Abscisic acid (ABA) is an sesquiterpene produced in plants that has been shown to mediate arrest of growth and development in processes such as control of stomatal aperture in leaves and the establishment of seed dormancy (1,2).

In Arabidopsis, an allelic series of mutations that reduce endogenous ABA levels demonstrates that seed dormancy correlates directly with the level of ABA synthesized by the embryo (3).

Genetic screens to identify ABA response genes in Arabidopsis have identified two protein phosphatases (ABI1, ABI2), a farnesyltransferase (ERA1), a novel protein (ERA3/EIN2)and two transcription factors (ABI3, ABI4). These results indicate that multiple levels of dormancy can be attained in the seed, which in turn suggests that the cellular response to ABA during the establishment of dormancy may be controlled at multiple levels.

 

References

1. Leung, J., Giraudat J. 1998. Abscisic acid signal transduction Ann. Rev. Plant Physiol. Plant Mol Biol. 49, 199-122.

2. Bonetta D. McCourt P. 1998. Genetic analysis of ABA signal transduction pathways. Trends Plant Sci. 3:231-235.

3. Karssen, CM, Brinkhorst-van der Swan DLC, Breekland AE, Koornneef M. 1983 Induction of dormancy during seed development by endogenous abscisic acid: Studies on abscisic acid deficient genotypes of Arabidopsis thaliana (L.) Heynh. Planta 157:158-165


ABA insensitive mutants of Arabidopsis were identified by their ability to germinate on concentrations of ABA that normally inhibit wild-type germination (1). Two ABA insensitive loci, designated abi1 and abi2 encode homologous protein type 2C phosphatases (2,3,4). This suggests that protein phosphorylation and dephosphorylation are involved in ABA signaling. To date, only one dominant allele of each of these genes confers an ABA insensitive phenotype. Biochemical characterization of abi1-1 protein suggests this mutation acts as a dominant-negative protein (5). The functional and molecular redundancy of these genes may explain why mutations in either gene must be dominant to confer ABA insensitivity.

abi1 suppressor screens

abi1 enhancer screens

 

References

1. Koornneef M, Reuling G,Karssen, CM. 1984. The isolation and characterization of abscisic acid-insensitive mutants of Arabidopsis thaliana., Physiol. Plant. 61:377-383.

2. Leung J, Bouvier-Durand M, Morris PC, Guerrier D, Chefdor F, Giraudat J. 1994. Arabidopsis ABA-response gene ABI1: features of a calcium-modulated protein phosphatase. Science 264:1448-1452.

3. Meyer K, Leube MP, Grill E. 1994. A Protein Phosphatase 2C involved in ABA signal transduction in Arabidopsis thaliana. Science 264, 1452-1455.

4. Leung J, Merlot S, Giraudat J. 1997. The Arabidopsis ABSCISIC ACID-INSENSITIVE2 (ABI2) and ABI1 genes encode homologous protein phosphatases 2C involved in abscisic acid signal transduction. Plant Cell 9:759-771.

5. Bertauche N, Leung J, Giraudat J. 1996. Protein phosphatase activity of abscisic acid insensitive 1 (ABI1) protein from Arabidopsis thaliana. Eur. J. Biochem. 241:193-200.


 

Recessive mutations in the ABI3 (ABA insensitive) gene allow the seed to germinate on exogenous ABA (1). However, abi3 seed also have other phenotypes in that they are unable to complete late embryogenesis and are desiccation intolerant (2). Furthermore, severe alleles of abi3 differ from dominant abi1 and abi2 because these mutant phenotypes are restricted to late seed maturation resulting in a green seed phenotype. Therefore, this gene may mark a developmental branch in ABA signal transduction.

The ABI3 gene appears to encodes a seed specific transcription factor whose expression patterns when first studied mostly reflects the abi3 mutant phenotypes (3). Recently it seems ABI3 may have functions outside of the seed but it is not clear if these functions relate to ABA signaling (4). Misexpression of ABI3 causes seed-specific mRNA transcripts to accumulate in vegetative tissues (5). These results suggest ABI3 may encode a developmental regulator that is necessary for correct implementation of seed ABA signaling (6).

References

1. Koornneef M, Reuling G,Karssen, CM. 1984. The isolation and characterization of abscisic acid-insensitive mutants of Arabidopsis thaliana., Physiol. Plant. 61:377-383.

2. Nambara E, Keith K, McCourt P, Naito S. 1995. A regulatory role for the ABI3 gene in the establishment of embryo maturation in Arabidopsis thaliana. Development 121:629-636.

3. Giraudat J, Hauge BM, Valon C, Smalle J, Parcy F, Goodman HM. 1992. Isolation of the Arabidopsis ABI3 gene by positional cloning. Plant Cell 4:1251-1261.

