How Wood Is Made

 

Arboreal Artistry:
The Amazing Talents of Wood Forming Cells 

By Margaret Helder

As the major component of trees, wood seems useful and even beautiful in finished products, but scarcely biologically dramatic. Yet recent research on the development of wood portrays an elaborate ballet which proceeds inside young developing xylem cells. This performance culminates in their producing “molecular ‘tracks’ for the cell-wall-producing machinery. This machinery moves along the microtubules like an asphalt paver and continuously deposits wall material on the outside of cells.”¹ Before we can appreciate these cellular dramas however, we must set wood in its biological context.

The global theatre

Trees are very important to life on earth. Their impact on the ecology, economy and comfortable living conditions can scarcely be overstated. In that trees moderate the climate and encourage rainfall, forests are highly desirable in many ways besides providing habitat for diverse creatures.

We take them for granted, but actually trees are very remarkable life forms. The ability to stand upright (and yet exhibit flexibility) comes from their ability to form wood. This product comes from the ability of these plants to drastically reinforce cell walls with cellulose. This chemical compound is a polymer of glucose, which consists of lengthy chains of six carbon sugar rings linked in an unbranching chain. It is no accident, given the great variety of trees, their sizes and widespread distribution, that cellulose is the most abundant polymer produced by living organisms.

Another widespread polymer of glucose is starch. This polymer is extremely important in human diets and it can take the form of unbranched or branched chains of glucose, but the linkage between sugar molecules is different from that of cellulose. In cellulose we find beta 1-4 bonds while in starch they are alpha 1,4 bonds. Another important polymer of glucose is glycogen, a highly branched polymer used to store carbohydrate energy in animals. Glycogen is a very complex branched chain of glucose units with alpha 1,4 and alpha 1-6 linkages. These molecules are up to 100,000 glucose units long with very many branches. Despite the fact that glycogen and starch are such important products of animal and plant biology, there is actually far more cellulose produced because trees are so large and so thickly dispersed.

The Venue

The tallest trees tend to grow in warm temperate to moderate tropical climates which exhibit lots of rain and sunshine. The tallest trees indeed achieve astonishing heights. The tallest flowering plant tree, a Eucalyptus regnans in southern Australia, has been measured at 330 ft (100 m) tall. The very tallest tree of all is a conifer in California, Sequoia sempervirens which reaches 379 ft (116 m).² Impressive as very tall trees are to look at, their really amazing talents are less obvious. It is a fact that the canopy contains the leaves which depend on water and mineral nutrients from the soil, which lies far below. How do trees raise these resources to the canopy? It is the ability of trees to produce cellulose reinforced tissue, or wood, that enables this amazing life form to exist at all.

Outfitting the venue – good plumbing required

It is apparent that trees need a plumbing system to move water up from the soil to the canopy, the most important part of which are the leaves. The leaves have to photosynthesize, and for this they need carbon dioxide and water (as well as sunlight). Since carbon dioxide is a gas, it has to diffuse into the leaves from the air. As water is scarce up there, the leaves utilize a protective layer to prevent unnecessary water loss. Thus, the leaves need controlled openings in leaf surfaces to let in the air including carbon dioxide. This also however allows water to evaporate and escape from inside the leaf.

The beauty of this is however that water molecules have a special feature called cohesion. They stick together through forces generated by hydrogen bonding. As water evaporates into the air from cells in the leaf interior, the escaping molecules tug at the water molecules behind them, and they tug at the ones farther into the plant. This evaporation process is called a Cohesion-Tension mechanism.³ It creates a negative pressure which holds the water column together under tension, pulling it up through the plant’s woody plumbing all the way from the root tips far below. Thus, as water escapes from photosynthesizing leaves, a steady column of water is drawn up from the soil into the canopy.

It is evident that trees need access to a lot of water. The process of evaporating water from the leaves needs to proceed at a fast pace in order to draw the water column up to major heights. Apparently, plants retain less than five percent of the water absorbed from the roots in order to provide for plant growth.  As the sun shines, the rest of the water passes through the plants directly into the atmosphere. One estimate suggests that some large rainforest trees can take up and lose nearly 320 gallons (1200 L) of water per plant in a single day. This is actually a good thing.⁴

Since the water column is held in such tension inside trees, it is evident that the plumbing design has to be very sophisticated indeed. The piping must not collapse inward under the negative pressure. The water conducting system (tissue) in plants is called xylem. In trees, a new layer of xylem (called secondary xylem) is added in a ring around the stem each year. Thus the tree progressively grows in diameter as more and more wood is accumulated. The xylem largely consists of two kinds of hollow conducting elements or cells. The smaller ones are tracheids, the larger are called vessels. Tracheids are shorter in length and of smaller diameter than vessels. These smaller elements are connected with cells above and below by means of overlapping tapering ends and numerous border pits.

