanswered for a long time in plant research, but also puzzle breeders in agricultural production practice.

In the agricultural production process, artificial selection accelerates the breeding process, and new varieties are constantly developed. However, some crops tend to lose excellent traits such as disease resistance during long-term breeding that tends to yield and quality.

Breeders resort to distant cross-breeding in order to retrieve lost excellent traits. Distant-cross is the crossing between different species, genera or more distantly related species, with the aim of selecting the best offspring.

However, reproductive isolation is a bottleneck problem in distant hybrid breeding, which greatly restricts the development and utilization of excellent genetic resources in distant species.

Duan Qiaohong introduced that most cruciferous plants such as Chinese cabbage have self-incompatibility and are typical cross-pollinating crops. According to incomplete statistics, Chinese cabbage is the largest number of vegetables stored in winter in northern China, with an annual sowing area of about 27 million mu, accounting for about 14.4% of the total sown area of vegetables in the country. Self-inbred incompatibility breeding and distant cross breeding are very important breeding techniques for cruciferous vegetables.

“Traditional distant cross breeding mainly selects hybrid parents through extensive crossing, but this method is labor-intensive and laborious, and the hybridization efficiency is very low or even zero. Because reproductive isolation prevents distant fertilization, distal hybrid embryos are often not available, and subsequent embryo rescue efforts will not help. Duan Qiaohong said that in order to solve the bottleneck problem in the field of self-inbred incompatible breeding and distant hybrid breeding, and make full use of the excellent genetic resources of distant species, the premise is a systematic analysis of their regulatory mechanisms.

Breaking down barriers to distant crossbreeding

How to break through the above dilemma?

The team found earlier that bok choy can inhibit the growth of self-pollen on the stigma by activating the stigma to produce reactive oxygen species. Since the pollen cannot grow down the pollen tube, it naturally cannot reach the ovary for insemination.

Therefore, they carried out a series of studies based on the inhibition of pollen by stigmas by stigma and unidirectional incompatibility of distant hybrids, which are manifested as the two phenomena of distant hybrid reproductive isolation and self-inbred.

Huang Jiabao, the first author of the paper and associate professor of Shandong Agricultural University, introduced that on the basis of the previous discovery that the self-inbreeding incompatibility reaction relies on stigma SRK receptor recognition to inhibit self-pollen, they found that distant pollen such as kale and European mustard can also activate the downstream FERONIA receptor kinase signaling pathway through the stigma SRK receptor of Chinese cabbage, increase stigma reactive oxygen species and inhibit distant pollen.

Self-inbred affinity plants have another mechanism to inhibit distant pollen. It turns out that the stigmas of inbred affinity plants lack functional SRK receptors, and distant pollen can pass through the stigmas. However, because intraspecific pollen reduces the inhibitory effect of stigma reactive oxygen species on pollen faster and more effectively than distant pollen, it “wins” in the competition with pollen of distant species, thereby maintaining reproductive isolation, that is, the phenomenon of “pollen of the same species”.

“It is exciting that we have developed a breeding technology that clears stigma reactive oxygen species to break the barrier of distant hybridization of cruciferous vegetables, and successfully obtained interspecific and intergenera hybrid embryos of Chinese cabbage, laying a solid foundation for the subsequent creation of breakthrough new germplasm.” Duan Qiaohong said that the technology can remove stigma reactive oxygen species in a variety of ways, including inhibition of SRK receptor or FERONIA receptor gene, spraying sodium salicylate and other inhibition of reactive oxygen species. It is reported that the technology has been patented.

A new chapter in distant cross-breeding

The seed industry is entering a new stage of modernization, and fully exploiting and utilizing excellent germplasm and genetic resources is the key to seed industry innovation. For cruciferous plants, self-inbred incompatible breeding and distant cross breeding are important breeding techniques.

Duan Qiaohong said: “Due to our insufficient understanding of its regulatory mechanism in the past, we have not been able to make breakthroughs in breeding technology, which has largely limited the selection of hybrid parents, resulting in the suspension of many meaningful breeding projects. ”

Researcher Huang Sanwen, president of the Chinese Academy of Tropical Agricultural Sciences, told China Science News that the research on the formation mechanism of distant hybrid reproductive isolation and the breeding technology to break reproductive isolation carried out by the team is of great significance for seed industry innovation. This study breaks through the traditional idea that Chinese cabbage mainly uses intraspecific hybrid breeding, introduces the excellent genetic resources of distant species into cultivars through new technologies, broadens the scope of germplasm inheritance, and will greatly promote the development and utilization of excellent genetic resources in cruciferous distant species, and lay a solid foundation for the creation of breakthrough germplasm and the cultivation of new varieties. The study also provides new ideas for the use of wild resources in other crops.

Professor Zou Xue, academician of the Chinese Academy of Engineering and president of Hunan Agricultural University, commented that the research systematically analyzed the formation mechanism of distant hybrid reproductive isolation, developed a distance breeding technology to break reproductive isolation, and was both theoretical innovation and breeding application value, which is a landmark achievement in the field of plant reproductive biology and hybrid breeding, and related theories and technologies will have a far-reaching impact on vegetable breeding.

Duan Qiaohong believes that the new breeding technologies brought by the discovery, including the breeding technology that breaks the self-incompatibility and the breeding technology that breaks the reproductive isolation of distant hybrids, will be used as the common key core technology of cruciferous plant breeding, bringing a lot of in-depth exploration into new fields. She hopes that more aspiring young people will join her research group and work together in related research. (Source: China Science News, Li Chen, Wang Jing)

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