In a previous post, I highlighted how cellulose microfibrils are a major component in the strength and organisation of the plant cell wall by discussing a recent paper by Jordi Chan & Enrico Coen in Current Biology. Whilst cellulose microfibril orientation is a major factor in controlling growth of plant cells, it’s probably not the whole story of how plant cell walls organise to control plant cell and plant body growth. To anyone who’s interested in plant science or commercial applications of plants, understanding more about the whole story is important. Pectins are an additional major component of plant cell walls and are made up complex series of different carbohydrates. In their recent paper in Science, Haas and colleagues examine the organisation of one type of pectin, homogalacturonan, in Arabidopsis cell walls and propose a new mechanism for how pectins may contribute to the geometry of pavement cells, one of the most complex cell shapes that plants produce.
Using the powerful microscopy technique 3D dSTORM and electron microscopy, Haas and colleagues examine the structure of homogalacturonan (HG) in the walls of Arabidopsis leaf epidermal ‘pavement’ cells, which produce an elaborate series of interlocking lobes. In the anticlinal cell walls of pavement cells (i.e. the cell walls perpendicular to the leaf surface) they use these microscopy techniques to show that HG is arranged in a series of tiny filaments running perpendicular to the leaf surface. This is surprisingly reminiscent of the well-described organisation of cellulose microfibrils in plant cell walls. A large body of previous work on pectins has shown that molecular modification of pectins through a process known as methylation can change their mechanical properties. The authors therefore investigated whether variation in methylation, which is not evenly distributed across the cell wall, can change the properties of the newly-identified HG filaments. Interestingly, they found that when methylation was reduced, the filaments were 1.4 times wider than when methylation was promoted.
So the pectin HG can form filaments in walls of Arabidopsis pavement cells and the width of these filaments can vary according to molecular modification of the filaments. From this, the authors propose a model in which spatially controlled expansion of these filaments is an important contributor to generating the lobing pattern of Arabidopsis pavement cells – the ‘expanding beam’ model. To test this model, they produce an in silico (computer) simulation of the growing cell wall of Arabidopsis pavement cells, in which they vary the thickness of idealised filaments in a spatially-varied manner. This in silico model is capable of producing the lobing pattern and variation in thickness of the cell wall seen in real Arabidopsis pavement cells. Finally, back in plants, the authors show that reducing methylation in Arabidopsis pavement cells (and therefore presumably causing wider pectin filaments) in the absence of turgor pressure can still cause some tissue expansion, consistent with a turgor-independent mechanism such as the filament width growth they propose.
The ‘expanding beam’ model proposed by Haas and colleagues is a major new model for the role of cell wall architecture in plant cell growth. Future work will no doubt test the robustness of this model and the extent to which this mechanism may contribute to supporting the growth of other plant cells. Development of such models also serves as a useful exploration of the wider complexity of plant cell walls beyond cellulose microfibril organisation, a topic that is relatively poorly understood compared to studies of cellulose and its function. This may just be the first step towards a more holistic understanding of the plant cell wall and its functions, but it’s an exciting place to start.