Characterization Of 'Fuzzy Tassel' Male Sterility Defects Implicates Novel MiRNA-Regulated Pathways In Maize Stamen Development

Abstract

The maize 'fuzzy tassel' ('fzt') mutant is caused by a mutation in 'dicer-like1' and has broad developmental defects. 'dicer-like1' encodes a key enzyme for microRNA (miRNA) biogenesis and many miRNAs are reduced in 'fzt' plants. 'fzt' plants have reduced stature compared to normal siblings and make shorter, narrower leaves. 'fzt' also has striking inflorescence defects; the inflorescence meristem is fasciated, and other meristem types in the inflorescence are indeterminate. In addition, 'fzt' is male and female sterile. To further investigate the 'fzt' stamen defects, we compared development of 'fzt' and normal siblings stamens in fixed, sectioned tissue. Early stamen development in 'fzt' was indistinguishable from normal siblings. However, later in development, 'fzt' stamens arrested after microspores became vacuolated but before dehiscence. At maturity, some 'fzt' stamens contained pollen that appeared morphologically normal, but most of the pollen was dead based on Alexander staining. Normal pollen is tricellular at maturity; 'fzt' pollen was a mixture of uni-, bi-, and tri- cellular pollen, indicating pollen development was arrested at multiple developmental stages. Pollen in normal siblings is loaded with starch before dehiscence. 'fzt' pollen, however, failed to accumulate starch, suggesting pollen that developed to late stages arrested before maturity. I hypothesize that misexpression of specific miRNA targets underlies the 'fzt' stamen defects. The GAMYBs are regulated by miR159 and have key roles in stamen development in other species, and misexpression of the GAMYBs might contribute to male sterility in 'fzt'. 'ZmMyb74' and 'ZmMyb138' encode GAMYBs that are regulated by miR159. I obtained a transposon insertion allele in 'ZmMyb138' ('ZmMyb138-m1::Mu1') and two insertion alleles in 'ZmMyb74' ('ZmMyb74-m1::Mu1'; 'ZmMyb74-m2::Mu1'). Single mutants in both genes are phenotypically normal, suggesting these genes might function redundantly. I am generating ZmMyb138 ZmMyb74 double mutants for further analysis. Better characterization of the phenotype and molecular interactions of the GAMYBs has the potential for improving maize breeding systems in industry in a similar fashion to patented male sterile alleles that allow selectivity in hybrid corn breeding.