Rice Genetics Newsletter, Vol. 17

Rgn17

3.	Dwarfism and reduced responses to gibberellic acid in transgenic rice plants expressing the Arabidopsis GAI gene, and the dominant negative allele gai
	X. FU, D.E. RICHARDs, N.P. HARBERD and P. CHRISTOU John Innes Center, Colney Lane, Norwich NR4 7UH, UK

Gibberellins (GAs) are tetracyclic diterpenoid hormones essential for plant growth and development. Mutants deficient for GA synthesis, perception and signal transduction have been isolated from a number of species, and the analysis of GA mutants in Arabidopsis has significantly advanced knowledge of the GA signaling pathway (Richards et al. 2000). Isolation of the Arabidopsis gai (gibberellic acid insensitive) mutant led to the cloning of the GAI gene (Peng et al. 1997). This encodes a transcription factor of the GRAS family, named after its three principal members: GAI, RGA and SCARECROW. The gai mutation is dominant to the wild type allele, and similar mutations have been identified in wheat and maize, causing genetically dominant dwarfism and reductions in the response to GA (Richards et al. 2000).

We generated transgenic rice plants carrying either the Arabidopsis dominant gai mutant allele or the normal wild type allele, GAI. We have previously reported dwarfism and reduced GA responses in transgenic Basmati rice expressing the mutant gai allele at a high level (Peng et al. 1999). More recently, however, we have demonstrated that even plants expressing very low levels of gai show severe dwarf phenotypes and lack a response to exogenously-supplied GA. This suggests that gai acts in a dominant negative manner, superimposing its activity over that of the wild type allele.

Surprisingly, high level expression of the wild type allele, GAI, also causes dwarfism

and reduced GA responses in rice, and the strength of the effect is proportional to the level of gene expression. Figure 1 shows the response to exogenously supplied GA in rice plants expressing the Arabidopsis GAI gene at low and high levels, in each case compared to a nontransformed wild type rice plant. As can be clearly seen, the dwarf phenotype resulting from low level GAI expression can be rescued by GA, whereas that caused by high level GAI expression cannot.

Since the dominant loss of function allele gai causes dwarfism and a reduced GA growth response in rice, it may seem surprising that overexpressing the wild type allele GAI has a similar effect. Furthermore, the phenotype of the gai transgene cannot be rescued by exogenous GA, whereas that of the GAI transgene can be rescued as long as it is expressed at low levels. These results confirm a derepressible repressor model, in which the normal function of the GAI protein is to suppress plant growth. In normal development, the activity of GAI is antagonized by GA. In plants where GAI protein is expressed as a transgene, the increased activity can be suppressed by increasing the doses of GA, at least up to a certain point. High level expression of GAI is resistant to GA, perhaps because the GA signaling pathway becomes saturated and there remains an excess of the active GAI protein. Presumably, the dominant gai allele encodes a repressor that cannot be antagonized by GA, so that even relatively low levels of the protein succeed in stunting plant growth (Richards et al. 2000).

These results indicate that the mechanism of GAI activity has been conserved over millions of years of plant evolution. The dominant GA-resistant allele from Arabidopsis (gai) can impose its repressive activity on the growth response genes in rice, and high level expression of the wild type allele has a similar effect. The resulting dwarf phenotypes are similar to those seen in the maize and wheat 'green revolution' genotypes (Peng et al. 1999), and it is hoped that similar increases in grain yield, at the expense of straw biomass, can be achieved in rice.

References

Peng, J.R., P. Carol, D.E. Richards, K.E. King, R.J. Cowling, G.P. Murphy and N.P. Harberd, 1997. The Arabidopsis GAI gene defines a signaling pathway that negatively regulates gibberellin responses. Genes & Dev 11: 3194-3205.

Peng, J.R., D.E. Richards, N.M. Hartley, G.P. Murphy, K.M. Devos, J.E. Flitham, J. Beales, L.J. Fish, A.J. Worland, F. Pelica, D. Sudhakar, P. Christou, J.W. Snape, M.D. Gale and N.P. Harberd, 1999. "Green revolution" genes encode mutant gibberellin response modulators. Nature 400: 256-261.

Richards, D.E., J.R. Peng and N.P. Harberd, 2000. Plant GRAS and metazoan STATs: one family? Bioessays 22: 573-577.