BZU2/ZmMUTE controls symmetrical division of guard mother cell and specifies neighbor cell fate in maize

Abstract

Intercellular communication in adjacent cell layers determines cell fate and polarity, thus orchestrating tissue specification and differentiation. Here we use the maize stomatal apparatus as a model to investigate cell fate determination. Mutations in ZmBZU2 (bizui2, bzu2) confer a complete absence of subsidiary cells (SCs) and normal guard cells (GCs), leading to failure of formation of mature stomatal complexes. Nuclear polarization and actin accumulation at the interface between subsidiary mother cells (SMCs) and guard mother cells (GMCs), an essential pre-requisite for asymmetric cell division, did not occur in Zmbzu2 mutants. ZmBZU2 encodes a basic helix-loop-helix (bHLH) transcription factor, which is an ortholog of AtMUTE in Arabidopsis (BZU2/ZmMUTE). We found that a number of genes implicated in stomatal development are transcriptionally regulated by BZU2/ZmMUTE. In particular, BZU2/ZmMUTE directly binds to the promoters of PAN1 and PAN2, two early regulators of protodermal cell fate and SMC polarization, consistent with the low levels of transcription of these genes observed in bzu2-1 mutants. BZU2/ZmMUTE has the cell-to-cell mobility characteristic similar to that of BdMUTE in Brachypodium distachyon. Unexpectedly, BZU2/ZmMUTE is expressed in GMC from the asymmetric division stage to the GMC division stage, and especially in the SMC establishment stage. Taken together, these data imply that BZU2/ZmMUTE is required for early events in SMC polarization and differentiation as well as for the last symmetrical division of GMCs to produce the two GCs, and is a master determinant of the cell fate of its neighbors through cell-to-cell communication. In the grasses, individual stomatal complexes comprise a pair of dumbbell-shaped guard cells associated with two subsidiary cells and the pore, which together play essential roles in the exchange of CO₂ and O₂, in xylem transport, and in transpiration. However, little is known about grass stomatal complex development. We have uncovered and characterized a key factor (BZU2/ZmMUTE) determining the formation of guard cells in Zea mays. Our data suggest that BZU2/ZmMUTE has a dual role, both as an important player in determining the formation of the guard mother cell, as well as being required for polarization and recruitment of the subsidiary mother cells.