Exine export in pollen

Abstract

Pollen as a sperm cell carrier is mainly protected by outer pollen wall (called exine) from physical and biological stresses. The major composition of exine is the highly resistant biopolymer sporopollenin, which mainly consists of hydrophobic lipids, phenylpropanoids, and aromatic compounds. The biosynthesis of these constituents has been shown to be catalyzed by enzymes preferentially expressed in the sporophytic tapetal layer, a nutritive tissue supporting pollen development. How the synthesized sporopollenin precursors are exported from tapetal cells onto the surface of microspore for pollen exine formation remains largely unknown. Here, we review the structure of tapetal cella and pollen exine in the model monocot rice (Oryza sativa) and the model dicot Arabidopsis thaliana. In addition, we highlight the update understanding on the role of ATP-binding cassette (ABC), lipid transfer protein (LTP), and multidrug and toxic efflux (MATE) transporters in trafficking of sporopollenin precursors across tapetal cells for exine development in rice and Arabidopsis. We also discuss the future research focus on the transport of sporopollenin precursors for exine synthesis.