Two duplicate CYP704B1-homologous genes BnMs1 and BnMs2 are required for pollen exine formation and tapetal development in Brassica napus

Abstract

S45A, a double recessive mutant at both the BnMs1 and BnMs2 loci in Brassica napus, produces no pollen in mature anthers and no seeds by self-fertilization. The BnMs1 and BnMs2 genes, which have redundant functions in the control of male fertility, are positioned on linkage groups N7 and N16, respectively, and are located at the same locus on Arabidopsis chromosome 1 based on collinearity between Arabidopsis and Brassica. Complementation tests indicated that one candidate gene, BnCYP704B1, a member of the cytochrome P450 family, can rescue male sterility. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of the developing anther showed that pollen-wall formation in the mutant was severely compromised, with a lack of sporopollenin or exine. The phenotype was first evident at the tetrad stage (stage 7) of anther development, coinciding with the maximum BnCYP704B1 mRNA accumulation observed in tapetal cells at stages 7-8 (haploid stage). TEM also suggested that development of the tapetum was seriously defective due to the disturbed lipid metabolism in the S45A mutant. A TUNEL assay indicated that the pattern of programmed cell death in the tapetum of the S45A mutant was defective. Lipid analysis showed that the total fatty acid content was reduced in the S45A mutant, indicating that BnCYP704B1 is involved in lipid metabolism. These data suggest that BnCYP704B1 participates in a vital tapetum-specific metabolic pathway that is not only involved in exine formation but is also required for basic tapetal cell development and function.