Morphogengineering roots: comparing mechanisms of morphogen gradient formation

We contrast three gradient-generating mechanisms, (i) a source-decay mechanism; and (ii) a unidirectional transport mechanism; and (iii) a so-called reflux-loop mechanism. Focusing on the dynamics of the phytohormone auxin in the root, we show that only the reflux-loop mechanism can generate a gradient that would be adequate to supply functional positional information for the Arabidopsis root, for biophysically reasonable kinetic parameters.We argue that traits that differ in spatial and temporal time-scales can impose complex selective pressures on the mechanism of morphogen gradient formation used for the development of the particular organism.Biological development is characterised by growth and differentiation of cells, which through cell divisions, cell shape changes and cell displacements ultimately shape and form an organism. To establish different tissues, both cell fate (the commitment to differential developmental programs) and cell differentiation (the actual modifications of the cell’s biochemical and biophysical properties) are essential. “Positional information” provides cells with directions for fate changes or cell differentiation due to the position of the cell within the embryo or tissuea. Within this context, Turing [1] coined the term “morphogen” to describe molecules whose spatial distribution within the organism determines patterns of gene expression as cells respond to differences in the concentration. He demonstrated the feasibility of de novo establishment of non-homogeneous morphogen patterns, arising without the need of pre-patterns such as localised sources or sinks. In contrast, Wolpert [2] argued that it is reasonable to assume pre-patterns, for example due to maternal factors or previously established cell polarity. He showed that a combination of pre-localised sources or sinks, diffusion of the morphogen and overall decay can result in a graded morphogen distribution that could supply positional information, using threshold concentrat