Acid growth: an ongoing trip.

Since its first formulation almost 50 years ago, acid growth has had a chequered past complicated by utilization of diverse species and organs for testing alongside necessary but coarse methodology. Within the past 25 years, we have gained new insights into the molecular mechanisms behind the transduction of the signal auxin into the reality of an apoplastic pH shift as well as the effect on cell wall mechanics and the biochemical players within the wall contributing to the resultant growth. In this review, we begin by discussing the historical work and its complications, move on to the modern work and its addition to acid growth, which we finally summarize in an updated model which includes new postulations and questions

On the role of pectin methylesterases in the development of the non-vascular land plant model Marchantia polymorpha

Morphogenesis is the emergence of form in a living thing from a set of processes occurring at different levels of organisation over time. For plant morphogenesis to be understood, it must therefore be studied at multiple scales, from subcellular elements to entire organisms. In this thesis, we examine how the gene family of pectin methylesterases (PMEs) contributes to the development of the non-vascular plant Marchantia polymorpha. Pectin is a highly abundant component in the cell walls of land plants and charophyte algae. From angiosperms, we know that the methylesterification of pectin polymers by PMEs is implicated in growth regulation as well as interactions with the surrounding environment, including microbes. However, little is known about how PMEs contribute to growth and biotic interactions beyond angiosperm models. Marchantia is an interesting system for studying plant development as it obtains a complex 3D morphology from a relatively simple disk-like propagule that is only a few cell-layers thick in its early development. This allows for direct imaging of growing tissues as well as easy characterisation of tissue growth when pectin de- esterification is perturbed. We have established young Marchantia thalli as a system to study pectin modifying enzymes. To that end, we have tested how different growth conditions influence the development of Marchantia and identified a suitable set of conditions for fast, flat thallus growth. We provide evidence for the presence of pectin in Marchantia tissues using immunochemical approaches. Following bioinformatic and experimental expression profiling of the 14 members of the PME family in Marchantia, we identified the MpPME11 member, which was most highly expressed in fast-growth regions compared to slow-growth regions. Promoter reporter assays support higher levels of MpPME11 expression in younger meristem-proximal tissues. Using CRISPR/Cas-generated gene knock-outs we obtained preliminary results suggesting that MpPME11 positively contributes to growth and development of gemmae and thalli, as well as resilience against infections by a filamentous pathogen. We anticipate that this research will extend our understanding of the developmental role of PMEs beyond what has traditionally been inferred from angiosperm-focussed studies. Comparative research utilising early-divergent lineages is essential to unravel the ancestral role of PMEs and to understand whether PMEs fulfil conserved functions in growth, development and plant-microbe interactions in land plants