Classical ultrastructural studies have revealed that the architectural structure of the pollen wall is composed of a series of concentric outer layers, although its shape, size and morphology are highly diverged among plant species. These layers are known as the exine, which is formed around the microspore during microsporogenesis. Detailed morphological studies have demonstrated that the exine consists of the simple inner nexine layer and the outer sexine portion, which determines the sculptured exine structure. Biochemical studies have shown that a constituent of the exine precursor, sporopollenin, potentially contains polymers of fatty acid, phenylpropanoids and phenolics derivatives. On the other hand, genetic and molecular studies employing Arabidopsis mutants defective in exine formation have provided new knowledge not only on the critical processes for this pattern formation, but also on the genes involved in the process. Characterization of these mutants has shown that they can be generally classified into three types from a morphological viewpoint: mutants defective in sporopollenin synthesis, primexine formation or callose wall formation. The genetic approach has demonstrated that Arabidopsis mutants lacking any of these three processes show failure in exine pattern formation. In other words, these three processes play critical roles in exine pattern formation in Arabidopsis. Here we review the mutants and genes related to exine pattern formation in Arabidopsis.
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