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Trees fight infections by building barricades, but how?

Trees fight infections by trapping pathogens in one part of the tree. Hugh Morris and colleagues have been reviewing how trees use secondary metabolites in order to control the process.

How do trees survive? They’re among the oldest things on the planet, but they’re static so they cannot dodge danger. They cannot bite or thump attackers. So their defences have to be very effective at fighting assaults over the long term. Hugh Morris and colleagues have been looking at how secondary metabolites, the tree’s chemical defences, work in the CODIT model.

Cells with infections and defences
A light microcopy image of a London plane (Plananus × acerifolia) infected by the white-rot fungus Inonotus hispidus. The spherical features observed in the fibre lumina are hyphae that have been successfully arrested by the reaction zone, an important part of the CODIT concept. In the reaction zone, living parenchyma undergo programmed cell death when producing secondary metabolites in response to pathogenic fungi or air ingress. The metabolite spread from parenchyma into neighbouring fibres, fortifying the region. The orange and red colours mark the polyphenols, produced by parenchyma. Image: Francis Schwarze.

Introducing CODIT

The CODIT model is how botanists have come to understand tree defence. It’s short for the Compartmentalisation of Damage/Dysfunction in Trees. The CODIT model says that when a tree is wounded, it starts to put up barricades to contain any infection. “The model was conceived by Dr. Alex Shigo in the early 1970s to help forest practitioners to understand basic tree defence processes aided by simplistic drawings,” Dr Morris said. “CODIT became one of the foundational and most important concepts in forest pathology in the 20th century. I must emphasize that CODIT is not a hypothesis or theory but a concept. The concept cannot be proven or disproven, but more research can help develop the concept further making it a better model to explain compartmentalization In woody plants.”

“The model was heavily criticized at the time by the academic community, but it was not originally developed as an academic treatise. The criticism of CODIT mostly hinged on the fact that trees can response to any desiccation-inducing threat (entrance of air into the system) and not only against pathogenic fungi. However, these developments since the original publication for practitioners should not undervalue the importance of the model but only help to develop it further.”

Since the development of CODIT, biologists have found Pattern recognition receptors in plants. These allow plants to sense molecules associated with pathogens. “Trees (plants) can recognize different levels of threat through pattern-triggered immunity,” Morris said. “For instance, trees can recognize differences in threat between mechanical damage and fungi but also between different classes of fungi, more evidence that supports the validity of the model. When explained to students, the model used to raise more questions than answers, as it was explained too simplistically. A light bulb moment only comes when you treat it as the complex subject that it is. Rather than make it just a theory lesson, good idea is to introduce the model to students as a workshop where they see the processes both macroscopically and microscopically using large pieces of infected wood and microscopes. When students see compartmentalization at the cellular level, things begin to make sense.”

What are secondary metabolites?

So by building barriers, trees can box in attackers and prevent them from spreading around the whole tree. To do this they need tools to build the barriers. This is where secondary metabolites are used. Metabolites are chemicals that a tree produces. A metabolite is a secondary if it’s not contributing directly to the tree’s growth. “A secondary metabolite is a chemical produced by the living cells in a plant in response to an environmental stimulus such as a fungus, an insect or mechanical damage,” explained Morris. “These chemicals are also produced by the plant during the formation of heartwood, an antimicrobial, usually dead, substance in the centre of trees that’s can make wood more durable depending on the amount of polyphenols. Polyphenols are a highly important class of beneficial secondary metabolites that give fruits colour, provide the ‘red’ in red wine and the amber complexion in whiskey. The whiskey colour derives from tannins in oak heartwood after years of harvesting in barrels. Polyphenols are essential for human health, with a huge range of benefits from brain function to digestion.”

“There are also different kinds of secondary metabolites, one being volatile organic compounds. If you tear a mint leaf, for example, the odour is, in fact, a defence compound that travels in the air to warn other mint plants nearby that a predator is lurking. A whole conifer forest can emit chemicals into the atmosphere to cool down in response to heat stress. Essentially, in plants, secondary metabolites are used as defence compounds that are already in place at the time of wounding (phytoanticipins) or are produced during wounding (phytoalexins). In human terms, phytoalexins behave like antibodies.”

