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 GA₈ were 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, GA₂₀ 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.