Botanists from São Paulo, working together with colleagues from the United States, have confirmed that the cell walls of various species of ferns exhibit a different composition to the outer coating on plants previously described. They believe that they have identified a third kind of cell wall, rich in mannose, a type of sugar that forms polymers called mannans that appear in low proportions in other types of wall. In addition to more information about the structure and evolution of the vegetable kingdom, the study may help with the utilization of other plants – that have less resistant external walls – for the purpose of producing biofuels and papers with special characteristics.
This project is the result of the collaboration between Giovanna Silva, Marcos Buckeridge and other researchers from the University of São Paulo’s (USP’s) Biosciences Institute, Jefferson Prado, from the Botanical Institute, and Nicholas Carpita, from Purdue University, in the United States. Carpita was one of the botanists that identified the first two types of cell walls, which implied the assumption – that no longer holds true – that all plants would have one type or the other.
As if it were an external skeleton, this coating structure of the vegetable cells gives them mechanical resistance, protection against predators and makes them porous to water, nutrients and sunlight, indispensable for photosynthesis. The form, composition and properties of the external wall of the cells of the root, the stalk or the leaves of an individual plant can be different, but essentially this structure consists of a web of microfibers of various sugars. The strongest fibers are those of cellulose, which is a long molecule composed exclusively of glucose. Hemicelluloses – more complex structures that contain various types of sugar – envelop the cellulose microfibrils or are intertwined with them. Lignin molecules stick the fibers together, working like cement, and increase the cell wall’s resistance. According to Buckeridge, this is one of the main reasons why tree trunks, which are rich in lignin, normally take a long time to decompose.
Carpita and David Gibeaut, who in 1993 was doing post-doctoral work with Carpita at Purdue and who is currently working at the Oregon State University, in the United States, had classified the cell walls into two types in 1993, after observing that the proportion of sugars between them varied a great deal, resulting in greater or lesser resistance and permeability. Type 1 is that of the eudicotyledons, a group that includes the majority of current plants.
In the cell wall of this group of plants, a sugar known as xyloglucan is the main component of the hemicellulose, which accounts for 20% of the total weight of sugars. There is a balance between the proportions of cellulose, hemicellulose and another combination of sugars known as pectin. In wall type 2, which is typical of members of the grass family, such as rice, wheat and sugarcane, another sugar, arabinoxylan, predominates in the hemicellulose (20% of the total) and there is less pectin than there is cellulose and hemicellulose.
This classification began to look dubious in 2004 when Giovanna Silva, who was then at the Botanical Institute, began to notice that the sugar mannose was the main component of the hemicellulose found in maidenhair ferns (adiantum raddianum), a common plant that is widely grown throughout the world, and in other species of ferns, a group of plants that date back a long way on the evolutionary scale of earthly flora. Giovanna, Buckeridge, Prado and Carpita were initially distrustful of the results obtained with a sample of 11 species of fern, but a larger sample, with 61 species, confirmed the conclusions. According to the study, published in the journal Phytochemistry, mannose accounted for 20% of the total wall or 35% of the total amount of sugars in one out of every three species examined.
Because it is so abundant in ferns, mannose might be used to produce biofuels. “Two enzymes might break down the mannan and produce free mannose, which can be fermented using yeast,” explains Buckeridge, who is a coordinator at the National Institute of Bioethanol Science and Technology (INCT), which is headquartered at USP. He is also scientific director at the National Laboratory of Bioethanol Science and Technology (CTBE), based in Campinas. The research that is in underway with a view to producing cellulosic ethanol is still coming up against difficulties to process pentoses such as xylose, one of the most abundant sugars in the cell wall of sugarcane, the member of the grass family that is most widely used in Brazil to produce ethanol. Breaking down the molecules of lignin continues to be an even more difficult problem.
More resistant paper
The information on cell wall variations might reveal not only the twisted paths of evolution, but also help to produce better quality paper. In a pilot study carried out at the Botanical Institute, Denis Lima, Rubens Oliveira and Buckeridge confirmed that the properties of paper varied in accordance with the addition of different types of hemicellulose. Those samples that got extra doses of mannan, the polymer formed by mannose, had greater resistance, tearing less easily than the others, with different sugars.
“The physical properties of the cell wall changed dramatically according to the composition,” states Jefferson Prado, a researcher at the Botanical Institute who took part in this project. He believes that a cell wall that is richer in mannose could result in greater permeability and porosity and, in addition to this, could enable ferns to grow faster. “Take a maidenhair fern like Adiantum and cut off all the leaves,” suggests Prado. “In a month it’s all grown back again!”
The most intriguing fact, which is yet to be explained, is that many species of ferns have a type 1 cell wall, while others have the recently discovered type 3 one. The botanists realized that – contrary to what they had imagined – the type of cell wall is unrelated to the environment or to the plant’s size: ground ferns and climbing ferns, the simplest and the most complex can have the same kind of coating.
What is making Prado and Buckeridge even more enthusiastic are the as yet unanswered questions. If there are two types of cell walls in ferns, other groups of plants may also have two types of cells. Or there might be many types of cell walls that have not yet been identified. The researchers from São Paulo and the United States are now on the lookout for new types of plant cell walls.
SILVA, G.B. et al. Cell wall polysaccharides from fern leaves: evidence for a mannan – rich Type III cell wall in Adiantum raddianum. Phytochemistry (on-line)