Ringing in another year – trees in springtime
“Early in May, the oaks, hickories, maples, and other trees, just putting out amidst the pine woods around the pond, imparted a brightness like sunshine to the landscape, especially in cloudy days, as if the sun were breaking through mists and shining faintly on the hillsides here and there.” from Walden by Henry David Thoreau (1817-1862)
The season of great change has come upon us. Snow banks are becoming but a memory. Wintry vistas gave way to the browns and yellows of frozen foliage, which now regain their vitality, proclaiming it in almost shocking emerald green. Flowers bloom, and overwintered insects buzz. Much of nature has wakened from its season of stasis – to grow and reproduce again.
There’s much in spring to attract the eye. In keeping with the name of the season, plants seem to spring from the ground. Foliage bursts from the soil, almost overnight it seems. Within short order, flowering occurs. First the early blooms – crocuses, irises, and narcissus – followed not long after by the furious colours of tulips. Simultaneously, the buds on trees burst, and their freshly minted leaves unfurl. The trees also flower – the air is first alive with pollen, replaced, over time, by a shower of the spent organs of plant sex. They festoon newly emerging herbs and grasses, and pepper carpets of moss. Spring, in brief, has sprung.
Spring seems to announce its presence loudly and boldly. Subtlety does not appear to be in spring’s repertoire. And yet, there are less obvious changes afoot.
While visually less dramatic, but no less awe-inspiring, spring ushers in a time of growth invisible to the human eye. Like a glacier, this growth is imperceptible when measured in hours or days, but makes its presence known over longer time scales. The initiation of another year’s growth-ring in the trunks of our neighbouring trees is amongst the most striking of these invisible changes.
Beneath the bark of trees is a layer of cells that will add to the girth of the tree throughout the coming months. This layer of cells lies like a giant sheath between the bark and the woody stem. It can be merely a few cells thick, this wood-enclosing glove – the thickness of 3 sheets if paper – but its activity will expand the diameter of a towering giant by centimetre increments per year, creating wood that can last longer than many human lifetimes.
The layer of cells under the bark is called the cambium. You may have seen or even touched it before – it is the watery, slippery layer of cells that you encounter if you strip the bark off a young branch. If you’ve ever peeled the green bark off a willow “whip”, you’ve encountered the cambium beneath.
The cambium is a special tissue in that it comprises cells that have the potential to become any kind of plant cell. These cells are said to be “totipotent” – they have the potential to become anything. The cells of the cambium are the plant version of animal stem cells. In some ways, cambium cells are both literally and figuratively stem cells. In the parlance of people who study plants, these are meristem cells, and the cambium itself is a meristem tissue.
Throughout the winter, the cambium cells were in a state of stasis. They were dormant. They neither divided, producing new cells, nor did they transform into another cell type. They just remained as they were – metabolising – waiting for signs that spring had arrived.
In springtime, temperature cues received by the roots, and light cues received by the bark above ground were conveyed to the cambium. They were conveyed in the form of sugars and amino acids that were mobilised from the roots and cellular reserves, and plant hormones that were transported from the roots and the aerial tissues. Together, they washed over cambium, informing it that a new season for growth had arrived.
On the basis of hormone cues, and making use of carbon skeletons provided by sugars and amino acids, the cambium set about a new year of business. On the bark side of the cambium, cell division created new cells that were transformed into new transporting cells – the cells of the phloem. In the coming year, these active cells will move sugars and other metabolites made in the leaves to other parts of the plant’s body – through stems all the way down to the roots. The metabolites will be used to construct the plant – to build new cells, to create new growth.
On the wood side of the cambium – towards the inside of the trunk – the cambium will divide to make cells that will become the plant’s water-conducting system – the xylem. Xylem cells are the plumbing system in a tree – transporting water and dissolved solutes from the roots to the aerial tissues, where the water is used in photosynthesis, and the dissolved solutes are used to support a vast array of cellular functions. Xylem cells are highly specialised – they are exquisitely well suited to perform several important functions – particularly to transport water over long distances and to support the weight of the plant body. In trees, this is not a trivial task. A lot of xylem cells are needed to sustain tree function. Wood is the composite that emerges from all of these xylem cells.
Given the extent to which trees embody life, it is somewhat ironic that almost all of the xylem cells are dead. In order to produce a living tree, the plant must deliberately kill – in a highly controlled fashion – a substantial proportion of its cells. The cambium creates the cells that are slated for death.
On the wood side of the cambium, the newly divided cells are follow a well defined development trajectory – gradually changing in their form to eventually become dead, water-conducting xylem cells, or, sometimes, xylem cells that function strictly to support the plant body – xylem fibre cells. As the new cells differentiate into these cell types, they first elongate and enlarge. The cells destined to become water-conducting cells acquire large interior diameter – perfect for functioning as pipes. Those whose fate is as a fibre become thin and elongate – like scaffold.
After they acquire their shape, the differentiating xylem cells dramatically increase the thickness of the wall that surrounds them. Plant cells are normally encased in a cellulose-rich cell wall. Xylem cells build on the cellulose foundation – adding more cellulose, as well as other compounds, like lignin, that strengthen the wall and make it resistant to degradation. At the end of this process, xylem cells are ready to meet their final fate, death.
Through a highly controlled mechanism, xylem cells release a cascade of enzymes whose job it is to digest the cellular contents. Some of these enzymes also create holes in defined regions of the cell wall of the water-conducting xylem cells – creating porous structures through which water can flow. These enzymes kill the cell. It will no longer differentiate. It will no longer metabolise. What is left is merely the skeleton of a cell – a corpse with a function.
