Recently, our team has made a new breakthrough in the study of plant pollen development, proving that the interaction between lectin receptor kinase osdaf1 and unknown functional protein osinp1 in rice determines the formation and development mode of pollen germination pore in monocotyledon rice. The related research results were published in nature plants with the title of "rice pollen aperture formation is regulated by the interplay between osinp1 and osdaf1". Dr. Zhang Xu and Dr. Zhao Guochao are the co first authors of the paper, and researcher Liang Wanqi is the corresponding author.

 

Pollen is the male gametophyte of flowering plants, which plays an important role in sexual reproduction, species reproduction and food production. Pollen morphology is diverse and species-specific, which is one of the important references for plant classification. However, the formation mechanism of pollen morphological diversity is not clear, which is an important scientific problem that has not been solved for a long time in the field of Botany. Pollen germination pore is the area of pollen surface without hard outer wall, which can promote pollen water absorption and pollen tube germination during pollination. The characteristics of pollen germination pores are mainly determined by size, shape, position and quantity. The most typical germinal pore of dicotyledons is composed of three pollen grooves arranged in equal distance and parallel on the pollen surface, while the characteristic germinal pore of monocotyledons is a single germinal pore distributed in the far end (Fig. 1).

 

Rice, corn, wheat and other cereal crops have similar morphology of pollen germination pore, which is composed of a single pore structure distributed in the distal part of pollen. Compared with the groove like pollen pores without pollen outer wall in Arabidopsis, the pollen pores of rice have complex accessory structures. The pollen free outer wall deposition area in the rice germinal pore is a ring, surrounded by a ring of protruding annulus structure similar to the bottle mouth, and the center is covered by a cap like operculum structure (Fig. 1). However, for a long time, the specific mechanism of morphological and structural differences between dicotyledons and monocotyledons has been unclear.

 

 

Fig. 1 Comparison of pollen morphology between Arabidopsis thaliana and rice

 

Inp1 (inaperturate pollen1) is a protein with unknown function, which has been proved to be a key factor in the formation of pollen germination pores in Arabidopsis thaliana. It gathers at the initial position of pollen germination pores and guides the formation of pollen germination pores. The lack of function will lead to the abnormal formation of pollen germination pores. In this latest study, the researchers proved that inp1 homologous gene in rice plays a conservative biological role in the formation of pollen germination pore in monocotyledonous rice, and osinp1 is located at the beginning of pollen germination pore to guide the formation of ring-shaped area without outer wall coverage. The researchers further found that osinp1 in rice can recruit osdaf1 (effective in aperture formation 1) to the position of the germinal pore to guide the formation of the pore ring structure around the germinal pore, but not in Arabidopsis. Osdaf1 protein belongs to the lectin receptor kinase family. When aggregated around the germinal pore, osdaf1 protein promotes the deposition of outer wall components, and the skeleton bifurcates, so that more outer wall materials are deposited at these positions to form outward protruding structures (Fig. 2).

 

 

Fig. 2 mechanism of osinp1 and osdaf1 synergistically controlling the formation of rice germination pores

 

This study is the first to report the important factors leading to the differences of pollen pore morphology among different species, which provides new clues for revealing the molecular mechanism of pollen surface pattern formation and pore polarity establishment during pollen development of cereal crops. The research was supported by national key R & D program and National Natural Science Foundation of China.

Building portals in pollen was published in nature plant.