Figure 8.

Model for AtMMS21 functions in stem cell niche maintenance and DNA damage responses in the root meristem. The subunit composition of SMC5/6 complexes in Arabidopsis is based on the model of Watanabe et al. (2009), and SMC5-SMC6 heterodimers associate with the δ-kleisin NSE4. Here, we demonstrate that AtMMS21 is an additional subunit of the SMC5/6 complex via direct binding of SMC5. AtMMS21 controls two important processes influencing root stem cell niche maintenance. First, AtMMS21 is essential for the proper identity of QC and stem cells by stable expression of the stem cell niche-defining transcription factors within the niches. Ishida et al. (2009) showed that AtMMS21/HPY2 functions downstream of PLT1/PLT2; therefore, we suggest that a feedback loop exists between AtMMS21 and PLT1/PLT2 in maintaining the root stem cell niche. Second, AtMMS21 is required to prevent cell death, early differentiation, premature endocycle onset, and misregulation of stem cell factors in the root meristem through a reduction in DSBs. Current knowledge about AtMMS21 highlights that AtMMS21-mediated regulation of the balance between cell division, cell endocycle, cell differentiation, and cell death is critical for the normal cellular organization and function of the root meristem. SCs, Stem cells; SCN, stem cell niche. [See online article for color version of this figure.]