Document Type

Article

Publication Date

2009

Abstract

Sporopollenin is the major component of the outer pollen wall (exine). Fatty acid derivatives and phenolics are thought to be itsmonomeric building blocks, but the precise structure, biosynthetic route, and genetics of sporopollenin are poorly understood.Based on a phenotypic mutant screen in Arabidopsis (Arabidopsis thaliana), we identified a cytochrome P450, designatedCYP704B1, as being essential for exine development. CYP704B1 is expressed in the developing anthers. Mutations in CYP704B1result in impaired pollen walls that lack a normal exine layer and exhibit a characteristic striped surface, termed zebraphenotype. Heterologous expression of CYP704B1 in yeast cells demonstrated that it catalyzes v-hydroxylation of long-chainfatty acids, implicating these molecules in sporopollenin synthesis. Recently, an anther-specific cytochrome P450, denotedCYP703A2, that catalyzes in-chain hydroxylation of lauric acid was also shown to be involved in sporopollenin synthesis. Thisshows that different classes of hydroxylated fatty acids serve as essential compounds for sporopollenin formation. The geneticrelationships between CYP704B1, CYP703A2, and another exine gene, MALE STERILITY2, which encodes a fatty acylreductase, were explored. Mutations in all three genes resulted in pollen with remarkably similar zebra phenotypes, distinctfrom those of other known exine mutants. The double and triple mutant combinations did not result in the appearance of novelphenotypes or enhancement of single mutant phenotypes. This implies that each of the three genes is required to provide anindispensable subset of fatty acid-derived components within the sporopollenin biosynthesis framework.

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