The relationship between expression of a negative regulator of GA signal transduction (RGL2) belonging to the DELLA gene family and repression of Arabidopsis seed germination has been studied (Lee S, Cheng H, King KE, Wang W, He Y, Hussain A, Lo J, Harberd NP, Peng J [2002] Genes and Development 16: 646-658). There is one DELLA gene (LeGAI) present in tomato (Lycopersicon esculentum Mill.), which is expressed in both vegetative and reproductive tissues. During germination of wild-type tomato seed, there was no decline in the expression of LeGAI in either the embryo or the endosperm. Rather, LeGAI transcripts increased in these tissues following imbibition and remained high during and following germination. A similar increase in LeGAI transcripts occurred in the endosperm and embryo of GA-treated gib-1 mutant seed during and following germination. Likewise in soybean (Glycine max) seed, there was no decline in the expression of two DELLA genes in the radicle before or after germination. Upon reexamination of RGL2 in Arabidopsis seeds, a decline in its expression was noted but only after radicle emergence, i.e. after germination had been completed. Taken together, these data are consistent with GA-induced down-regulation of DELLA genes not being a prerequisite for germination of tomato, soybean, and Arabidopsis seeds.

The procera (pro) mutant of tomato exhibits a well-characterized constitutive gibberellic acid (GA) response phenotype. The tomato DELLA gene LeGAI in the pro mutant background contains a point mutation that results in an amino acid change in the conserved VHVID putative DNA-binding domain in LeGAI to VHEID. This same point mutation is in four different genetic backgrounds exhibiting the pro phenotype, suggesting that this mutation co-segregates with the pro phenotype. Complementation of the mutant with a constitutively expressed wild-type LeGAI gene sequence was not conclusive due to the infertility of transgenic plants. The pro mutation alters tomato branching architecture through differential suppression of axillary bud development, indicating a role for DELLA proteins in the regulation of plant structure. Isolated gib-1 pro double mutant embryo axes, which are unable to synthesize GA, germinate faster than their wild-type counterparts, and exert greater embryo growth potential. The pro mutation is therefore regulating GA responses within the tomato embryo. Transient expression of a LeGAI-GFP (green fluorescent protein) fusion protein in onion epidermis results in its location to the nucleus, and this protein is rapidly degraded by the proteasome in the presence of GA.