Probing cytokinin homeostasis in Arabidopsis thaliana by constitutively overexpressing two forms of the maize cytokinin oxidase/dehydrogenase 1 gene

Engineering transgenic plants with reduced cytokinin (Ck) contents is a way to analyze the role of these hormones in growth and development. Cytokinin oxidase/dehydrogenase (CKO) genes are good candidates to promote Ck deficiency. They code for enzymes degrading Cks and generally belong to multigene families. Plants constitutively expressing naturally occurring CKO genes that code for secreted or vacuolar enzymes have been described. We report on Arabidopsis transgenics constitutively overexpressing the secreted native form or an engineered non-secreted form of the maize CKO1 enzyme. Severity of phenotype symptoms (increased root system, reduced size of aerial parts and defects in seed development) was clearly correlated with the level of enzyme activity. Aerial part was especially affected in plants overexpressing the non-secreted enzyme, even at low activity level. In all strong overexpressers, zeatin-type metabolites were highly depleted compared to isopentenyladenine-type metabolites. AtIPT genes involved in Ck biosynthesis were found to be up-regulated in those transgenics while all AtCKO genes were down-regulated except At5g21482, coding for a putative cytoplasmic enzyme. Cytokinin deficiency in transgenics was not counter-balanced by a higher sensitivity: expression of a cytokinin receptor and type-A response regulators was decreased as well as plant response to benzyladenine.