During plant embryogenesis, the zygote divides asymmetrically to produce a small apical cell and a large basal cell. In Arabidopsis, this asymmetric cell division is regulated by a mitogen-activated protein kinase (MPK) cascade involving MPK6. Now, Yutaka Sato and colleagues report that, in rice, MPK6 is not required for asymmetric cell division but instead controls cell differentiation during embryogenesis. The authors first analyse rice gle4 mutants, which exhibit defective organogenesis, and discover that GLE4 encodes OsMPK6, an orthologue of Arabidopsis MPK6. Their further characterisation of gle4 mutants reveals that, while early cell division patterns are normal, the expression of basal markers in embryos is lost. Indeed, transcriptomic analyses confirm that basal markers are downregulated in gle4 mutants, with cells acquiring an apical signature instead. Finally, the researchers examine the timing of cell differentiation and show that the entire wild-type embryo initially acquires a basal identity, with apical markers appearing subsequently in just the apical region; in gle4 mutants, however, basal markers fail to be expressed and the whole embryo expresses apical markers. Together, these findings suggest that OsMPK6 is essential for establishing basal identity, and highlight that asymmetric cell division and cell differentiation can be regulated separately during plant embryogenesis.
