Proper gene regulation is critical for normal development. Useful models of gene regulation include developmental switches, such as the decision to flower. Unfortunately, relatively little is known about the regulation of this critical developmental switch in crops of agronomic importance such as maize. Increasing evidence suggests that the decision to flower, as well as many other developmental decisions, are heavily controlled by epigenetic regulation. Epigenetic regulation involves modifications superimposed upon the primary DNA sequence that may alter the tertiary structure / accessibility of the DNA. Furthermore, a significant role for small RNAs in this level of regulation is becoming increasingly evident.

Paramutation is an epigenetically based allelic interaction, and represents an excellent system for gaining a mechanistic understanding of epigenetic regulation. Two novel mutations that affect paramutation also have interesting effects on development, including delayed flowering and feminization of the male inflorescence (a phenotype referred to as tasselseed). The effect of both mutations on flowering time is reproducible, but variability and heritability of the other developmental effects further support the epigenetic nature of these mutants. These mutations serve as useful tools to dissect epigenetic regulation and connections to development. One of these mutations, mediator of paramutation1 (mop1), has recently been cloned, and encodes an RNA-dependent RNA polymerase, fully supporting the concept that small RNAs play a significant role in such regulation.

 The research in my lab is focused on understanding the regulatory mechanisms surrounding two developmental decisions in maize, both of which are affected by the novel mutations described above, and on further characterizing the connection between each mutant and the altered developmental decisions that they make.  We are studying gene expression of several candidate genes likely to influence flowering time. We currently have several putative flowering time genes that are differentially expressed in one or both of our mutants, and are focusing on characterizing their tissue-specific gene expression and demonstrating the functional role of these genes in flowering time.  One gene of interest is orthologous to CONSTANS in Arabidopsis and Heading Date1 in rice both of which exhibit circadian rhythms and play a role in the photoperiod sensitivity of those species. Maize, while evolved from a short-day plant, is considered to be day-neutral, i.e. unaffected by varying day-length.  Thus, additional experiments are currently focusing on the expression of the maize gene in varying photoperiods, and on determining whether it exhibits a circadian rhythm. Our most recent results suggest that it behaves differently in distinct photoperiods, and exhibits some unique patterns of expression relative to its homologs.  We are optimistic that these differences will help shed light on the beneficial evolutionary change that has rendered maize day-neutral.

Once the floral transition has been made, the decision to produce the correct inflorescence structure also remains enigmatic. We have been studying the tasselseed phenotype observed in our mutants, trying to dissect the developmental regulation involved in this decision. Maize produces separate male (tassel) and female (ear) inflorescences, but initially those structures are quite similar.  Both the tassel and the ear produce perfect florets, i.e. containing both male (stamens) and female (gynoecia) structures, but the unnecessary structure aborts during the later development of each inflorescence. There are at least nine maize developmental mutants that exhibit stamens in the ear (anther ear), or gynoecia in the tassel (tasselseed).  A few of the corresponding genes have been cloned, most of which encode gene products required to produce or perceive hormones. Thus, the only clear connection regarding this developmental decision is a role for hormones, but the upstream regulatory mechanisms that control these decisions remain obscure.  We have been studying the tasselseed phenotype observed in our mutants, seeking to determine the cause of the phenotype by identifying those genes that show differential expression. Our most recent experiments reveal high expression of three members of the Squamosa-Promoter Binding (SPB) family specifically in mop1 mutants showing tasselseed phenotypes relative to much lower expression in their wild-type siblings.

Several members of this gene family have been shown to be subject to miRNA regulation, and thus future experiments will be directed toward determining whether the RNA-dependent RNA polymerase encoded by the mop1 gene has a direct or indirect effect on the expression of these SPB genes.

jane.dorweiler@marquette.edu