The role of auxin response in patterning the thallus of the liverwort Marchantia polymorpha
2017-02-28T03:17:55Z (GMT) by
In the liverwort Marchantia polymorpha, the phytohormone auxin controls the development of specific organs in different developmental stages. Pharmacological experiments show that in the haploid generation young gemmalings develop rhizoids in response to auxin, while in a mature thallus, auxin promotes the growth of gemmae cup rims, controls the spacing of gemmae cups and affects the formation of air chambers. A phylogenetic evaluation of genes involved in auxin biology across land plants indicates that most major families involved in auxin synthesis, transport, conjugation and response exist in Marchantia as single representatives, possibly resembling the genome of the common ancestor of all land plants. Auxin insensitive plants were created by overexpression of a chimeric protein consisting of the endogenous TOPLESS (MpTPL) co-repressor fused to protein-protein interaction domains 3 and 4 (D34) encoded by AUX/IAA or ARF genes of 6 Marchantia loci. Overexpression of these fusion proteins creates extremely small plants that lack a clear establishment of dorsi-ventral polarity and exhibit an overall disorganized patterning. Only the D34 of MpARF3 did not produce aberrant patterning phenotypes, suggesting that the MpARF3 protein has low interaction affinities with MpARF1, MpARF2 and MpAUX/IAA. In Marchantia, TPL likely acts as a co-repressor in the auxin signaling pathway, as ectopic expression of TOPLESS inhibits the formation of gemma cups, while a dominant negative version of TPL (TPLN176H) promotes meristem production (bifurcation), as well as abnormal gemmae cups in the thallus. To begin the dissection of specific roles of ARFs, we focus on MpARF3, a transcription factor orthologous to class C ARFs (ARF10/16/17) of Arabidopsis and which is regulated by microRNA160. Overexpression of the endogenous MpmiR160 greatly inhibits meristem bifurcation and produces gemmae-less plants. Conversely, expression of miR-resistant MpARF3 transforms the thallus into juvenile undifferentiated tissue, and when expressed at only low doses increases bifurcation rates of the thallus. Thus, miR160-regulation of class C ARFs existed in the common ancestor of all land plants and it is plausible that the ancestral role of class C ARFs was the promotion undifferentiated developmental stages, as this is a shared character with Arabidopsis. Taken together, my experiments demonstrate a crucial role of auxin as a morphogenetic trigger across all land plants. In addition, I assign specific roles for auxin in nearly all life stages and tissues of the gametophyte of Marchantia, a member of the basal most lineage of land plants.