Measuring Cytokinin In Zeptomole Level

Phytohormone cytokinin is well studied since its discovery by Folke Skoog and colleagues around 60 years ago. Cytokinin promotes cell division (cytokinesis), cell growth, differentiation and affect apical dominance, axillary bud growth, leaf senescence and so on. Most interestingly, it interacts with other hormones like auxin, ethylene, abscisic acid, gibberellins, and strigolactones. More precisely, the balance between cytokinin and auxin underlies their critical, antagonistic roles in regulating organ initiation, embryogenesis, meristem function, and other crucial processes. Phytohormone levels are thought to be tightly regulated both temporally and spatially.

In the root of Arabidopsis thaliana, an auxin gradient is observed. But, such a gradient study for cytokinin has not done yet.  This is challenging for cytokinin, because cytokinins are present at extremely low levels (pmol/ g-21 fresh weight, 100-fold lower than auxin levels) and consist of several related molecules and derived metabolites, different forms of which can interconvert via enzymatic reactions. 

Antoniadi et al. (2015) used a powerful technique to construct a map of the intracellular distribution of cytokinin and cytokinin metabolites in the Arabidopsis root tip based on precise measurements of cytokinin pools in different cell types. To construct this map, the authors measured cytokinin levels in protoplasts prepared from four different populations of root tip cells. The cell populations were labeled by expressing green fluorescent protein (GFP) in root tips under the control of four different cell-type-specific promoters driving expression in different regions of the root tip, including the distal root (root cap, columella, columella initials, and quiescent center), endodermis, stele, and cortex/epidermis regions.

An intracellular gradient of CKs and CK metabolites was detected in the apical part of the primary root, with maximum levels in the lateral root cap, columella, columella initials, and quiescent center cells. Strikingly, the authors estimated the concentration of CK metabolites in a single root cell to be in the zeptomole range (between 3 X 10-21 and 100 X 10-21 mol per cell), highlighting the exquisite sensitivity of their detection system.

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