Topic 8: example of a signal transduction pathway found within a plant

      Ethylene serves as a gaseous hormone in plants, and is perhaps most widely known for its role in the ripening of such fruit as tomatoes, bananas, and apples. The ethylene signal transduction pathway is probably the best understood hormone signaling pathway, whose elucidation of this pathway has been greatly aided by the isolation of mutants that are either blocked in ethylene response or display a constitutive ethylene response in the absence of the gas. In the absence of ethylene, the ethylene receptors constitutively activate CTR, a Raf protein kinase, and the ensuing signaling cascade culminating in the repression of the positively acting EIN2 protein. Binding of ethylene to the receptors disrupts the interaction between the receptors and CTR and somehow inactivates the latter, as well as the ensuing MAP kinase cascade. This relieves the repression of the EIN 2 protein leading to ethylene response gene activation. Production of the ETR1 protein in bacteria has facilitated its biochemical characterization. In vitro studies demonstrated that the hydrophobic Nterminal domain of ETR1 is responsible for ethylene binding, whilst the C terminus contains a domain with histidine kinase activity which is dependent on Mn2. The precise mechanism by which the signal is relayed from the ethylene receptor to CTR is, however, not known. Like all other MAP kinases, full-length CTR1 may not be active because of an auto-inhibitory effect involving the N-terminal domain.

     Four plants hormones:

1.  Auxin, stimulates stem elongation; promotes the formation of lateral and adventitious roots; regulates development of fruit; enhances apical dominance; functions in phototropism and gravitropism; promotes vascular differentiation; retards leaf abscission.
2. Cytokinins, regulates cell division in shoots and roots; modify apical dominance and promote lateral bud growth; promote movement of nutrients into sink tissues; stimulate seed germination; delay leaf senescence.
3. Gibberellins, stimulate stem elongation, pollen development, pollen tube growth, fruit growth, and seed development and germination; regulate sex determination and the transition from juvenile to adult phases.
4.  Strigolactones, promote seed germination, control of apical dominance, and the attraction of mycorrhizal fungi to the root.