The cytochrome P450 enzyme MpCYP78E1 inhibits meristem initiation and activity in Marchantia polymorpha

Plant shoot branches are formed by the initiation and activation of generative centers known as meristems. In dichotomously branching plants, such as many bryophytes and lycophytes, new meristems are formed when a pre-existing meristem splits into two daughter meristems. These meristems may be active and produce shoot branches or may be inactive. Here, we show that in conditions where meristem inactivation occurs, such as simulated shade, the position of the inactive meristem alternated between either side of the plant body in the liverwort Marchantia polymorpha. Using this predictable pattern, we generated transcriptomes of active and inactive meristems and identified the cytochrome P450 monooxygenase, MpCYP78E1, as a novel regulator of meristem activity. MpCYP78E1 reporter expression was higher in active meristems than in inactive meristems. More meristems were active in loss-of-function mutants than in wild type, while fewer meristems were active in gain-of-function mutants, indicating that MpCYP78E1 inhibits meristem activity. Furthermore, unlike wild type, Mpcyp78e1 loss-of-function mutants produced supernumerary meristems from the center of the mature plant body. We conclude that MpCYP78E1 inhibits both meristem initiation and activity to modulate shoot branching architecture.