The gaseous hormone ethylene is a two-carbon olefin that elicits a broad range of effects on plant growth and development (1). Ethylene protects the plant from a number of environmental stresses ranging from flooding to pathogen attack. It also can strengthen stems, allows fruits to ripen and causes senescence. From a genetic standpoint, exposure of dark grown germinating seedlings to ethylene causes radial swelling of the hypocotyl and inhibition of root and hypocotyl growth. Collectively known as the triple response, the reproducibility of this response has allowed geneticists to screen for mutants that show an altered triple response in the presence or absence of the gas (2). Screening has involved identifying insensitive mutants that do not show a triple response in the presence of ethylene (ETR1, ETR2, EIN4, EIN2, EIN3, EIN5, EIN6, EIN7) or identifying mutants that show a constitutive triple response in the absence of the hormone (CTR1, ETO1, ETO2).

 

References

1. Abeles, F. Morgan, P., Saltveit M., 1992. Ethylene in Plant Biology, San Diego: Academic

2. Johnson, PR, Ecker JR., 1998 The ethylene gas signal transduction pathway: a molecular perspective. Ann Rev. Genet. 32, 227-254.


 

 

Recessive ctr1 mutants of Arabidopsis display the ethylene triple response in dark grown seedlings and constitutive expression of ethylene-regulated genes in the absence of ethylene (1). Mutants are compact dwarves with small roots that ectopically express root hairs. Based on genetic studies it appears the CTR1 gene product acts as a negative regulator of ethylene signaling and as a consequence loss-of -function mutants confer an ethylene constitutive phenotype. The amino acid sequence of CTR1 closely resembles the Raf kinase family of mammalian protein kinases (2). The amino terminal end of CTR1, however, diverges suggesting this protein may have other novel functions in plants. It appears CTR1 and ETR1 interact in a yeast two hybrid assay so these two proteins may interact in plants (3)

References

1. Roman G, Lubarsky B, Kieber JJ, Rothenberg M, Ecker JR. 1995. Genetic analysis of ethylene signal transduction in Arabidopsis thaliana: five novel mutant loci integrated into a stress response pathway. Genetics 139:1393-1409.

2. Kieber JJ, Rothenberg M, Roman G, Feldmann KA, Ecker JR. 1993.CTR1, a negative regulator of the ethylene response pathway in Arabidopsis, encodes a member of the raf family of protein kinases. Cell 72:427-441.

3. Clark KL, Larsen PB, Wang X, Chang C. 1998. Association of the Arabidopsis CTR1 Raf-like kinase with the ETR1 and ERS ethylene receptors. Proc. Natl. Acad. Sci. U S A 95:5401-5406.


Dominant mutations in the ETR1 gene were identified by the inability of these mutants to perform the triple response in the presence of ethylene (1). Other phenotypes associated with etr1 are poor germination, decreased senescence of detached leaves and reduced ethylene-induced gene expression in vegetative tissue. The ETR1 gene encodes a protein with sequence similarity to bacterial two component histidine kinases (2). Biochemical studies on ETR1 suggest this gene binds ethylene and is therefore an ethylene receptor (3). To date four other ETR1 like genes have been cloned from Arabidopsis (ETR2, EIN4, ERS1, ERS2), and mutations in a number of these confer an ethylene insensitive phenotype (4). Based on loss of function mutants in these same four genes it appears ethylene binds ETR1-like genes and inactivates their function which is to positively regulate CTR1 (5)

References

1. Bleecker AB, Estelle MA, Somerville C, Kende H. 1988. Insensitivity to ethylene conferred by a dominant mutation in Arabidopsis thaliana. Science 241:1086-1089.

2. Chang C, Kwok SF, Bleecker AB, Meyerowitz EM. 1993. Arabidopsis ethylene-response gene ETR1: similarity of product to two-component regulators. Science 262:539-544.

3. Schaller GE, Bleecker AB. 1995. Ethylene-binding sites generated in yeast expressing the Arabidopsis ETR1 gene. Science 270:1809-1811.

4. Kieber JJ. 1997. The ethylene response pathway in Arabidopsis. Ann. Rev. Plant Physiol. Plant Mol. Biol. 48:277-296.

5. Hua J, Meyerowitz EM. 1998. Ethylene responses are negatively regulated by a receptor gene family in Arabidopsis thaliana. Cell 94,261-271.


Recessive mutations in the Arabidopsis EIN2 locus result in a strong insensitivity to ethylene and this is seen at the morphological, physiological and molecular levels (1). That loss-of-function ein2 mutants are the strongest ethylene insensitive mutants identified so far suggests this gene is a key positive regulator of ethylene responses. The EIN2 gene product is novel and weak homology to the integral membrane domain of the Nramp family of metal transporters (2). Expression of the carboxyl-terminus of EIN2 is sufficient to constitutively activate responses. Genetic analysis places EIN2 downstream of CTR1 and upstream of EIN3 (3). Although ein2 mutants are ethylene insensitive, other genetic screens do uncover ein2 mutations. In screening for enhanced response to ABA mutants (era) we identified two new alleles of ein2 which we had designated era3 (4). Interestingly although ein2/era3 alleles are supersensitive to ABA at the level of germination these alleles are insensitive to ABA at the level of root growth. EIN2/ERA3 mutants are also identified as suppressor of abi1-1 at the level of germination. Furthermore ctr1 mutants enhance ABA insensitivity of abi1-1. Subsequent analysis of this ABA/ethylene interaction suggests in the absence of ethylene, ABA can signal through the ethylene response pathway.

References

1. Guzman P, Ecker JR. 1990. Exploiting the triple response of Arabidopsis to identify ethylene-related mutants. Plant Cell 2:513-523.

2. Alonso JM, Hirayama T, Roman G, Nourizadeh S, Ecker JR. 1999. EIN2, a bifunctional transducer of ethylene and stress responses in Arabidopsis. Science 284, 2148-2152.

3. Roman G, Lubarsky B, Kieber JJ, Rothenberg M, Ecker JR. 1995. Genetic analysis of ethylene signal transduction in Arabidopsis thaliana: five novel mutant loci integrated into a stress response pathway. Genetics 139:1393-1409

4. M. Ghassemian, E. Nambara, S. Cutler, H. Kawaide, Y. Kamiya P. McCourt (2000) Regulation of abscisic acid signaling by the ethylene response pathway in Arabidopsis. Plant Cell 12,1117-26.


Recessive mutations in the ein3 gene of Arabidopsis have similar phenotypes to dominant mutations in etr1 in that these mutants show a reduced response to ethylene (1). The EIN3 gene encodes a novel nuclear-localized protein with similarity to known eukaryotic transcriptional activators (2). All ein3 alleles sequenced so far suggest this gene is a positive regulator of the ethylene response. Furthermore, over expression of ein3 causes a constitutive ethylene response similar to ctr1.

References

1. Roman G, Lubarsky B, Kieber JJ, Rothenberg M, Ecker JR. 1995. Genetic analysis of ethylene signal transduction in Arabidopsis thaliana: five novel mutant loci integrated into a stress response pathway. Genetics 139:1393-1409.

2. Chao Q, Rothenberg M, Solano R, Roman G, Terzaghi W, Ecker JR. 1998. Activation of the ethylene gas response pathway in Arabidopsis by the nuclear protein ETHYLENE-INSENSITIVE3 and related proteins. Cell 89:1133-1144.