The molecular circuitry controlling asymmetric cell division in  roots resembles a flip-flop switch.
  (Phys.org)For organisms to grow and develop, they must  produce tissues with distinct functions, each one made up of  similar cells. These different tissues are derived from stem  cells. How stem cells divide to create new cell types is known as  asymmetric cell division, and is obviously crucial to the overall  development of the organism. In plants, whose cells cannot  migrate, the location where a stem cell undergoes asymmetric cell  division must also be crucial to ensuring tissues develop in the  correct place.
    In research published in the journal Cell, a    collaboration between theoretical biologists and    experimentalists, headed by Stan Mare of the John Innes    Centre, and Ben Scheres, of the University of Utrecht, the    Netherlands, uncovered a molecular switch that integrates signals to    ensure these asymmetric cell divisions happen in the right    place and at the right time, to produce layers of specialised    tissue in the root.  
    "Through an experimental-modelling cycle, we have unravelled    how stem cells in the Arabidopsis root regulate    asymmetric cell divisions that give rise to two new cell    identities at the correct position," said Dr Stan Mare of the    John Innes Centre, which is strategically funded by the    Biotechnology and Biological Sciences Research Council. "We    dissected the underlying molecular circuit which operates in    each cell, and found that it presented a highly robust bistable    behaviour, due to two positive feedback loops involving the    proteins SHR, SCR and the cell-cycle related players RBR and    CYCLD6;1. In other words, we showed that the circuit behaves    like a switch."  
    Bistable systems, which can only exist in one of two states,    are found in nature where tight control is needed. Positive feedback loops are common features of    them as they help make the rapid switch from one state to    another.  
    Having identified this switch, the next step was to work out    how the plant turns it on and off, so that only the correct    stem cells perform asymmetric division, and in the right    location for the overall development of the plant.  
    To do this, Dr Stan Mare together with Dr Vernica Grieneisen    constructed a mathematical model, an in silico version of the    root and the molecular circuitry behind the switch.  
    The physical location of an asymmetric cell division relies on the interaction of the    plant hormone auxin and the protein SHR. Previous work by Dr    Grieneisen had shown how auxin accumulates in the root tip    through a reflux-loop mechanism established by polarly    localized auxin efflux carriers in cells, and that the    concentration of auxin declines the further from the root tip,    forming a gradient with its highest peak at the stem    cells.SHR protein sets up a similar gradient, but    perpendicular to the auxin gradient, radiating out.  
    "We found that the cells that undergo these special cell    divisions are located right at the crossroads of these two    gradients," said Dr Grieneisen.  
    The cell divisions also trigger protein degradation, which    turns the switch off again. This is needed to prevent    uncontrolled development.  
Read this article:
'Flip-flop' switch discovered behind key cellular process