Overexpressing AtCKX1 in Potato Plants grown In Vitro: The Effects on Cytokinin Composition and Tuberization

Abstract

Over the last years, cytokinin deficiency has been studied in a variety of plant species, using transgenic expression of cytokinin oxidase/dehydrogenase genes (CKX). In tobacco and Arabidopsis thaliana, overexpression of A. thaliana CKX genes caused changes in plant morphology known as the "cytokinin deficiency syndrome", including stunted shoot growth, decreased apical dominance, and enhanced root growth. For a valid comparison with the previously obtained in vitro grown AtCKX2-transgenic potato lines, we report here on the construction of transgenic lines of the cultivar Désirée constitutively expressing the gene AtCKX1 under control of the CaMV 35S promoter. Three lines expressing the transgene in both shoot and root tissues were used for analysis. Analyses of CKX activity of tissue extracts, cytokinin levels, morphology, and tuberization parameters revealed similarities as well as differences from the AtCKX2-transgenic plants, including lower levels of CKX activity, and more pronounced morphological effects of cytokinin deficiency. One of the three AtCKX1-overexpressing potato lines was able to start forming tubers before 30 days of growth in long day conditions (16 h/8 h photoperiod) as has been previously observed in some AtCKX2 overexpressors. The extension of the experimental period to 120 days of growth in vitro showed that the other two AtCKX1-overexpressing lines showed impaired tuberization in terms of the number of tubers per plant, and tuber size. We conclude that cytokinin deficiency has distinct effects on tuber induction and tuber initiation/growth, possibly mediated by distinct signaling mechanisms.