DIFFERENTIAL GENE EXPRESSION OF VARIANT TASSEL PHENOTYPES IN Zea mays mop1-1 MUTANTS

Judd Hultquist
Marquette University
Milwaukee, WI 53233
Mentor: Dr. Jane E. Dorweiler

In recent years, the central role of small-interfering RNA’s in epigenetic gene regulation has become apparent. By manufacturing and amplifying a signal of small RNA’s, a cell can efficiently down-regulate the gene or genes of complimentary sequence. In maize, mediator of paramutation1 is hypothesized to play an important role in this regulatory pathway due to the varied and abnormal phenotypes displayed by plants homozygous for the recessive mop1-1 mutation of that gene. Based on sequence homology and these atypical, mutant phenotypes, it has been hypothesized that mop1 encodes an RNA-dependent RNA polymerase used subsequently in the amplification of the siRNA signal.

One of the most debilitating phenotypic effects of the mop1-1 mutation in maize is the loss of sexual viability in the tassel due to complications arising in sexual differentiation during development. In severe cases, the typically male tassel will either become barren or it will become feminized, growing kernels and silks, a phenomenon known as tasselseed. Knowledge of which genes are expressed during sexual differentiation and are also affected by the mop1-1 mutation would provide significant insight as to the developmental transition from the vegetative to reproductive stages in maize and also in the role of the mop1 protein in epigenetic gene regulation. Unfortunately, very few genes have been recognized as playing a role in the sexual differentiation of maize, but a selection of those that have, as well as homologs to a few identified in Arabidopsis, have been identified and assayed for gene expression in both wild type and mutant plant tassels. 

To get an accurate picture of gene expression, tassels from a family segregating for the mop1-1 mutation were dissected shortly after the transition of the shoot apical meristem to tassel. RNA isolation was followed by DNAse treatment and reverse transcriptase reaction to obtain clean cDNA copies. Amplification by Real-Time polymerase chain reaction allowed for quantification of expression level. Expression levels of the candidate genes were compared between mutant and wild type plants to identify those genes affected by the mop1-1 mutation and also hypothesized to be responsible for the variant phenotypes in the tassel. Early analysis indicates possible differential gene expression of a few candidates in the younger tassels, but a definite conclusion has yet to be reached. 
 
 
 
 

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