Plant cell walls are important for a whole range of industrially important processes, from food processing to paper production and as a sustainable source of material for biofuels and renewable chemical production. Our focus has been on the woody, secondary cell wall since this constitutes the majority of plant biomass and we have two mains areas of research. Firstly, to maximize the potential of woody biomass by overcoming the particular problems associated with biomass composition and breakdown particular problems efficient use of the constituent. The secondary cell wall is composed predominantly of three polymers: cellulose, xylan and lignin. While cellulose and xylan may are potential sources of sugars for biofuels production lignin must be removed and utilized separately. Currently, our lack of basic understanding of how cell wall polymers are made, what controls cell wall quantity and composition and what physical properties different polymers confer upon the wall limit our ability to make beneficial changes to cell wall composition. Secondly, understanding the regulation of vascular tissue development with the aim of improving plant growth and biomass production. This is an excellent system for addressing fundamental question in plant development including how what regulates both the extent and orientation of cell division. A better understanding of this process will enable us to generate even bigger plants with greater biomass and/or crop yield.
Unlocking the potential of cellulose
Cellulose is the most abundant biopolymer on the planet and is increasingly recognised as having the potential to be a renewable source of feedstock for the production of biofuel, chemicals and materials. Despite the importance of cellulose in the regulation of plant growth and in shaping the world around us many basic questions about how cellulose is synthesised remain. Cellulose synthesis is unique in that a very large enzyme complex moves through the plasma membrane synthesising the many chains of cellulose that make up the cellulose microfibril, the basic unit of cellulose found in plants. We are currently combining genetics, cell biology and biochemistry to understand the composition and organization of this large complex with the aim of understanding how this complex determines both the structure and orientation of the microfibril that it produces. Genetics has allowed us to identify three related, but distinct, members of a family of proteins known as CESA proteins that catalyse cellulose synthesis in the secondary cell wall (Kumar et al. 2015, Kumar et al. 2016).
We recently demonstrated that the cellulose synthase complex is heavily modified by the addition of many fatty acid (acyl) groups to cysteine residues within the CESA complex. This work was published recently in Science (Kumar et al. 2016). This explains how the complex remains locked within the plasma membrane and has important implication for the properties of the complex and its partitioning in the plasma membrane. We are currently looking at how other proteins involved in directing cellulose synthesis are modified and how this affects partitioning within the plasma membrane. In order to facilitate this work and to move the field forward we have recently generated comprehensive atlas of Arabidopsis proteins modified by S-acylation from 6 different tissues in order to facilitate the study of this important but elusive modification. The analysis suggest that more than 10% and up to 20% of Arabidopsis proteins may be modified in this way and that S-acylation plays an important role in many metabolic and signalling pathways (Kumar et al. 2020).
Our aims it to increase cellulose synthesis, modify cellulose microfibril structure that will allow us to exploit it more easily and to engineer plants to produce complex novel types of cellulose.
Understanding the microtubule orientation and its role in determining plant growth
Cellulose orientation is essential in determining plant cell shape and the orientation of cellulose is determined by the organization of the cortical microtubules that sit just below the plasma membrane. Microtubules in expanding cells all show similar orientation and this is essential for proper cell shape. We have demonstrated a role for the microtubule severing protein katanin at this process. In plants katanin appears to sever at sites where microtubules cross one another and so removes unaligned microtubules. This process is regulated by the microtubule binding protein SPR2 by a unique mechanism that involved big changes in SPR2 mobility (Wightman et al. 2013). More recently we have identified a class of short highly dynamic microtubules that result from microtubule severing and appear to be important in helping microtubule alignment (Chomicki et al. 2016). We have recently found that modification of microtubules and associated proteins (MAPS) are also modified by S-acylation (Kumar et al. 2020). Currently we are looking into how this may affect the organization of the cytoskeleton and relationship between the cytoskeleton and cellulose synthesis.
Manipulating plant vascular development to increase productivity
We have identified a novel pathway involving a receptor kinase (PXY) that is involved in regulating the orientation of cell divisions in the procambium. The ligand for PXY as a short peptide encoded by the CLE41 gene and we have demonstrated how CLE41 expression must be specifically localised in phloem cells in order for it to be perceived by PXY and used to generate the spatial information essential for regulating the proper orientation of cell division (Etchells and Turner, 2010). Using transcriptional profiling, we have demonstrated that a pxy compensatory mechanism exists and that a number of transcription factors are up regulated in pxy mutants. We have been able to show that vascular cell division is regulated by crosstalk between PXY-CLE and ethylene signaling (Etchells et al. 2012) and demonstrated the importance of WOX genes and the receptor kinase ER in determining vascular cell proliferation and orientation (Etchells et al. 2013). The receptor kinase PXY and ER also act to regulate the coordination between cell layers during radial growth (Wang et al 2019). Recently this work has led to the development of a transcriptional network controlling plant vascular development (Smit et al. 2020)
We have demonstrated the application of this work by demonstrating its utility in trees. Carefully tissue-specific engineering of both the CLE and PXY genes allowed us to double the production of vascular tissue and plant biomass. Furthermore, these trees were also 50% taller than the controls and possessed leaves that were 50% larger (Etchells et al. 2015).