Class III HD-Zip activity coordinates leaf development in Physcomitrella patens.

Land plant bodies develop from meristems, groups of pluripotent stem cells, which may persist throughout the life of a plant or, alternatively, have a transitory existence. Early diverging land plants exhibit indeterminate (persistent) growth in their haploid gametophytic generation, whereas later diverging lineages exhibit indeterminate growth in their diploid sporophytic generation, raising the question of whether genetic machinery directing meristematic functions was co-opted between generations. Class III HD-Zip (C3HDZ) genes are required for the establishment and maintenance of shoot apical meristems in flowering plants. We demonstrate that in the moss Physcomitrella patens, C3HDZ genes are expressed in transitory meristems in both the gametophytic and sporophytic generations, but not in the persistent shoot meristem of the gametyphyte. Loss-of-function of P. patens C3HDZ was engineered using ectopic expression of miR166, an endogenous regulator of C3HDZ gene activity. Loss of C3HDZ gene function impaired the function of gametophytic transitory meristematic activity but did not compromise the functioning of the persistent shoot apical meristem during the gametophyte generation. These results argue against a wholesale co-option of meristematic gene regulatory networks from the gametophyte to the sporophyte during land plant evolution, instead suggesting that persistent meristems with a single apical cell in P. patens and persistent complex meristems in flowering plants are regulated by different genetic programs.