Specification of the basal region identity after asymmetric zygotic division requires mitogen-activated protein kinase 6 in rice

Asymmetric cell division is a key step in cellular differentiation in multicellular organisms. In plants, asymmetric zygotic division produces the apical and basal cells. Mitogen-activated protein kinase (MPK) cascade in Arabidopsis act in asymmetric divisions such as zygotic division and stomatal development, but whether the effect on cellular differentiation by this cascade is direct or indirect following asymmetric division is not clear. Here, we report the analysis of a rice mutant, globular embryo 4 ( gle4 ). In two- and four-cell-stage embryos, asymmetric zygotic division and subsequent cell division patterns were indistinguishable between the wild type and gle4 mutants. However, marker gene expression and transcriptome analyses showed that specification of the basal region was compromised in gle4 . We found that GLE4 encodes MPK6 and GLE4/ MPK6 is essential in cellular differentiation rather than in asymmetric zygotic division. Our findings provide a new insight into the role of MPK in plant development. We propose that the regulation of asymmetric zygotic division is separate from the regulation of cellular differentiation that leads to apical-basal polarity.