Quint Lab:Research

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genetics of phytohormone responses

our knowledge about the mechanisms of signal transduction pathways triggered by plant hormones has dramatically increased within the last decade or so. some pathways, such as auxin signaling seem to be resolved from perception to gene expression (Quint and Gray, Current Opinion in Plant Biology 2006; Delker et al. Planta 2008). however, the multitude of different responses triggered by the same molecule is as amazing as it is poorly understood. hormone-induced expression of sometimes hundreds of genes seems to be a key aspect of these responses. but which genes or clusters of genes are responsible for which responses? why do ecotypes from different geographical and climatic backgrounds respond differently to the hormone stimulus ... and what are the genetic factors underlying this variation?

TIR1-dependent auxin signaling

to identify novel components of SCF complex regulation and/or auxin signaling we used the f-box protein and auxin receptor mutant tir1-1 for a second site forward genetic screen. in a previous screen in bill gray's lab several enhancers of tir1-1-mediated auxin resistance had been identified (see ito and gray, plant physiology 2006; quint et al., plant journal 2005; chuang et al., plant cell 2004; gray et al., plant cell 2003). Vice versa, we are screening for suppressors of the root growth defect on auxin-supplemented (2,4-D, artificial auxin) media. we identified appr. 15 independent tir1-1 suppressor (tis) mutants that restored the wild-type response and are currently cloning the underlying gene/s and charactarize the physiological and genetic features of the mutants.

natural variation and quantitative genetics of hormone responses

we have revealed extensive natural variation for auxin responses in the root in world-wide arabidopsis ecotype collections (delker et al., planta 2008) and could recently determine the first quantitative trait loci (QTLs) involved in the inheritance of this genetic variation. the question that arises is which genes are underlying the QTLs and what are the allelic variants responsible for the variation? to address these questions we are fine-mapping the target intervals and make use of the vast genetic resources of arabidopsis thaliana to come up with a reasonable number of candidate genes that can be tested for their ability to functionally complement the differences in auxin response. we have observed that ecotypes with a high degree of auxin insensitivity in the root do not necessarily display the same insensitivity in other organs like the hypocotyl. hence, it is likely that the various factors responsible for this variation are downstream components and we are therefore also interested in transcriptional differences in response to auxin between ecotypes.

f-box proteins

the evolutionary conserved f-box motifs can be found in various organisms ranging from fungi, insects, fish, and mammals to plants. f-box proteins are subunits of SCF-type E3 ubiquitin ligases and selectively recruit target proteins via their protein-protein interaction domain for ubiquitination and subsequent proteasomal degradation. therefore, this system represents a straight forward mechanism for simple regulation of signal transduction pathways by removal of target proteins. furthermore, the members of the TIR1 f-box protein family in arabidopsis perceive auxinic compounds and thereby constitute a previously unknown novel class of intracellular receptors for small molecules in eukaryotes. the arabidopsis genome encodes appr. 700 f-box proteins which makes this gene superfamily one of the largest in eukaryotes. however, a biological function has been assigned to less than 30 genes/proteins of the 700 members. a major reason for this seems to be functional redundancy due to evolutionary emergence by gene duplication which disqualifies forward genetics as the approach of choice for the characterization of f-box proteins in plants. we are applying reverse genetic approaches to biologically characterize two subfamilies of f-box proteins to place them into the regulatory networks in which they are active.

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