The net result of this process is that the xylem cells are dead, hollow tubes of reinforced cellulose – strongly constructed pipes through which water can flow, without the obstruction of cellular constituents. They are a perfect water-conducting system.
Throughout the growing season, the cambium will continue to produce xylem cells. Many cells with large diameters will be made when the growing conditions are best – in the spring and early summer. In the later summer and autumn, as resources become more limiting, the cambium will begin to slow down, producing fewer cells, with smaller diameters.
The two gears of cambium activity produce earlywood and latewood respectively – the light and dark bands seen in a tree ring – the marker of the passage of another year. The thickness of the bands is a reflection of the conditions the cambium was functioning under at a given time of the year, over the year. Come autumn, the cambium will enter its dormant state again, and wait for another springtime.
The activity of the cambium marks the change of the seasons. It tracks the passage of time. It makes a record the prevailing conditions. It is like a great internal clock that literally rings in one growing year to the next.
There’s something fitting about cambium activity marking the passage of time, because its activity is itself as age old process – one that predates the emergence of trees on this planet. Recent research from the laboratory of Taku Demura has found that the genetic program that shapes xylem cell formation is older than the evolutionary emergence of xylem cells themselves – harkening back to mosses.
Mosses are ancient plants. Their lineage did not acquire the evolutionary innovation of a full-blown xylem. This said, mosses have cells that resemble xylem cells. For instance, mosses have water-conducting cells know as hydroids. Hydroids have a thickened cell wall, but lack reinforcing compounds like lignin. They also have lost their cellular constituents, but they do not have the porous holes that xylem cells have for the smooth passage of water.
Mosses also have reinforcing cells, like fibres, called stereids. Stereids are reinforced dead cells that function to support the moss body, just like xylem fibres do in wood.
Given the parallels between xylem cells and hydroids and stereids, Taku Demura and colleagues wondered whether they were made in using the same cellular program. Mosses don’t have a cambium, but they do have meristem cells that give rise to hydroids and stereids, so it is reasonable to hypothesise that the last common ancestor of mosses and trees may have used genetic programs to direct the formation of these specialised cell types from meristem cells.
The process to make xylem cells is exquisitely controlled. It is an active process that involves the action of many genes. Genes that encode information necessary for cell elongation are used early in xylem development; whereas, genes that are important in making cell walls are brought into play later in the process. Finally, a whole suite of genes that encode the enzymes necessary to create the functional corpses are brought into play – these genes are said to invoke programmed cell death.
Most of the genes involved in making xylem are like a parts list – make this or make that. But some genes function to orchestrate the construction of xylem. Rather than providing instructions to make a component of a xylem cell, these genes provide information about where and when parts need to be constructed and assembled – or where and when they need to be deconstructed and die.
One group of genes that are involved in orchestrating xylem formation in plants like trees are VND genes. VND genes are needed to direct the formation of xylem cells from those freshly divided cambium cells. Some VND genes inform the cells to become water-conducting cells. Other VND genes inform the cells to differentiate into xylem fibres.
Taku Demura and his colleagues found that the genome of the moss, Physcomitrella patens, contains VND-like genes. They called these genes VNS genes. They found that a subset of these genes seemed to function in the water transporting regions of the moss – its veins.
When moss was engineered so that specific VNS genes were no longer were able to function, the moss lost its ability to make functional hydroids. When hydroids were made, they did not undergo programmed cell death. Consequently, water wasn’t transported properly, and the moss plants wilted.
Similarly, when moss was engineered so that all of the vein-active VNS genes were disabled, the moss not only could not make hydroids, but it also lost its ability to make stereids. Stereids in these plants didn’t have a thickened cell wall, and they remained full of cellular constituents.
Taku Demura and colleagues also engineered moss so that the VNS genes functioned in moss at times and in places when they shouldn’t. When they did this, the moss produced extra hydroids and stereids in places and at times when they should not be produced. What’s more, these plants were shown to orchestrate the activity of genes that were very similar to those that are orchestrated by VND genes when xylem cells are made.
Taken together, the findings suggest that plants have long possessed the cellular program to make water-conducting cells. While these programs are deployed at a different time in the life cycles of mosses and trees, the VND/VNS genes underpin a means by which plants are able to construct their major water-conducting cells – from hydroids to xylem. This particular program has enabled plants to thrive in a terrestrial environment – to take water extracted from the ground and transport to aerial tissues where it can be used in the capture of carbon, and the use of that carbon in the growth and development of remarkable organisms.
Xylem – wood – provides a wonderful connection with the passage of time. It is a reminder of deep time – of the remarkable ability of plants to colonise the terrestrial environment. It is also a reminder of the passage of time within our lifetimes.
While outwardly imperceptible to the human eye, the activity of the cambium nonetheless makes a record for us. We can see the record in tree rings. But less intrusively, we can, if we pause long enough, see it in the changed world around us. The forest that was but a collection of seedlings only a handful of years ago. The sapling that lent us support on a hilltop, now a stately giant that provides us will shade. These incremental changes are no less impressive than the dramatic flare of springtime we see every year, they just take a little more effort to see. It’s an effort worth making.
Images: All photographs by Malcolm M. Campbell.
Xu B, Ohtani M, Yamaguchi M, Toyooka K, Wakazaki M, Sato M,...& Demura T (2014) Contribution of NAC transcription factors to plant adaptation to land. Science 343: 1505-1508