Upstream Events In Ethylene Signal Transduction In Arabidopsis
Shockey, Jason Alan
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Ethylene gas has profound effects on the growth and development of higher plants. The understanding of how plants can sense this gas, and react in the appropriate manner is important for both agricultural purposes as well as the basic understanding of plant biology. While many components of this signaling pathway have been identified using classical genetics, we have little understanding of how these components work together. My work has focused on the understanding of early events in ethylene signal transduction. The interaction between the ETR1 ethylene receptor and the CTR1 Raf-like kinase was the first clue that the ethylene signaling pathway diverged from that of the yeast HOG1 osmo-sensing pathway. In this thesis, I examined the functional relevance of this interaction in the regulation of CTR1's activity. My work suggests that although CTR1 demonstrates the novel interaction with two-component receptors, the biochemical regulation of CTR1 may be similar to that of Raf1. Recent studies have suggested that histidine kinase activity of ETR1 may not play a major role in ethylene signal transduction, despite the remarkable degree of sequence conservation with functional histidine kinases from bacteria and yeast. In order to better understand the role of this highly conserved domain, either in ethylene signaling or other possible functions, I utilized biochemical assays, protein interaction studies and transgenic plants. My work indicates that phospho-relay plays no observable role in most ethylene responses, but plays an important role in recovery from ethylene treatment. Important members of this signaling system may yet be unidentified. A gene previously identified in the Chang lab, D2, was shown to have a probable role as a scaffolding protein in ethylene signaling using multiple reverse genetic techniques. This gene is unique to plants and cyanobacteria, as is the ethylene binding fold suggesting the two may have evolved together. The emerging paradigm of the ethylene signaling system reveals the pathway to be much more complex than originally thought.