Phytochrome regulates gibberellin biosynthesis during germination of photoblastic lettuce seeds

Germination of lettuce (Lactuca sativa L.) seed is regulated by phytochrome. The requirement for red light is circumvented by the application of gibberellin (CA). We have previously shown that the endogenous content of GA(1), the main bioactive GA in lettuce seeds, increases after red-light treatment. To clarify which step of GA, synthesis is regulated by phytochrome, cDNAs encoding CA 20-oxidases (Ls20ox1 and Ls20ox2, for L. sativa CA 20-oxidase) and 3 beta-hydroxylases (Ls3h1 and Ls3h2 for L. sativa GA beta-hydroxylase) were isolated from lettuce seeds by reverse-transcription polymerase chain reaction. Functional analysis of recombinant proteins expressed in Escherichia coli confirmed that the Ls20ox and Ls3h encode GA 20-oxidases and 3 beta-hydroxylases, respectively. Northern-blot analysis showed that Ls3h1 expression was dramatically induced by red-light treatment within 2 h, and that this effect was canceled by a subsequent far-red-light treatment. Ls3h2 mRNA was not detected in seeds that had been allowed to imbibe under any light conditions. Expression of the two Ls20ox genes was induced by initial imbibition alone in the dark. The level of Ls20ox2 mRNA decreased after the red-light treatment, whereas that of Ls20ox1 was unaffected by light. These results suggest that red light promotes CA, synthesis in lettuce seeds by inducing Ls3h1 expression via phytochrome action.