Defective Kernel 1 ( DEK 1) is required for three‐dimensional growth in P hyscomitrella patens

Summary Orientation of cell division is critical for plant morphogenesis. This is evident in the formation and function of meristems and for morphogenetic transitions. Mosses undergo such transitions: from two‐dimensional tip‐growing filaments (protonema) to the generation of three‐dimensional leaf‐like structures (gametophores). The Defective Kernel 1 ( DEK 1) protein plays a key role in the perception of and/or response to positional cues that specify the formation and function of the epidermal layer in developing seeds of flowering plants. The moss P hyscomitrella patens contains the highly conserved DEK1 gene. Using efficient gene targeting, we generated a precise PpDEK1 deletion ( ∆dek1 ), which resulted in normal filamentous growth of protonema. Two distinct mutant phenotypes were observed: an excess of buds on the protonema, and abnormal cell divisions in the emerging buds resulting in developmental arrest and the absence of three‐dimensional growth. Overexpression of a complete PpDEK1 cDNA , or the calpain domain of PpDEK1 alone, successfully complements both phenotypes. These results in P . patens demonstrate the morphogenetic importance of the DEK 1 protein in the control of oriented cell divisions. As it is not for protonema, it will allow dissection of the structure/function relationships of the different domains of DEK 1 using gene targeting in null mutant background.