4. Rohde A, De Rycke R, Beeckman T, Engler G, Van Montagu M, Boerjan W. 1999. ABI3 Affects Plastid Differentiation in Dark-Grown Arabidopsis Seedlings. Plant Cell 12, 35-52.

5. Bonetta D. McCourt P. 1998. Genetic analysis of ABA signal transduction pathways. Trends Plant Sci. 3:231-235.

6. Parcy F, Valon C, Raynal M, Gaubier-Comella P, Delseny M, Giraudat J. 1994. Regulation of gene expression programs during Arabidopsis seed development: roles of the ABI3 locus and of endogenous abscisic acid. Plant Cell 6:1567-1582.


Similar to abi3 mutants, abi4 mutants have pleiotropic defects in seed development, including decreased sensitivity to ABA inhibition of germination and altered seed-specific gene expression (1). This mutation, however, does not confer a reduced desiccation tolerance or an inability to complete late embryogenesis as is seen in severe abi3 alleles. The ABI4 gene shows sequence similarity to a plant-specific family of transcriptional regulators characterized by a conserved DNA binding domain, the APETALA2 domain (2). Interestingly, the expression of this gene is not limited to seed development. Furthermore a number of other genetic screens including increased salt tolerance and sugar sensing mutants identify new abi4 alleles (3).

References

1. Finkelstein RR. 1994. Mutations at two new Arabidopsis ABA response loci are similar to the abi3 mutations. Plant J. 5:765-771

2. Finkelstein RR, Wang ML, Lynch TJ, Rao S, Goodman HM. (1998). The Arabidopsis abscisic acid response locus ABI4 encodes an APETALA 2 domain protein. Plant Cell 6, 1043-1054.

3. Quesada V, Ponce MR, Micol JL. (1999). Genetic Analysis of Salt-Tolerant Mutants in Arabidopsis thaliana. Genetics 154,421-436.


 

Exploiting the dosage sensitivity of exogenous ABA on Arabidopsis germination, we isolated a collection of mutations, designated enhanced response to ABA (era) which, unlike the insensitive abi mutants, show enhanced sensitivity to ABA (ref 1; era1 screens). Molecular cloning of one of the era loci, ERA1, has shown this gene encodes the b-subunit of a heterodimeric protein farnesyl transferase. Protein farnesylation involves the attachment of a farnesyl (15-carbon) isoprenoid to a select group of molecules thereby facilitating protein interaction with membrane lipids and/or other proteins (2). The era1 mutations are loss-of-function suggesting a negative regulator of ABA signaling must be farnesylated to function in the seed. Studies in collaboration with Dr. Julian Schroeder at the University of California San Diego have shown the era1 mutation results in ABA hypersensitivity of guard cell anion activation and of stomatal closing (3).The era1 mutations not only affect ABA-related processes such as water relations and seed dormancy but also show unexpected phenotypes. When grown under day/night conditions, the apical meristems of the era1 plants become fasciated due to enlargement of the meristematic dome. era1 mutants also show reduced initiation of axillary meristems, a reduction in lateral branches and reduced fertility.

Two hypotheses can explain these novel phenotypes (4). Possibly both ABA-dependent and ABA-independent farnesylated targets exist and, therefore, loss of ERA1 function affects multiple signaling pathways. Alternatively, ABA may impinge on a number of developmental pathways, thus, the altered ABA sensitivity causes the pleiotrophy. The Arabidopsis genome contains over 600 potential targets for farnesylation (5). This suggests the pleiotrophy of era1 mutants is because many farnesylated gene products are affected. To test this hypothesis suppressor mutations of era1 defects are been identified .in order to determine which targets function in which processes.

era1 screens

era1 suppressor screens

1. Cutler, S. Ghassemian, M. Bonetta, D. Cooney, S. McCourt, P. (1996). A protein farnesyl transferase involved in abscisic acid signal transduction in Arabidopsis. Science 273, 1239-1241.

2. Zhang, F. L. Casey, P. J. (1996). Protein prenylation: molecular mechanisms and functional consequences. Annu. Rev. Biochem. 65, 241-269.

3. Pei, Z. M. Ghassemian, M. Kwak, C. M. McCourt, P. Schroeder, J. I. (1998). Role of farnesyltransferase in ABA regulation of guard cell anion channels and plant water loss. Science 282, 287-290.

4.Bonetta, D, Bayliss, P, Sun, S., SageT., McCourt, P. (2000) Farnesylation is involved in meristem organization in Arabidopsis. Planta, 211:182-190

5. Nambara E. McCourt P. (1999) Protein farnesylation in plants: a greasy tale. Curr. Opin. Plant Biol. 2, 388-392.