A diagram of the biosynthesis and transport of pollen coat formation in monocotyledons and dicotyledons.
The male gametophyte of angiosperm is protected by a solid pollen wall. In the outermost layer of the pollen wall, there is a hydrophobic mixture called "pollen coat" that plays an important role in protecting male gametes from various environmental stresses and microorganism attacks, and in pollen-stigma interactions.
The previous research of Prof. Qi's group from Institute of Botany. CAS, found that a tapetum-specific triterpene metabolic pathway regulated the pollen coat formation in Poaceae, and the lost-of-function mutation of OsOSC12, a key enzyme for biosynthesis of triterpene, led to the humidity-sensitive genic male sterility (HGMS). Study on the pollen coat formation mechanism can reveal not only the basic scientific question of important reproductive organs formation, but also the genetic and molecular mechanisms of HGMS, providing valuable male sterile lines and theoretical basis for the utilization of crop heterosis. Therefore, members of Prof. Qi's group have systematically reviewed the recent research progresses on biosynthesis and transport of pollen coat precursors in angiosperms.
In this review, a detailed comparison of the morphology, composition, and function of pollen coat among plant species with different pollination types showed that the morphology and composition of pollen coat are related to pollination modes of different plant species with clear lineage specificity. Pollen coat proteins and small molecules, including long and very long chain fatty acids and their derivatives, are important pollen coat components. This review summarized the genes and coding proteins involved in the biosynthesis of pollen coat precursors, and confirmed that they are mainly involved in the metabolisms of lipid, phenol, triterpene and sterol in the middle and late stages of anther development. Among them, the triterpene and sterol metabolisms vary widely in different species, indicating possible lineage specificity., The processes of tapetum degradation with pollen wall formation between monocotyledons and dicotyledons were compared at the cellular level with special attention to the morphological changes of two oil-enriched organelles at the late anther development stage.
This review also discussed several urgent scientific questions. For example, what are the exact components of pollen coat? How are the pollen coat precursors synthesized and transported? And how are the biosynthesis and transport of pollen coat precursors coordinately regulated during anther development? These issues will be addressed with improvements in methods such as extensive plant genome mining, single cell and spatial multiomics, and tracing of pollen coat proteins and metabolites. At the end, the application pollen coat-defective mutants with HGMS in crop heterosis utilization was discussed. HGMS lines could be used as a male sterile female parent to produce hybrid seeds in dry areas (such as Xinjiang, China), and propagated by self-pollination in humid and rainy areas (southern China). This would expand the geographical range of two-line hybrid seed production, making extensive usage of crop heterosis for crop production.
This review was published online in Nature Plants on May 25, 2023. Prof. Xiaoquan Qi from the Institute of Botany of the Chinese Academy of Sciences (CAS) is the corresponding author, a Ph.D. student Yuyuan Qiao and Dr. Bingzhu Hou are the co-first authors. This work was supported by the National Key Research and Development Program of China (no.2022YFF1003500), and the National Natural Science Foundation of China (no.32101769, 31530050, 3191001081).