Genes Specifying Auxin and Cytokinin Biosynthesis in Prokaryotes

Abstract

Much effort has been expended to understand the biosynthesis and mode of action of the auxins and cytokinins, with only moderate success. For example, although cytokinin structures, internal distribution and transport within plants are documented (64,37), cytokinin catabolism is well understood (47), and the morphological and physiological consequences of cytokinin application have been described (37), we still do not know fully how they are synthesized. Putative plant cytokinin biosynthetic enzymes have been observed (14,13), but none has been purified to homogeneity. Further, we do not know when and where such enzymes are expressed in the plant or even if they are responsible for plant cytokinin synthesis in vivo. A longstanding debate continues (7; see Chapter B3) as to whether cytokinins are synthesized directly or via an indirect pathway involving tRNA catabolism. Plant genes that encode biosynthetic enzymes for both pathways remain to be isolated. A similar situation obtains for auxins.