The Physiology of Flowering Plants - KHAM PHA MOI

198 PLANT GROWTH HORMONESBox 7.2Mutations on the ETR1 proteinresult in plants becoming ethyleneresistant (etr) phenotype.Theunmutated, functional protein isreferred to as ETR1. See Appendix.copper-binding proteins and have a number of hydrophobic regionstypical of proteins located in a membrane. Recent studies have shownthat the ETR1 receptor (Box 7.2) is located in the endoplasmic reticulum(Chen et al. 2002). When ethylene binds to an ethylene receptor,the receptor interacts with other proteins in the signal transductionpathway. Protein phosphorylation is a common component of signaltransduction pathways in many organisms and a number of proteinsinvolved in the ethylene and other hormone transduction pathwaysare known to be kinases or phosphatases. In the absence of ethylene,the receptor activates CTR1. This leads to the repression of the activityof the protein EIN2. When ethylene binds to the receptor this repressionis released and gene expression is activated, leading eventuallyto the observed responses. Hence mutations in the early componentsof the chain, such as CTR1, lead to the pathway being permanentlyactivated, causing the constitutive triple-response phenotype.Although the term ‘triple response’ suggests that ethylene causesjust three responses, in fact many more genes will be activated(Section 7.4.5). It is thought that part of the ethylene signal transductionchain consists of a series of DNA-binding proteins which willbind to specific sequences in the promoters of ethylene-responsivegenes causing them to be expressed. In this way the activation of justa few ethylene-response-element binding proteins (EREBPs) can activatemany different genes. Similar binding proteins and sequenceshave been found for other hormone response pathways such asauxin, ABA and GA, and this is likely to be a common feature of allsignal transduction pathways leading to alterations in geneexpression.The mutagenic approach has been widely applied to studies of allthe plant growth hormones and has uncovered components of manysignal transduction pathways. However, relatively few receptors forhormones other than ethylene have been discovered in this way.Where putative receptors have been found (e.g. a cytokinin receptorwhich shares many features with the ethylene receptors) hormonebinding has not yet been demonstrated. This failure to identifyreceptors might result from the presence of multiple receptorgenes encoding proteins with complementary functions; hencelesions in one receptor will be compensated for by others, the mutationsmay be lethal, or the situation may require a major reassessmentof how some growth hormones are perceived.7.4.4 Rapid events following hormone applicationAlthough relatively few hormone receptors have been identified, awide range of events which are activated soon after the application ofhormones to plant cells have been discovered. One of the best-studiedsystems has been guard cells and their role in the closure of stomatafollowing exposure to ABA. Stomata provide an excellent experimentalsystem as the guard cells are readily accessible at the leaf surfaceand the response is rapid and readily quantifiable. The closure of