Vessels are much larger cells. They too have border pits in the side walls, They are stacked end to end with ends entirely open or with perforation plates. Vessels exhibit diameters approximately that of a human hair. These water conduits may be about 2 inches (5 cm) long or as long as 10 m (33 ft). The water moves upward through the tracheids and vessels, and sideways through border pits to new vessels and tracheids as upward progress through individual extensions ends. Border pits are safely values. Thus “Bordered pits are cavities in the thick secondary cell walls of both vessels and tracheids that are essential components in the water-transport system of higher plants. The pit membrane, consisting of a modified primary cell wall and middle lamella, lies at the center of each pit, and allows water to pass between xylem conduits while limiting the spread of air bubbles.”⁵

 Plumbing specifications

The compound that confers strength to support the height of trees and strength to allow water to move under negative pressure up to the canopy tops, is cellulose. We take trees for granted, and their ability to form the thickening on xylem vessels and tracheids may seem like old news, but recent research has revealed that it is a very complex process. Both vessels and tracheids obviously start out as living cells, it is only once they have achieved mature form, that they die and become hollow. It is the development of these cells before they die that is so interesting.

Around the periphery of a woody stem is a thin layer of living cells just under the bark. Called the cambium, this layer of cells divides on an inner plane and an outer plane. The outer plane produces cells which are not destined to die. These form the phloem that transports organic nutrients down from the canopy to the roots. The interior plane produces xylem cells which are eventually destined to die. But first, they have to assume a proper form.

Onto the stage

A future xylem vessel or tracheid initially is an ordinary living cell equipped with all the usual interior organelles. The outer boundary of the cell is a typical plasma membrane made up of a phospholipid bilayer with various proteins situated in the expanse. Lying immediately below the plasma membrane and parallel to it are microtubules. These tiny protein tubes are dynamic structures, always forming new material at the front end and shrinking at the back end. Thus, the tubules are continuously advancing forward under the plasma membrane.

Penetrating through the plasma membrane from the cell interior to its exterior are very large protein structures called cellulose synthetic complex (CSC). According to an article on these structures, current estimates suggest that this is “one of the largest protein complexes”⁶ that we know about. Electron micrograph images reveal six hexagonally arranged particles arranged on the surface like a rosette, but they are really rectangular molecules that penetrate the plasma membrane from cell interior to the outside. Closer examination however reveals that these are elaborate structures:

[E]ach of the six lobes of the rosette in turn consists of six cellulose synthases [enzymes expediting formation of cellulose], that each polymerizes a single glucan chain using UDP-glucose as a substrate. Those individual chains are then assembled into one crystalline CMF [cellulose microfibril], which by implication consists of 6 X 6 = 36 chains [of cellulose].”⁷

As soon as each unbranched cellulose chain is produced, it binds tightly to adjacent chains through hydrogen bonding.

The astonishing relationship of the CSC to the microtubules is that the rosettes move through the plasma membrane following the forward path of the microtubules. The rosettes thus leave a tight strand of cellulose behind as each follows the path of their guiding microtubule. The young cell initially deposits a thin primary layer of cellulose followed by other layers each of which lie interior to the previous one, while still lying outside the plasma membrane.

Dance of the microtubules

The microtubules inside the xylem cell and the rosette CSCs embedded in the plasma membrane, together cooperate to produce strong walls that are deposited in intricate patterns taking the form of rings, spirals, nets, and pitted patterns. But how do the microtubules know how to guide the machinery depositing cellulose outside the cell wall?  As long ago as 1975 one specialist suggested:

It is proposed that plasmalemma [plasma membrane] located cellulose synthase enzyme complexes are free to move in the plane of the membrane. Their directed movement may  …. generate a sliding force which moves the entire complex through the membrane utilizing the microtubule as a rigid guiding track and thus laying down, in the wake of the complex, cellulose fibrils whose orientation mirrors that of the microtubules.⁸

How amazing is that! Here are large molecular machines able to move through the plasma membrane! And even more confounding to our imaginations is the idea of microtubules arranging themselves into various elaborate patterns in order to guide the depositing of cellulose outside the cell. It is just recently that scientists from the Netherlands and Germany have described the elaborate dance of the microtubules which leads to the fancy xylem wall patterns.⁹ How do they do it? You have probably seen dance moves involving two steps forward, one step back, or one step forward and two steps back. Well microtubules exhibit similar patterns of motion which are no less an art form.

