Acidic Cell Elongation Induces Cell Differentiation in the Arabidopsis Root

In multicellular organisms, a stringent control of the transition between cell division and differentiation is crucial for correct tissue and organ development. In the Arabidopsis root this transition occurs at the Transition Zone (TZ), a developmental boundary where cell from the root meristem loose their capacity to divide and start differentiating. Meristem cells originate from a stem cell niche near the apex of the meristem. Stem cells continuously originate daughter cells that eventually differentiate into the mature root tissues. The root meristem is therefore a dynamic structure where a balance between cell differentiation and cell division must be achieved to guarantee continuous root growth. We have previously shown that the control of cell differentiation initiation, the position of the TZ and thus root and root meristem size depends on the activities of two plant hormones: auxin and cytokinin. Cytokinin shapes the graded distribution of auxin positioning and auxin minimum in the topmost root meristematic cell. This auxin minimum acts as a positional signal triggering cell differentiation. Initiation of cell differentiation in the root meristem is marked by characteristic changes in cell size: cells of all root tissues start to expand in one direction (elongate) increasing their size. Whether this sudden change in size is a prerequisite for the initiation of cell differentiation, or rather an early consequence of it is still unknown. Data will be presented to show that cytokinin directly controls, via ARR1, the activity of the EXPA1 gene and, at the same time, of the AHA1 and AHA2 proton pumps. This creates the pH conditions at the TZ for the EXPA1 protein to induce cell wall loosening and consequently cell elongation thus controlling cell differentiation initiation, the position of the TZ and consequently root and root meristem size.