Cytokinin oxidase/dehydrogenase inhibitors: progress towards agricultural practice

Cytokinin oxidase/dehydrogenase (CKX) inhibitors reduce the degradation of cytokinins in plants and thereby may improve the efficiency of agriculture and plant tissue culture-based practices. Here, we report a synthesis and structure-activity relationship study of novel urea derivatives concerning their CKX inhibitory activity. The best compounds showed sub-nanomolar IC50 values with maize ZmCKX1, the lowest value yet documented. Other CKX isoforms of maize (Zea mays) and Arabidopsis were also inhibited very effectively. The binding mode of four compounds was characterized based on high-resolution crystal complex structures. Using the soil nematode Caenorhabditis elegans, and human skin fibroblasts, key CKX inhibitors with low toxicity were identified. These compounds enhanced the shoot regeneration of Lobelia, Drosera, and Plectranthus, as well as the growth of Arabidopsis and Brassica napus. At the same time, a key compound (namely 82), activated a cytokinin primary response gene ARR5:GUS and cytokinin sensor TCSv2:GUS, without activating the Arabidopsis cytokinin receptors AHK3 and AHK4. This strongly implies that the effect of compound 82 is due to the upregulation of cytokinin signalling. Overall, this work presents highly effective and easily prepared CKX inhibitors with a low risk of environmental toxicity for further investigation of their potential in agriculture and biotechnology.

Cytokinin oxidase/dehydrogenase inhibitors: outlook for selectivity and high efficiency

Inhibitors of cytokinin oxidase/dehydrogenase (CKX) reduce the degradation of cytokinins in plants, and this effect can be exploited in agriculture and in plant tissue culture. In this study, we examine the structure-activity relationship of two series of CKX inhibitors based on diphenylurea. The compounds of Series I were derived from the recently published CKX inhibitors 3TFM-2HM and 3TFM-2HE, and we identified key substituents with increased selectivity for maize ZmCKX1 and ZmCKX4a over AtCKX2 from Arabidopsis. Series II contained compounds that further exceled in CKX inhibitory activity as well as in the ease of their synthesis. The best inhibitors exhibited half-maximal inhibitory concentration (IC50) values in low nanomolar ranges with ZmCKX1 and especially with ZmCKX4a, which is generally more resistant to inhibition. The activity of the key compounds was verified in tobacco and lobelia leaf-disk assays, where N6-isopentenyladenine was protected from degradation and promoted shoot regeneration. All the prepared compounds were further tested for toxicity against Caenorhabditis elegans, and the assays revealed clear differences in toxicity between compounds with and without a hydroxyalkyl group. In a broader perspective, this work increases our understanding of CKX inhibition and provides a more extensive portfolio of compounds suitable for agricultural and biotechnological research.

Diphenylurea-derived cytokinin oxidase/dehydrogenase inhibitors for biotechnology and agriculture

Increasing crop productivity is our major challenge if we are to meet global needs for food, fodder and fuel. Controlling the content of the plant hormone cytokinin is a method of improving plant productivity. Cytokinin oxidase/dehydrogenase (CKO/CKX) is a major target in this regard because it degrades cytokinins. Here, we describe the synthesis and biological activities of new CKX inhibitors derived mainly from diphenylurea. They were tested on four CKX isoforms from maize and Arabidopsis, where the best compounds showed IC50 values in the 10-8 M concentration range. The binding mode of the most efficient inhibitors was characterized from high-resolution crystal complexed structures. Although these compounds do not possess intrinsic cytokinin activity, we have demonstrated their tremendous potential for use in the plant tissue culture industry as well as in agriculture. We have identified a key substance, compound 19, which not only increases stress resistance and seed yield in Arabidopsis, but also improves the yield of wheat, barley and rapeseed grains under field conditions. Our findings reveal that modulation of cytokinin levels via CKX inhibition can positively affect plant growth, development and yield, and prove that CKX inhibitors can be an attractive target in plant biotechnology and agriculture.

Concomitant polymorphs of 1,3-bis(3-fluorophenyl)urea

Hydrogen bonding between urea functionalities is a common structural motif employed in crystal-engineering studies. Crystallization of 1,3-bis(3-fluorophenyl)urea, C13H10F2N2O, from many solvents yielded concomitant mixtures of at least two polymorphs. In the monoclinic form, one-dimensional chains of hydrogen-bonded urea molecules align in an antiparallel orientation, as is typical of many diphenylureas. In the orthorhombic form, one-dimensional chains of hydrogen-bonded urea molecules have a parallel orientation rarely observed in symmetrically substituted diphenylureas.