Stomatal regulators are co-opted for seta development in the astomatous liverwort Marchantia polymorpha

Stomatal evolution and plant adaptation to future climate

Global climate change is affecting plant photosynthesis and transpiration processes, as well as increasing weather extremes impacting socio-political and environmental events and decisions for decades to come. One major research challenge in plant biology and ecology is the interaction of photosynthesis with the environment. Stomata control plant gas exchange and their evolution was a crucial innovation that facilitated the earliest land plants to colonize terrestrial environments. Stomata couple homoiohydry, together with cuticles, intercellular gas space, with the endohydric water-conducting system, enabling plants to adapt and diversify across the planet. Plants control stomatal movement in response to environmental change through regulating guard cell turgor mediated by membrane transporters and signaling transduction. However, the origin, evolution, and active control of stomata remain controversial topics. We first review stomatal evolution and diversity, providing fossil and phylogenetic evidence of their origins. We summarize functional evolution of guard cell membrane transporters in the context of climate changes and environmental stresses. Our analyses show that the core signaling elements of stomatal movement are more ancient than stomata, while genes involved in stomatal development co-evolved de novo with the earliest stomata. These results suggest that novel stomatal development-specific genes were acquired during plant evolution, whereas genes regulating stomatal movement, especially cell signaling pathways, were inherited ancestrally and co-opted by dynamic functional differentiation. These two processes reflect the different adaptation strategies during land plant evolution.

Convergent evolution of water-conducting cells in Marchantia recruited the ZHOUPI gene promoting cell wall reinforcement and programmed cell death

A key adaptation of plants to life on land is the formation of water-conducting cells (WCCs) for efficient long-distance water transport. Based on morphological analyses it is thought that WCCs have evolved independently on multiple occasions. For example, WCCs have been lost in all but a few lineages of bryophytes but, strikingly, within the liverworts a derived group, the complex thalloids, has evolved a novel externalized water-conducting tissue composed of reinforced, hollow cells termed pegged rhizoids. Here, we show that pegged rhizoid differentiation in Marchantia polymorpha is controlled by orthologs of the ZHOUPI and ICE bHLH transcription factors required for endosperm cell death in Arabidopsis seeds. By contrast, pegged rhizoid development was not affected by disruption of MpNAC5, the Marchantia ortholog of the VND genes that control WCC formation in flowering plants. We characterize the rapid, genetically controlled programmed cell death process that pegged rhizoids undergo to terminate cellular differentiation and identify a corresponding upregulation of conserved putative plant cell death effector genes. Lastly, we show that ectopic expression of MpZOU1 increases production of pegged rhizoids and enhances drought tolerance. Our results support that pegged rhizoids evolved independently of other WCCs. We suggest that elements of the genetic control of developmental cell death are conserved throughout land plants and that the ZHOUPI/ICE regulatory module has been independently recruited to promote cell wall modification and programmed cell death in liverwort rhizoids and in the endosperm of flowering plant seed.

Hormonal and genetic control of pluripotency in bryophyte model systems

Land plant meristems are reservoirs of pluripotent stem cells where new tissues emerge, grow and eventually differentiate into specific cell identities. Compared to algae, where cells are produced in two-dimensional tissues via tip or marginal growth, land plants have meristems that allow three-dimensional growth for successful exploration of the terrestrial environment. In land plants, meristem maintenance leads to indeterminate growth and the production of new meristems leads to branching or regeneration via reprogramming of wounded somatic cells. Emerging model systems in the haploid dominant and monophyletic bryophytes are allowing comparative analyses of meristem gene regulatory networks to address whether all plants use common or diverse programs to organise, maintain, and regenerate meristems. In this piece we aim to discuss recent advances in genetic and hormonal control of bryophyte meristems and possible convergence or discrepancies in an exciting and emerging field in plant biology.

The stomatal fates: Understanding initiation and enforcement of stomatal cell fate transitions

In the stomatal lineage, repeated arcs of initiation, stem-cell proliferation, and terminal cell fate commitment are displayed on the surface of aerial organs. Over the past two decades, the core transcription and signaling elements that guide cell divisions, patterning, and fate transitions were defined. Here we highlight recent work that extends the core using a variety of cutting-edge techniques in different plant species. New work has discovered transcriptional circuits that initiate and reinforce stomatal fate transitions, while also enabling the lineage to interpret and respond to environmental inputs. Recent developments show that some key stomatal factors are more flexible or potentially even interchangeable, opening up avenues to explore stomatal fates and regulatory networks.

Liverwort bHLH transcription factors and the origin of stomata in plants

Stomata are distributed in nearly all major groups of land plants, with the only exception being liverworts. Instead of having stomata on sporophytes, many complex thalloid liverworts possess air pores in their gametophytes. At present, whether stomata in land plants are derived from a common origin remains under debate.^1,2,3 In Arabidopsis thaliana, a core regulatory module for stomatal development comprises members of the bHLH transcription factor (TF) family, including AtSPCH, AtMUTE, and AtFAMA of subfamily Ia and AtSCRM1/2 of subfamily IIIb. Specifically, AtSPCH, AtMUTE, and AtFAMA each successively form heterodimers with AtSCRM1/2, which in turn regulate the entry, division, and differentiation of stomatal lineages.^4,5,6,7 In the moss Physcomitrium patens, two SMF (SPCH, MUTE and FAMA) orthologs have been characterized, one of which is functionally conserved in regulating stomatal development.^8,9 We here provide experimental evidence that orthologous bHLH TFs in the liverwort Marchantia polymorpha affect air pore spacing as well as the development of the epidermis and gametangiophores. We found that the bHLH Ia and IIIb heterodimeric module is highly conserved in plants. Genetic complementation experiments showed that liverwort SCRM and SMF genes weakly restored a stomata phenotype in atscrm1, atmute, and atfama mutant backgrounds in A. thaliana. In addition, homologs of stomatal development regulators FLP and MYB88 also exist in liverworts and weakly rescued the stomatal phenotype of atflp/myb88 double mutant. These results provide evidence not only for a common origin of all stomata in extant plants but also for relatively simple stomata in the ancestral plant.