The plant hormone gibberellin was first identified in rice and appears to play an essential function in determining plant stature in a large number of plants (1)

In Arabidopsis, severe mutations in genes acting early in GA biosynthesis (GA1, GA2, GA3) display a number of GA-rescued phenotypes (2). These include, failure to germinate, growth of the plant as a dark green dwarf, underdeveloped petals and stamen accompanied by reduced fertility, an increased number of buds per inflorescence, delayed flowering, reduced apical dominance, and delayed senescence.

In principle, GA-insensitive mutants should reflect the spectrum of phenotypes seen in severe GA biosynthetic mutants, but fail to be rescued by GA. A number of Arabidopsis mutants with decreased GA signal transduction has been described including a transcriptional activator (gai1) and a yet unidentified gene (sly1) Genetic studies of GA response in Arabidopsis have also used mutations causing increased GA signal transduction. These include a glycosyl transferase (spy1) and a gai-like transcription factor (rga1).

 

References

1. Hooley R.1994. Gibberellins: perception, transduction and responses. Plant Mol. Biol. 26:1529-1555.

2. Koornneef M, van der Veen. 1980. Induction and analysis of gibberellin sensitive mutants of Arabidopsis thaliana (l.) Heynh. Theor. Appl. Genet. 58:257-263


A semidominant gai1-1 mutation causes Arabidopsis plants to grow as dark green semi dwarves, with reduced fertility and germination (1). Although these phenotypes are similar to GA -deficient mutants, the lack of rescue of gai1-1 by GA application suggests this mutation results in reduced GA responsiveness. Intrallellic suppressor mutations of gai1-1 have been identified and the only prevalent phenotype of these mutants is their increased insensitively to the GA biosynthetic inhibitor paclobutrazol. The GAI1 protein has sequence identity to a number of known transcriptional activators, and the semidominant gai1-1 allele contains an inframe 51 base pair insertion mutation. One possible model of GAI1 action is GAI1 negatively regulates cell elongation and GA inhibits this function. Interestingly, many of the dwarf phenotypes developed in breeding programs for the green revolution of crop food production are gai -like mutants (3).

References

1. Koornneef M, van der Veen. 1980. Induction and analysis of gibberellin sensitive mutants of Arabidopsis thaliana (l.) Heynh. Theor. Appl. Genet. 58:257-263.

2. Peng J, Carol P, Richards DE, King KE, Cowling RJ, et al. 1997. The Arabidopsis GAI gene defines a signaling pathway that negatively regulates gibberellin responses. Genes Dev. 11:3194-3205.

3. Peng J, Richards DE, Hartley et al. 1999. 'Green revolution' genes encode mutant gibberellin response modulators. Nature 6741, 256-261.


Recessive rga1 mutations were identified by a suppressor screen of an Arabidopsis GA biosynthetic mutation ga1-3 (1). Mutations in the rga1 gene restore most of the vegetative phenotypes associated with the GA auxotrophy of ga1-3 but do not suppress the germination defect. In wild-type plants rga1 mutants are slightly etiolated. The RGA1 gene shows high sequence similarity to GAI1 and this molecular redundancy may explain why single loss-of-function alleles of GAI1 and RGA 1 are phenotypically subtle (2).

 

References

1. Silverstone AL, Mak PY, Martinez EC, Sun TP. 1997. The new RGA locus encodes a negative regulator of gibberellin response in Arabidopsis thaliana. Genetics 146:1087-1099

2. Silverstone AL, Ciampaglio CN, Sun T-p. 1998. The Arabidopsis RGA gene encodes a transcriptional regulator repressing the gibberellin signal transduction pathway. Plant Cell 10: 155-169


The first spindly1 (spy1) mutations were identified by the ability of Arabidopsis seed to germinate in the presence of inhibitory concentrations of a GA biosynthetic inhibitor (1). Recessive mutations at the spy1 locus result in partial suppression of all defects associated with GA auxotrophy including germination. Moreover, unlike loss-of-function rga1 and gai1 alleles, spy1 mutations in a wild-type genetic background have a reduced GA requirement for germination, an increased internode length and a floral timing defect. The recessive nature of the spy1 alleles suggests this gene encodes a negative regulator of GA action. The SPY1 gene shows sequence similarity to SER (Thr)-O-linked acetylglusamine (O-GlcNAc) transferases. These enzymes glycosylate proteins and in some cases, this modification can interferes with phosphorylation of proteins (2). Experiments carried out with the barley homolog of SPY1 (HvSPY1) suggest this gene is not only involved in GA signaling but may also be involved enhancing ABA response in barley aleurone cells. That spy1 mutants are slightly insensitive to ABA and suppress era1 defective germination supports this contention.

References

1. Jacobsen SE, Olszewski NE. 1995. Mutations at the SPINDLY locus of Arabidopsis alter gibberellin signal transduction. Plant Cell 5:887-896.

2 Jacobsen SE, Binkowski KA, Olszewski NE. 1996. SPINDLY, a tetratricopeptide repeat protein involved in gibberellin signal transduction in Arabidopsis. Proc. Natl. Acad. Sci. U S A 93:9292-9296.


Genetic screens at the level of germination for suppressor mutations of the dominant insensitive mutation abi1-1 enrich for mutations that are defective in GA synthesis or response (1). The GA non-responsive mutants are phenotypically similar to GA auxotrophs and define one complementation group designated sleepy1 (sly1). These are the first recessive GA-insensitive mutants reported in Arabidopsis. That mutants defective in GA biosynthesis or response have increased ABA responsiveness at the level of germination is the opposite to the observation that suppression of GA auxotrophy can allow the identification of ABA response and biosynthetic mutants. These results suggest GA and ABA combine to determine the overall dormancy of the seed in a type of push and pull fashion with GA encouraging germination while ABA encourages dormancy. Recently we have discovered that the germination defects of sly1 and ga auxotrophs can partially suppressed by the addition of brassinosteroids (2). Therefore, brassinosteroids application could be used in principle to identify more GA responsive mutants.

References:

1. Steber CM, Cooney SE, McCourt P. 1998. Isolation of the GA-response mutant sly1 as a suppressor of ABI1-1 in Arabidopsis thaliana. Genetics 149:509-521.

2. Steber CM, McCourt P, 2001 A role for brassinosteroids in germination in Arabidopsis thaliana. Plant Physiol. 125: 763-769