Chinese
Team members are discussing the significance and difficulties of distant cross breeding
Distant breeding is an important approach to germplasm innovation, but reproductive isolation is the barrier in distant breeding, which largely limits the utilization of desirable traits in distantly species.
Phenotypes of interspecific reproductive barrier
On January 26, 2023, the international academic journal Nature published online the research from vegetable reproduction mechanism and breeding application team of Shandong Agricultural University, which is led by Professor Qiaohong Duan.
This study reveals the molecular mechanism of interspecific reproductive barrier in Brassica vegetable crops (such as Chinese cabbage), which is by regulating reactive oxygen species (ROS) level in stigma. Duan et al. also developed a new breeding technique to break the interspecific barrier, and finally obtained interspecific hybrid embryos of Chinese cabbage. These results represent a remarkable breakthrough and would open up a new way of distant breeding
To solve challenging problem on distant breeding, the key is to overcome limitations on theoretical cognitions
As the largest vegetable for winter storage in northern China, the annual planting area of Chinese cabbage is about 1.8 m hectares, accounting for about 14.4% of the total vegetable planting area in China. Self-incompatibility breeding and distant breeding are very important techniques for Chinese cabbage and other Brassicaceae vegetables.
Extensive test cross is mainly applied for hybrid parental selection in traditional distant breeding, which is laborious and low efficiency (or even zero) to obtain hybrids. Due to interspecific reproduction barrier, it is hardly successful to obtain distant hybrid embryo, even with embryo rescue technique. Therefore, to solve the challenge in distant breeding and make full use of genetic resources of interspecies, the prerequisite is to systematically analyze the theoretical mechanism of interspecific barriers.
Most Brassicaceae plants are typical cross-pollinated crops and self-incompatible. Bees are usually used for pollination to produce F1 generation hybrids in production, while bees may carry a variety of pollen, including self-pollen from the same species, cross-pollen from the same species, and pollen from other species.
Bee pollination
The corresponding author Professor Qiaohong Duan described the complexities of mate choice in Brassicaceae: the stigma is the first barrier to pollen entry. The stigma, on the one hand, accepts cross-pollen but suppresses self-pollen to encourage outcrossing, and on the other hand, avoids pollen from distant species to maintain the genetic stability of the species. The common phenomenon of unilateral incompatibility in distant hybridization is also puzzling: why do pistils of self-incompatible plants avoid pollen of self-compatible plants, while in turn pistils of self-compatible plants accepted pollen from self-incompatible plants? How can stigmas suppress self-pollen of the same species through self-incompatibility? How do stigmas of self-incompatible plants suppress pollen from distant species? How do self-compatible plants accept pollen from distant species, but could keep species stability? Is there some mechanism about 'homozygous pollen preference' on the stigma-pollen recognition? So far, little is known about the molecular mechanisms regulating these compatibility systems and their interconnections in the Brassicaceae.
Breaking barrier of distant hybridization and obtaining hybrid embryos
How to break out of dilemma above?
Duan’s team previously discovered that reactive oxygen species (ROS) level in stigma was activated by self-pollen, so that self-pollen was avoided in Chinese cabbage. This study takes the two phenomena of reproductive isolation and self-incompatibility as start, which are stigma inhibition of pollen, and unidirectional incompatibility.
The self-incompatibility response in Chinese cabbage is recognized by the stigma through the SRK receptor and thus inhibits self-pollen. Duan’s team found that pollen from distant species such as Brassica oleracea and Barbarea vulgaris can also activate SRK receptor and the downstream FERONIA signaling pathway, and eventually increase ROS boost in stigma. Because of the lack of functional SRK receptors in stigmas of self-compatible plants, they accept pollen from distant species, but exhibits “homozygous pollen preference” phenomenon, which is a fact that intraspecific pollen is faster and more effective than pollen from distant species in reducing the pollen inhibitory effect of stigma ROS and eventually fuse with the egg cell.
The reviewers of Nature considered that the article as an important advance in the systematic study of fertilization mechanisms in Brassicaceae and that it would stimulate much discussion and further research in the related fields.
Model and hybrid embryos obtained by distant breeding technique
“It is exciting that we have successfully obtained interspecific and intergeneric hybrid embryos of Chinese cabbage through a new breeding technique which is developed from our theoretical research in Brassicaceae vegetables. I believe it would greatly help to create groundbreaking new germplasm by distant hybridization in Brassicaceae.” PhD student Lin Yang gladly told the reporters. The new breeding technique is now being applied in Brassicaceae vegetable breeding.
A new chapter in distant breeding by making full use of the excellent germplasm resources of distantly related species
The seed industry is entering a new phase of modern breeding, and the full exploitation of superior germplasm and genetic resources is the key to innovation in the seed industry. Although genome maps of many crops and their closely related wild species have been published, gene flow between crops and their closely related wild species is limited by reproductive isolation, and it is still a challenge to make effective use of the excellent genetic resources in distant species.
Self-incompatibility breeding and distant breeding are very important breeding techniques for Brassicaceae vegetable crops. Due to our insufficient knowledge of their regulatory mechanisms in the past, we were unable to make breakthroughs in breeding techniques, which largely restricted the selection of hybrid parents and led to many meaningful breeding projects being forced to stall. Prof. Qiaohong Duan explained.
This research provide an systematic analysis of the regulatory mechanisms of self-incompatibility and distant incompatibility in Brassicaceae crops, and provide theoretical support for important breeding work such as cross breeding, distant breeding and germplasm resource innovation in Brassicaceae crops. The new practical breeding techniques to break self-incompatibility and distant incompatibility, are key technologies for Brassicaceae vegetable breeding, which greatly improve the efficiency of parent propagation, promote the exploitation of vegetable germplasm resources, and open a new chapter of distant hybrid breeding.
Expert review
Professor Xuexiao Zou, the academician of the Chinese Academy of Engineering and the president of Hunan Agricultural University, commented that this research systematically analyzed the formation mechanism of interspecies reproduction barrier, and developed a new breeding technique to break distant incompatibility, which is a landmark achievement in the field of plant reproductive biology and hybrid breeding.
Professor SanWen Huang, President of the Chinese Academy of Tropical Agricultural Sciences, commented that Professor Qiaohong Duan's research on the formation mechanism of reproductive isolation in distantly related hybrids and the breeding techniques to break reproductive isolation is of great significance for seed industry innovation. The research breaks through the current traditional thinking pattern, and introduces superior genetic resources of distant species into cultivars through new technique, which will greatly promote the exploitation of superior genetic resources in distant species of the Cruciferae family and lay a solid foundation for the creation of breakthrough germplasm and breeding of new varieties. This research also provides new ideas for the use of wild resources in other crops.
Shandong Agricultural University is the first affiliation of the paper, with Professor Qiaohong Duan from Shandong Agricultural University and Professor Alice Y. Cheung from the University of Massachusetts, USA, as co-corresponding authors, Associate Professor Jiabao Huang and PhD student Lin Yang as co-first authors. Professor Xiansheng Zhang and Nian Wang from Shandong Agricultural University, Professor Xiaowei Zhang, Yuxiang Yuan and Xiaochun Wei from Henan Academy of Agricultural Sciences, Professor Chaozhi Ma, Pengwei Wang and Cheng Dai from Huazhong Agricultural University and Professor Fei Cheng from Qingdao Agricultural University participated in the study. The work was supported by grants from the National Natural Science Foundation of China, Shandong Natural Science Foundation, National Major Scientific Research Program of China.