Developmental regulation of the gibberellin pathway in pea shoots

To investigate gibberellin (GA) biosynthesis in mature tissue of pea (Pisum sativum L.) in the absence of potentially GA-producing meristematic tissue we grafted wild-type scions to rootstocks of the GA-deficient ls-1 mutant and later decapitated the shoot. After 2 d, decapitated shoots contained as much GA(19) (a precursor of the bioactive GA(1)) as comparable tissue from intact plants, even though applied [C-14]GA(19) was metabolised rapidly during this time. These results show that the pool size of endogenous GA(19) was maintained, probably by de novo GA(19) synthesis. We also found that the LS gene, which catalyses an early step in GA biosynthesis, is expressed in mature tissue, as are the shoot-expressed GA 20-oxidase and GA 3-oxidase genes. Nevertheless, mature tissue contained very low levels of GA(1) and GA(20) compared with immature tissue. Levels of GA(19), GA(29) and GA(8) were less affected by tissue age. Metabolism studies using C-14-labelled GAs indicated that mature tissue rapidly converted GA(19) to GA(20) and GA(20) to GA(1); the latter step was promoted by IAA. However, the 2-oxidation steps GA(1) to GA(8), GA(20) to GA(29) and GA(29) to GA(29)-catabolite appear to proceed very rapidly in mature tissue (regardless of IAA content), and we suggest this is the reason why GA(1) and GA(20) do not accumulate. This is supported by the high level of expression of a key GA 2-oxidase gene in mature tissue.