For crustaceans such as crabs and lobsters, their hard armoured protective shells restrict their body growth and shedding this layer of exoskeleton through moulting is a critical phase in their lives. Plants face a similar problem during their growth as their cells are encased in stiff cell walls which are rich in polysaccharides (which are long chains of carbohydrate molecules) such as cellulose and pectins. So how do plant cells achieve cellular growth while being trapped inside the walls? Lacking the ability to move, plants cannot just wiggle themselves out from the walls before expansion like the crustaceans do. On the contrary, plants have acquired unique mechanisms that enable them to remodel their cell walls to soften the wall for cellular expansion. Before we could begin to understand this mechanism, it is important to take a look at the cell wall structures:

The main compositions of plant cells walls are cellulose microfibril, hemicellulose and pectins which provide the fundamental support as the plant exoskeleton. Despite its significance as a major global carbon sink (where final photosynthetic products are stored), plant biologists still cannot agree on the arrangement of these carbon-rich molecules in the wall. Scattered throughout the walls are some proteins which are highly decorated with sugars, collectively called glycoproteins. These glycoproteins play mysterious roles in plant growth as overexpressing them often leads to peculiar changes in the plant growth. There are also receptor proteins kinases which are embedded within the plasma membrane but span beyond the cell wall-plasma membrane interface to help relay signalling information from the wall to the inside of the cell. Some famous receptor proteins such as FERONIA and WAK (wall associated kinases) are known to interact with polysaccharides from the cell wall and signal important changes downstream to alert the plant cells to cope with the ever-changing environment.

When associating the cell wall structures with cellular growth in plants, a few questions become imminently apparent:

(1) When plant cells are ready to expand, how do the cells instruct the cell walls to loosen up? Subsequently, how is the loosening halted when cell expansion has ceased?

(2) As the cell wall is being remodelled (softening/rigidification) to accommodate for cellular growth, how are compositional changes within the wall sensed by the cell?

Figuring out these answers is not easy and requires robust tools as well as a model than can be genetically manipulated. Our lab is primarily using Physcomitrium patens (P. patens), a moss as a model to answer the above questions. Moss belongs to the bryophyte group, which is a sister group to all the vascular plants on land.

Most plant biologists agree that land plants evolved from the fresh water algae in the Cambrian era (515-494 million years ago). Splitting from a common ancestor, the bryophytes represent the earliest plants that conquered and survived on the barren land. Imagine arriving at a highly desiccated environment compared to its previous “water world”, the bryophytes must have invented some tricks that allows them to survive on the dry land. These tricks are likely to be cell wall-related and aim to prevent unregulated water loss. Hence, moss is a valuable system for the investigation of cell wall signalling, especially during the early phase of land plant evolution.

Using the moss, we have recently made some progress

(1) We showed that a cell wall glycoprotein called SLEEPING BEAUTY (SB) could affect the signalling output of an auxin-dependent transcription factor, named ARFC (Teh et al., 2022). The ARFC would control the expression of cell wall modifying enzymes such as PECTIN METHYLESTERASES (PMEs). The PMEs are ubiquitous cell wall enzymes that modify and determine the degree of methylation in the cell wall pectins. Pectins with varying degree of methylation are often used to explain the stiffness of the cell wall and hence its methylation status is a crucial regulator during cell wall remodelling. We have begun to investigate the functions of PME in moss and found that overexpression of particular PMEs could have a profound effect on the plant growth (unpublished), suggesting that pectin dynamics is a key driver for increased architectural complexity.

(2) We recently demonstrated that a type of small signalling peptide, Rapid Alkalinisation Factor (RALF) is a key factor for apical tip growth in the filamentous structures of moss (Febrianto and Teh et al., 2022). Moss colonies that are defective of RALFs could not elongate properly and gave rise to colonies with smooth edges. Intriguingly, the RALF peptide exhibited a polarized localiation at the growing tip. As RALFs in the flowering plant Arabidopsis are known to bind to FERONIA, a well-known cell wall integrity sensor, we are pursuing the connection between RALFs and cell wall integrity regulation. 

The first step in our investigation is to identify the FERONIA-equivalent in the moss. We have taken an explorative phosphoproteomic approach using mass spectrometry to analyse peptides that became phosphorylated when RALF is overexpressed. Receptor-like kinases that became phosphorylated are termed RAREs (RALF receptors). We identified 13 RAREs and intriguingly one of these RAREs, RARE12, when overexpressed in tobacco leaves leads to cell death (unpublished). We are currently investigating how a growth-promoting kinase could cause an opposite phenotype when present in a large quantity.