• Methods We used a statistics approach, multiple structural change algorithm (MSC), for estimating the number and position of developmental transitions in the growing portion of the root apex. Once the positions of the transitions between domains and zones are determined, linear models are used to estimate the critical size of dividing cells (LcritD) and other parameters.
• Key Results MSC approach enabled identification of three discrete regions in the growing part of the root that correspond to the proliferation domain (PD), the transition domain (TD), and the elongation zone (EZ). Simultaneous application of MSC approach and G2-to-M transition (CycB1;1DB:GFP) and endoreduplication (pCCS52A1:GUS) molecular markers confirmed the presence and position of the TD. We also found that MADS-box gene XAANTAL1(XAL1) is required for the wild type (wt) PD increase in length during the first two weeks of growth. Contrary to wt, in the xal1 loss-of-function mutant this increase and acceleration of root growth were not detected. We also found alterations in LcritD in xal1 compared to wt which was associated with longer cell cycle duration in the mutant.
• Conclusions The MSC approach is a useful, objective, and versatile tool for identification of the PD, TD, and EZ and boundaries between them in the root apices and can be used for phenotyping of different genetic backgrounds, experimental treatments or developmental changes within a genotype. The tool is publicly available at www.ibiologia.com.mx/MSC_analysis.