Cytokinins in higher plants: biosynthesis and interaction with auxins

Cytokinin biosynthesis and cytokinin/auxin interactions were studied in wildtype and hormone-overproducing plants. To enable the physiological studies to be performed, analytical tools based on liquid chromatography/ mass spectrometry (LC/MS) were developed, allowing cytokinins in minute amounts of plant tissue to be analysed.A method for in-vivo deuterium labelling of cytokinins provided evidence for the existence of an alternative, iPAMP-independent pathway for cytokinin biosynthesis in plants. The alternative pathway was active both in wildtype plants and in plants expressing the isopentenyltransferase (ipt)-gene from Agrobacterium tumefaciens. In the ipt-expressing, cytokinin overproducing plants, the rate of ZRMP production was found to be 66 times higher than that of iPAMP formation. This indicates that iPAMP is not the major precursor of ZRMP in the bacterial pathway.IAA biosynthesis was also studied, in tobacco, to determine the relative importance of tryptophan-dependent and tryptopan-independent biosynthesis. From tracer studies, it was concluded that the independent pathway contributed the majority of the IAA synthesised during vegetative growth.​​​​​​​In another series of experiments, cytokinin/auxin interactions in hormoneoverproducing plants were studied. Cytokinin overproduction was shown to decrease the rate of auxin biosynthesis, resulting in lower levels of IAA. Likewise, lower cytokinin levels were found in plants that overproduced auxins. In order to investigate the interactions further, cytokinin and auxin overproducing plants were crossed. The offspring expressed all the phenotypic traits associated with both of the parental lines, giving rise to a mixed phenotype. Surprisingly, did not the hormone levels of the cross significantly differ from wildtype plants. Thus, were the increased hormone levels of the parental lines downregulated in the cross but the plant line did still express the phenotypical traits associated with auxin- and cytokinin-overproduction