The Arabidopsis ABCG26 transporter: a tool for investigating the nature of sporopollenin

Sporopollenin, a highly resistant biopolymer in plant spore and pollen walls, protects these key reproductive structures from environmental stresses. From the data available, sporopollenin is a polymer of fatty acids and oxygenated aromatic compounds, coupled by ester and ether linkages, which provide the biopolymer with its characteristic resistance to chemical degradation. However, our knowledge of sporopollenin structure, composition and assembly are far from complete. Analyses of Arabidopsis male sterile mutants defective in pollen wall formation have revealed genes required for sporopollenin biosynthesis and/or deposition, including MS2, ACOS5, PKS-A/PKS-B, TKPR1 and ABCG26. ABCG26, an ABC transport protein, is thought to function in sporopollenin export from tapetum cells. However, the substrate transported by ABCG26 is unknown. In the abcg26 mutant, sporopollenin precursors are predicted to accumulate in the tapetum. Through the analysis of abcg26 by live-cell two-photon microscopy, lipidic and autofluorescent compounds in anthers can be visualized over the course of pollen development. No differences between wild type and abcg26 tapetum lipids were observed when stained with Nile Red. Conversely, abcg26 exhibits autofluorescence in tapetum vacuole-like bodies, not observed in wild type. Transmission electron microscopy supports these findings, with enlarged, debris-filled vacuoles in the tapetum of abcg26 mutants. Identification of the autofluorescent components accumulating in abcg26 tapetum cells by biochemical methods will provide a unique opportunity to examine the composition of sporopollenin in planta and the mechanisms of sporopollenin export from tapetum cells in Arabidopsis. In addition to improving our understanding of one of the most inert organic compounds known, this work has demonstrated the successful application of two-photon microscopy for brighter, deeper imaging in plant samples.