Cytosolic Heat Shock Protein 90 in Plant Hormone and Environmental Stress Response

From recent findings, it was revealed that cytosolic heat shock protein (HSP) 90 plays important roles in plant signal transduction during hormone sensing and environmental stress response. The various types of signaling proteins are identified as substrate for cytosolic HSP90 in plants; auxin receptor complex, auxin transporters, jasmonate receptor complex, brassinosteroid receptor complex, brassinosteroid-related transcription factors, heat shock transcription factors, receptors for pathogen infection, and components for circadian system. From the overview of these HSP90 substrates, cytosolic HSP90 works essentially in terms of sensing by receptor and output by transcription. It is also obvious that HSP90 facilitates SCF-type ubiquitin ligase complexes (e.g., auxin and jasmonate receptors) together with co-chaperone SGT1 in plants. This implies that cytosolic HSP90 regulates various SCF-type ubiquitin ligase complexes in other plant signaling systems. This chapter reviews recent advances in our understandings of cytosolic HSP90 function in plant hormones and environmental stress signaling.