In contrast, a triple mutant of subfamily 2 receptors is primarily distinguished by an increase in its ethylene sensitivity such that it exhibits a partial triple response phenotype due to its responsiveness to endogenous ethylene levels in the seedling [ 22].
It contains a Constitutive Triple Response 1 (CTR1) domain (Additional file 2: Figure S1), confirming it as an ortholog of the CTR1 gene: a potential-MAP3K that in A. thaliana is negatively regulated by the ethylene responsive histidine kinase ETR1 [ 97].
A constitutive ethylene triple response is achieved by expressing putative C-terminal EIN2-cleaved fragments [ 45].
Among all of the described phytohormones, ethylene, a naturally occurring triple response growth regulator (shoot elongation, stem thickening and horizontal growth habit) in seedlings, has been studied since ancient times [ 6].
By exploiting the triple response in a genetic screen, the ethylene-insensitive etr1-1 mutant was isolated, which led to the identification of the receptor ETR1 (Bleecker et al. 1988; Chang et al. 1993; Schaller et al. 1995).
Moxon et al. [ 19] validated that a member of the constitutive triple response (CTR) family is involved in tomato fruit ripening, which is the target of the novel miRNA, and a higher expression pattern of miR390 was found in small fruit than in leaves and flowers.
The A. thaliana Raf-like members, Enhanced Disease Resistance 1 (EDR1) and Constitutive Triple Response 1 (CTR1), shared a homology with mammalian Raf-like MAPKKKs, were reported to participate in ethylene-mediated signaling and defense responses [ 23- 26].
