Genes encoding GA20-oxidases have been found in many plant species, reviewed by Hedden and Kamiya (1997). Deficiency of this enzyme from the GA20-ox2 gene leads to dwarfism in 'green revolution' rice (IR8) (Sasaki et al., 2002). It is most highly expressed in mature leaves and so appears to be primarily involved in vegetative growth (Hedden, 2003). However, GA20-ox1 is preferentially expressed in the reproductive organs (Sasaki et al., 2002). In Arabidopsis, overproduction of GA20-oxidase led to a decrease in seed dormancy, increased stem elongation, earlier flowering and an increase in seed set (Huang et al., 1998), possibly resulting from elevated levels in GA. In sorghum, expression was suppressed by ABA (Pérez-Flores et al., 2003). GA20-oxidase was also identified as a candidate gene controlling the preharvest sprouting QTL on barley (Hordeum vulgare L.) chromosome 5H (Li et al., 2004). When Calvo et al. (2004) isolated GA20-ox1 from F. sylvatica, expression was undetectable in dry dormant seeds, low when imbibed under conditions that would break dormancy (i.e. stratified at 4°C or imbibed with GA), but significantly higher after treatments that inhibited germination - exposure to paclobutrazol (a GA3 inhibitor) or amino oxyacetic acid (an ethylene biosynthesis inhibitor). Further treatments indicated that there was an interaction with gibberellins and ethylene (Calvo et al., 2004).
On the other hand, the 'green revolution' gene from wheat, reduced height (Rht), is an allele associated with the GA signalling pathway via a different mechanism, which causes a reduced response to GA. It encodes a mutant form of a DELLA protein, a GA signalling repressor and so also causes GA malfunction (Peng et al., 1999; Hedden, 2003; DELLA is an acronym of the single letter codes of five amino acids conserved in non-mutant 'DELLA' proteins). There are many Rht orthologues (e.g. gibberellin insensitive (GAI), repressor-of-GA (RGA), RGA-like1-2 (RGL1-2), slender1 (SLN1); Olszewski et al., 2002). By studying various mutants, RGL2 from Arabidopsis was found to be important in regulating seed germination and was enhanced by GAI, RGL1 and RGA (Lee et al., 2002). This is backed up by Tyler et al. (2004), who concluded that RGL2 is the most important protein controlling seed germination. It is suggested that DELLA proteins regulate the interaction of seed germination and the environment (Lee et al., 2002).
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