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 GA19 (a precursor of the bioactive GA1) as comparable tissue from intact plants, even though applied [14C]GA19 was metabolised rapidly during this time. These results show that the pool size of endogenous GA19 was maintained, probably by de novo GA19 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 GA1 and GA20 compared with immature tissue. Levels of GA19, GA29 and GA8were less affected by tissue age. Metabolism studies using 14C-labelled GAs indicated that mature tissue rapidly converted GA19 to GA20 and GA20 to GA1; the latter step was promoted by IAA. However, the 2-oxidation steps GA1 to GA8, GA29 to GA29 and GA29 to GA29-catabolite appear to proceed very rapidly in mature tissue (regardless of IAA content), and we suggest this is the reason why GA1 and GA20 do not accumulate. This is supported by the high level of expression of a key GA 2-oxidase gene in mature tissue.