**4.2 Ethylene signaling in fruit ripening**

It is well established fact that apart from several crucial role in plants ethylene also induces triple response in several species like Arabidopsis, pea and other plants. Triple response includes inhibition of hypocotyl elongation, initiation of swelling (radial elongation) & inhibition of root elongation and appearance of prominent apical hook. The pathway of ethylene signal transduction has been well established and extensively studied since long time and a number of genes have been identified. In Arabidopsis, the ethylene-resistant1 (ETR1) was first isolated ethylene receptor. Further, 5ethylene receptorsETR1, ETR2, ERS1, ERS 2 and EIN4 has been discovered. These ethylene receptors consist of 2 subfamilies with N-terminal transmembrane ethylene binding domain and C-terminal histidine kinase domain according to amino acid sequence. ETR2, ERS2 and EIN4 are subfamilies of ethylene receptors with extra N- terminal transmembrane domain. ERS1 has only histidine kinase activity whereas ERS1 has both histidine kinase and serine threonine kinase activity. Subfamily1 has major role in ethylene signal transduction as CTR1 (Constitutive triple response1) strongly interact with subfamily1 along with ETR1 & ERS1 as compared to subfamily2 [21, 22]. In tomato researchers have identified 6 ethylene receptors genes [LeETR1, LeETR2,LeETR3 (NR, never ripe), LeETR4-6] in which LeETR1-3 consists of type1 receptors while LeETR4-6 type2 receptor. Gene LeETR1 and LeETR2 are expressed constitutively during developmental phases, LeETR5 in fruits and flowers during biotic stress conditions, NR& LeETR4 in reproductive phase, fruit ripening and senescence. However, ethylene sensitivity can be affected by repression all above genes except LeETR4. Arabidopsis ETR1 forms dimer in endoplasmic reticulum where it requires copper as cofactor for ethylene binding. Responsive-to- antagonist1 (RAN1) acts as copper transporter and its mutation forms inactive receptors lacking copper. CTR1 is a negative regulator of ethylene signaling and shows homology with Raf family of mitogen activated protein kinase kinase kinase (MAPKKK). LeCTR1-4 are homologs of CTR1 identified in tomato where LeCTR1 functionally complement CTR1 in Arabidopsis. Mutation in EIN2 which lies downstream ethylene responses in Arabidopsis plant when ethylene binds to ligand, it trigger activities of several genes via EIN3 &ERF1 family of transcription factors. Primary ethylene response elements (PEREs) stimulate ethylene promoting genes and modulates ripening and senescence related genes as well. However, homodimer of EIN3 binds to PERE which in turn bind to GCC-box present in promoters of stress responsive

### *Role of Post-Harvest Physiology in Evolution of Transgenic Crops DOI: http://dx.doi.org/10.5772/intechopen.94694*

gene, enabling downstream process. Four EIN3 (LeEIL1-4) genes are identified in tomato in which LeEIL3 functionally complement EIN3 mutation in Arabidopsis. In absence of ethylene, activated form of CTR1 binds to ethylene receptor &retards the downstream ethylene signal transduction process thus, suppressing ethylene stimulated gene expression. When ethylene binds to receptor, conformation change in receptor begins which enables dissociation of CTR1 and CTR deactivation, releasing downstream pathway from suppression. EIN2 protein gets activated to stimulate downstream signal transduction and perception [23].
