Antagonism between blue and red light-signalling controls thallus flatness in Marchantia polymorpha

The growth orientation of the Marchantia polymorpha thallus – a system of dorsiventralized, indeterminate axes – is modulated by light. We show that red and blue light act antagonistically to control thallus flatness, with red light signalling promoting epinasty and blue light signalling promoting hyponasty. We found that loss-of-function mutations in the blue light receptor MpPHOT led to epinasty, while loss-of-function mutations in the red light receptor MpPHY resulted in hyponasty. We hypothesize that these antagonistic activities of blue and red light signalling are balanced in white light, resulting in the development of flat thalli. Using time-resolved transcriptomics, we identified genes that were rapidly induced upon light exposure. Among these genes were all six members of the M. polymorpha BBX gene family. Mutants harbouring loss-of-function mutations in two of the six MpBBX transcription factors developed defective thalli. Mpbbx1 loss-of-function mutants formed hyponastic thalli, while Mpbbx5 loss-of-function mutants developed epinastic thalli. Double mutants Mpbbx1 Mpbbx5 grew flat, supporting the hypothesis that they function antagonistically. Together, these data indicate that phototropin-mediated blue light and phytochrome-mediated red light signalling antagonistically modulate thallus flatness, and that BBX transcription factors also act antagonistically to regulate thallus flatness. Overall design: To examine light-responsive gene expression in Marchantia polymorpha, wild-type was grown under white light conditions, followed by a dark adaptation period and subsequent exposure to distinct light qualities. Plants were subjected to white, blue, red, or far-red light, as well as continued darkness, and samples were collected across a time course to capture early and late transcriptional responses. Total RNA was isolated from biological replicates for each condition and time point, and transcriptome-wide gene expression was quantified by RNA sequencing. Sequencing data were processed using a bioinformatic pipeline to map reads to the M. polymorpha reference genome and quantify transcript abundance. Gene expression levels were compared across treatments relative to the dark reference to identify differentially expressed genes, enabling a systematic analysis of light-quality- and time-dependent transcriptional responses.