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 centres 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 a predictable pattern of active and inactive meristems develops along the thallus in the liverwort Marchantia polymorpha. In conditions where meristem inactivation occurs – such as simulated shade and older plants – one daughter meristem was active and produced a shoot branch, and the other daughter meristem was inactive after dichotomy. The position of the inactive meristem alternated between either side of the plant axis. Using this predictable pattern, we generated transcriptomes of active and inactive meristems. A comparison of these transcriptomes identified the cytochrome P450 monooxygenase, MpCYP78E1, as a novel regulator of meristem activity. MpCYP78E1 reporter expression was higher in active meristems than inactive meristems. More meristems were active in loss of function mutants than wild type, and fewer active meristems developed 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 centre of the mature plant body.  We conclude that MpCYP78E1 inhibits both meristem initiation and activity to modulate shoot branching architecture. Overall design: RNA-seq analysis of pairs of active and inactive M. polymorpha meristems at each of the three stages of meristem inactivation, as well as thallus (non-meristematic) samples