Feminized tassels of maize mop1 and ts1 mutants exhibit altered levels of miR156 and specific SBP-box genes
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Development of the unisexual maize inflorescences requires the abortion of pistillate primordia in the florets of the developing tassel and the arrest of staminate primordia in the florets of the developing ears. Mutations of many genes that lie within this sexual differentiation pathway, such as tasselseed1 (ts1), or that influence this pathway, such as mediator of paramutation 1 (mop1), result in feminization of the normally male tassel. Here, we show the loss of mop1 or ts1 function results in increased mRNA levels for several members of the SBP-box gene family. Our analyses of this family expand the number of maize SBP-box genes from 9 to 31 members. Intron–exon structures as well as phylogenetic data support the division of these family members into six groups. The SBP-box genes upregulated in feminized tassels fall into two groups, share common structural motifs and include the presence of a target site for miR156. Small RNA blots show miR156 levels are decreased in both mop1 and ts1 mutants. While there is a correlation between miR156 levels and SBP-box gene transcript levels, this correlation is not absolute, and thus we hypothesize that decreased levels of miR156 may provide competency for SBP-box gene upregulation by other common factors yet to be identified. We present a model that provides a putative link between ts1, ts2, ts4, Ts6, and mop1 in the sex-determination pathway.
KeywordsInflorescence development Squamosa promoter binding protein Tasselseed RT-PCR Sexual differentiation Sex determination
We thank the Maize Genome Sequencing Consortium for making genomic sequence available prior to publication, Amanda R. Robinson for assistance collecting some of the tassel RNA samples and Theresa A. Miller, Michael Schläppi, and anonymous reviewers for helpful comments on the manuscript. This project was supported by the National Research Initiative of the USDA Cooperative State Research, Education and Extension Service, grant number 2004-35301-14495.
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