Building the pollen wall : the role of tapetum transport proteins and sporopollenin translocation in the formation of the specialized cell wall encasing Arabidopsis thaliana pollen

A durable framework of sporopollenin and a lipid-rich pollen coat form the outer (exine) wall encasing pollen grains. The sporopollenin and pollen coat constituents of the exine are contributed by surrounding sporophytic tapetal cells by pre- and post-mortem events, respectively. The objective of this study was to investigate the transport and assembly of exine components from tapetal cells to developing pollen in the anthers of Arabidopsis thaliana. High-pressure freezing/freeze substitution and transmission electron microscopy revealed the sequence of developmental events in the anther that lead to sporopollenin deposition to form the exine and the dramatic differentiation and death of the tapetum, which produces the pollen coat. Through its tight co-expression with genes required for sporopollenin biosynthesis, a member of the ATP-binding cassette transporter superfamily, ABCG26, was found to be expressed preferentially in tapetal cells during early exine formation, and identified as a candidate gene encoding a sporopollenin export protein. Phenotypic abnormalities in the abcg26 mutant were first apparent in early uninucleate microspores as a lack of exine formation and sporopollenin deposition, and indicated that ABCG26 is required for normal male fertility, exine formation and pollen maturation. Further, two-photon microscopy of abcg26 anthers revealed large fluorescent vacuoles in tapetal cells with corresponding loss of fluorescence on microspores, consistent with abnormal ABCG26-mediated export activity in the mutant. These tapetum inclusions were not observed in double mutants of abcg26 and genes encoding the proposed sporopollenin polyketide biosynthetic metabolon (ACOS5, PKSA PKSB and TKPR1), providing a genetic link between transport by ABCG26 and polyketide biosynthesis. Genetic analysis also showed that hydroxycinnamoyl spermidines, known components of the pollen coat, were exported from tapeta prior to programmed cell death in the absence of polyketides, raising the possibility that they are incorporated into sporopollenin prior to pollen coat deposition. I propose a model where ABCG26-exported polyketides traffic from tapetal cells to form the sporopollenin backbone, in coordination with trafficking of additional constituents prior to tapetum programmed cell death.Science, Faculty ofBotany, Department ofGraduat