Fig. 1.

Overview of protophloem SE differentiation within the root meristem of Arabidopsis thaliana. 4 zones in the root tip can be distinguished: Meristem, Transition, Elongation and Differentiation zone (Ivanov and Dubrovsky 2013). The gradient of protein abundance of the PLTs is shown on the left side (Mähönen et al. 2014). The protophloem SE cell file is fully differentiated before the end of the transition zone as shown in the enlargement. The timing of differentiation progress has not been investigated yet, thus indicated zones within the SE cell stand are approximations. The first cell of the expression domains of known SE regulators is indicated with arrow heads. The yellow and red cell mark the first and second periclinal cell division, respectively. The transit amplifying cells divide several times anticlinally. The differentiating cells elongate and undergo cell wall thickening, sieve plate morphogenesis and autolysis. The green cell is enucleating