Scene of the dance

Young xylem cells start out with microtubules evenly arranged in multiple directions parallel to the surface of the plasma membrane. The CSC are already arranged in the membrane above the microtubules and a thin primary cellulose wall is deposited with cellulose fibrils randomly arranged like the orientation of the microtubules below. But microtubules are  dynamic. While the initial scene finds the microtubules evenly dispersed and facing in multiple directions, they end up all oriented in the same direction with gaps between bands of microtubules. The result is that the microtubules “directionally and spatially template the cellulose synthesis machinery during cell wall deposition.”¹⁰ This process has to be directed and coordinated.

Microtubules are constantly on the move. However, this is an elaborate progression involving “catastrophes” and “rescues”. Catastrophes represent the occasion when forward growth of a tubule actually stops and disintegration of the tip (depolymerization) takes place. Rescues result in forward growth starting again. Obviously, it is the rate at which these processes occur which will impact the pattern of where microtubules end up. The recent European study revealed that “We found that microtubules underwent frequent depolymerization due to catastrophes when they were located in gaps. By contrast, the majority of microtubules in bands appeared to grow, with low depolymerization frequency.”¹¹

How this happens is as follows: the counter-intuitive fact is that the “microtubules display more vigorous dynamics in the gaps as compared to the bands.”¹² It actually takes more effort to evacuate the gaps than to continue to grow placidly ahead in the bands. So, the speed with which the microtubules grew and shrank was higher in the gaps than in the bands. Thus, microtubules in the band regions underwent catastrophes and rescues at rates that were less than half of those in the gaps.¹³

Who is the producer?

Of course, the next question is how does the cell know how to control microtubule arrangements? The authors admit that “the principles by which the dynamic microtubule network is re-organized during transition from primary to secondary wall deposition in proto-xylem remain elusive.”¹⁴ There are some preliminary suggestions however. In plants, there are signaling proteins which serve to recruit other proteins to regulate microtubule advance or retreat. Of course, this just puts the question of control one step farther back, what controls the signaling proteins. Control at one stage or other has to be based on a plan/program and information on how to achieve that plan.

The nature of trees and their major impact on life on Earth depends upon their ability to manufacture woody stems. The ability of plants to funnel certain cells into the wood developing process, is as important as are the amazing processes that lead to the appearance of wood itself. We should be very grateful for these capacities of trees, and for the super intelligent mind who decreed that these things should be so.

References

1. Max-Planck-Gesellschaft. January 28, 2021. https://www.mpg.de/16342677/how-plants-stabilize-their-water-pipes
2. regnans means “reigning” and sempervirens means “always green or flourishing”
3. Andrew J. McElrone, Brendan Choat, Greg A. Gambetta, and C. R. Broderson. 2013. Water Uptake and Transport in Vascular Plants. Nature Education Knowledge 4 (5): 12. See p. 6.
4. McElrone et al. 1.
5. McElrone et al. 6.
6. Fabiana Diotallevi and Bela Mulder. 2007. The Cellulose Synthase Complex: a Polymerization Driven Supramolecular Motor. Biophysical Journal 92 April: 2666-2673. See 2666.
7. Diotallevi and Mulder p. 2666.
8. Brent Heath. 1975. A unified hypothesis for the role of membrane bound enzyme complexes and microtubules in plant cell wall synthesis. J. Theoretical Biol. 48 (2): 445-9. See p. 445.
9. Rene Schneider et al. Long-term single-cell imaging and simulations of microtubules reveal principles behind wall patterning during proto-xylem development. Nature Communication 28 1 21  doi:10.1038/s41467-021-20894-1
10. Schneider et al. 4.
11. Schneider et al. 7.
12. Schneider et al. 8.
13. Schneider et al. 7 “microtubules in the band regions underwent catastrophes and rescues at average rates of 0.096 +/- 0.066 and 0.054 +/- 0.048 events per microtubule end per minute, respectively.” p. 8 “higher rates and variations were found in gaps with average r_cat and r_res of 0.258 +/- 0.282 and 0.120 +/- 0.120 +/- 0.138 events per microtubule-end per minute, respectively.”
14. Schneider et al. 4.

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Margaret Helder completed her education with a Ph.D. in Botany from Western University in London, Ontario (Canada). She was hired as Assistant Professor in Biosciences at Brock University in St. Catharines, Ontario. Coming to Alberta in 1977, Dr Helder was an expert witness for the State of Arkansas, December 1981, during the creation/evolution ‘balanced treatment’ trial. She served as member of the editorial board of Occasional Papers of the Baraminology Study Group in 2001. She also lectured once or twice a year (upon invitation) in scheduled classes at University of Alberta (St. Joseph’s College) from 1998-2012. Her technical publications include articles in the Canadian Journal of Botany, chapter 19 in Recent Advances in Aquatic Mycology (E. B. Gareth Jones. Editor. 1976), and most recently she authored No Christian Silence on Science (2016) which promotes critical evaluation of scientific claims. She is married to John Helder and they have six adult children.