Loss-of-function, dominant-negative, and change-of-function genetic approaches were used to investigate the role played by the Arabidopsis mitogen-activated protein (MAP) kinase MPK6 throughout development. Plants homozygous for T-DNA null alleles of MPK6 displayed reduced male fertility and abnormal anther development. In addition, a portion of the seed produced by mpk6 plants was found to contain embryos that had burst out of their seed coats. To address potential functional redundancy, a dominant-negative version of MPK6 was constructed by changing the TEY activation loop motif to the amino acid sequence AEF. Plants expressing MPK6AEF via the MPK6 native promoter were found to produce excessive stomata, consistent with the recently described role of MPK6 in stomatal patterning. A novel floral phenotype characterized by abnormal sepal development was also observed in MPK6AEF lines. The gene expression pattern of the MPK6 native promoter was determined using a YFP-MPK6 fusion construct, and expression was observed throughout most plant tissues, consistent with a role for MPK6 in multiple developmental processes. The YFP-MPK6 construct was found to rescue the fertility phenotype of mpk6 null alleles, indicating that the fusion protein retains its biological activity. It was also observed, however, that plants expressing YFP-MPK6 displayed reduced apical dominance and a shortening of inflorescence internodes. These results suggest that the YFP tag modifies the activity of MPK6 in a manner that affects inflorescence development but not anther development. Taken together, the present results indicate that MPK6 is involved in the regulation of multiple aspects of plant development.