Leung (CNRS, Gif-sur-Yvette) described functional studies of ABI1, previously cloned by chromosome walking and shown to be a member of the protein phosphatase IIC family (Leung et al 1994; Meyer et al. 1994). The dominant abi1-1 allele was introduced into Nicotiana benthamiana and shown to disrupt ABA-regulation of guard cell potassium channels, producing a wilty phenotype similar to that seen in the Arabidopsis mutant. The defect in guard cell response could be partially overcome by exposure to protein kinase inhibitors, supporting the view that this ABA signaling pathway involves a protein phosphorylation cascade.
Cutler (recently of U Toronto) presented a characterization of the first Arabidopsis mutant with enhanced response to ABA (era1), also isolated by a germination-based screen and exhibiting seed-specific effects. The ERA1 gene was cloned by virtue of having a T-DNA tagged allele; sequence analysis of genomic and cDNA clones showed it to encode a beta-subunit of a protein farnesyl transferase. Farnesylation is known to be important to membrane localization and/or function of RAS and trimeric G proteins, consistent with a role for G-proteins or some other membrane-anchored signaling component in ABA response.
GA signaling was discussed by Harberd (John Innes Centre, Norwich) and Jacobsen (recently of U Minnesota). The semi-dominant gai mutation of Arabidopsis confers gibberellin (GA)-insensitive dwarfism and identifies a gene (GAI) whose product is likely to be involved in GA-related signal transduction (Koornneef et al. 1985). Harberd's group has isolated both intra- and extragenic (gas1 and gas2) suppressors of the gai dwarf phenotype. Some of the intragenic suppressors were created by insertion of a mobilized linked Ds element, thereby tagging the GAI locus; DNA sequencing of GAI, gai and associated cDNAs is in progress. Singly, the gas mutants only partially suppress the gai phenotype, but the triple homozygote (gas1, gas2, gai) has wild-type height. The gas1-1 mutation appears to reduce the dependency of plant growth on GA since, although growing taller than the gai progenitor, the gai gas1-1 double mutant does not display enhanced GA responsiveness. In contrast to this suppressor screening, the SPINDLY (SPY) locus was identified by screening for germination/elongation in the presence of the GA biosynthesis inhibitor paclobutrazol (Jacobsen et al. 1993). The spy-4 allele was created by T-DNA insertion, facilitating cloning of the locus. Sequence analysis shows significant homology to a gene of unknown function from C. elegans, leading Jacobsen et al. to suggest that these are members of a family of regulatory genes.
Finkelstein, R.R. 1994. Mutations at two new Arabidopsis ABA response loci are similar to the abi3 mutations. The Plant J. 5, 765-771.
Finkelstein, R., T. Lynch, D. Zarek. 1995. Genetic analysis of abscisic acid response. Sixth International Conference on Arabidopsis research, Madison, WI.
Grill, E., T. Ehrler, K. Meyer, and M. Leube. 1993. Steps of abscisic acid action. Fifth International Conference on Arabidopsis Research, Columbus, Ohio.
Jacobsen, S.E. and N.E. Olszewski. 1993. Mutations at the SPINDLY locus of Arabidopsis alter gibberellin signal transduction. Plant Cell 5, 887-896
Koornneef, M., G. Reuling, and C.M. Karssen. 1984. The isolation and characterization of abscisic acid-insensitive mutants of Arabidopsis thaliana. Physiol. Plant. 61, 377-383.
Koornneef, M., A. Elgersma, C.J. Hanhart, E.P. van Loenen-Martinet, L. van Rijn, and J.A.D. Zeevaart. 1985. A gibberellin insensitive mutant of Arabidopsis thaliana. Physiol. Plant. 65, 33-39.
Leung, J., M. Bouvier-Durand, P.-C. Morris, D. Guerrier, F. Chefdor, and J. Giraudat. 1994. Arabidopsis ABA response gene ABI1: Features of a calcium-modulated protein phosphatase. Science. 264, 1448-1452.
Meyer, K., M. Leube, and E. Grill. 1994. A protein phosphatase 2C involved in ABA signal transduction in Arabidopsis thaliana. Science. 264, 1452-1455.