PUB4 calls time on root cell division

In the plant root meristem, a highly orchestrated pattern of cell divisions controls both root growth and cell fate. A large number of signalling factors and transcriptional regulators have been found to control proliferation and differentiation in the root meristem, including small peptide ligands of the CLV3/CLE family. However, the mechanisms by which these peptides act remain poorly understood. Now (see p. 444), Shinichiro Sawa and colleagues identify the E3 ubiquitin ligase PUB4 as acting downstream of CLV3/CLE signalling to regulate cell division in the Arabidopsis root. The authors identify pub4 mutants in genome-wide screens for mediators of CLV3/CLE activity, and characterise the mutant lines in detail – finding defects in a number of lineages that result in overproliferation and patterning phenotypes. Mechanistically, the authors show that the expression of a D-type cyclin is disrupted in the mutant and that auxin levels are altered. Although much remains to be learned about how PUB4 acts in this context, the data intriguingly point to a role for PUB4 in regulating the timing of asymmetric cell divisions and provide further evidence for an important function for CLV3/CLE signalling in controlling root meristem activity.