“The drawback for the tree is that a build-up of polyphenols in the cells is toxic and kills the cells. The plant cells sacrifice themselves in the fight against an outside intruder, thus limiting the damage caused to whole plant function. The limitation of damage in plants is called compartmentalization.”

“I would not be exaggerating by stating that all plant processes are governed by secondary metabolites.”

How serious are threats to the xylem?

Secondary metabolites are transported through the xylem, which is a bit like the veins of a tree. But this important feature of a tree has its weaknesses. “Xylem transports water, solutes and metabolites around the tree and this is usually safe providing air is kept out of the system,” Morris said. “Air can enter the xylem through a number of ways, including through mechanical damage of the bark and xylem tissue from branch breakage or main stem damage, or through drought stress resulting in embolism formation and spread. Emboli are gas bubbles that develop through negative pressure in xylem sap.” When these bubbles get trapped in the xylem, they break the flow of sap. It’s the tree equivalent of getting ‘The Bends‘ and left untreated can be fatal. Morris explained how a bad situation is made worse by attacking fungi.

“Pathogenic fungi require oxygen to survive and spread, so the entrance of air is often followed by pathogens taking advantage of a dysfunctional system. When air enters the system, susceptibility of spread depends on many factors from the genetics of the host tree to the level of stress factors involved, which promote hydraulic dysfunction.”

Fighting the entrance of air is a top priority, and some trees can act better than others, Morris said. “Some tree species have greater reserve supplies of carbohydrates than others, which can be used for conversion into secondary metabolites in response to pathogens, thus rapidly forming a boundary around the damaged region. Also, the interconnectivity of symplast (living cell connectivity) and apoplast (dead cells and dead substances embedding the symplast) play a large role to the ease in which water, solutes and metabolites are transported. Greater living cell interconnectivity means faster transport of defence signals and more diverse options available to transport them. When you have greater vessel (dead cells that transport water) interconnectivity, this allows for faster water transport and in greater quantities but comes with a greater risk of embolism spread and therefore fungal spread. These are all trade-offs that we must consider in trees.”

Morris added that these defences can vary according to season. The metabolites are mobile, when the sap flows so the CODIT system only works during the growing season in temperate climate zones.

Not all trees are the same in how they compartmentalise

Deeper genetic differences can have bigger effects on how CODIT works. In the paper, the authors say: “It is important to mention that there is remarkable variation among and between angiosperms and gymnosperms in the organization of xylem and in the presence and frequency of cell types, and this should be considered when applying the CODIT model.” Morris explained some more.

“I think the key differences lie in the wood and its anatomical and physiological structure. Wood in conifers is much more uniform and mostly comprised of two cell types, tracheids and parenchyma, but mostly the former (approx. 85%). Tracheids provide two major functions in conifers, transport of water and mechanics. To perform both, tracheids must have the ability to keep their shape under huge pressure exerted by gravity and hydraulic forces. Tracheids are short-narrow and unicellular, and have a torus-margo pit membrane system (the pit membrane is the part of the cell wall that fluids pass through).”

“The torus can block the pit aperture through the flexibility of the margo in response to changes in pressure brought about by stress. This makes it very difficult for air to enter the system and when it does, it can be easily contained through the torus-margo mechanism and therefore the spread of pathogens. This makes conifers a safer system but water flow is extremely slow, unlike that of angiosperm trees. Also, conifers do not have the same carbohydrate storage capabilities as angiosperms.”

“We know that there is a strong correlation between living cell fractions and carbohydrate reserves in wood, which means a higher potential for metabolite production. Also, conifers have very few axial living cells and many conifers lack them completely, which connect vessel-to-vessel and vessel-to-ray cells in wood. Axial living cells have diverse functions in wood, with a role in defence being one of them. Resin ducts in conifers have just one function and that is in defence, where living cells block up the canals and prevent axial spread of pathogens. Both conifer and angiosperm defence systems suit the environment in which they are in with neither having a superior defence system, just different.”

Trees do not fight pathogens alone

The authors also draw on the holobiont concept in examining CODIT. “The holobiont concept is a very important concept to understand, all the way from a higher ecological to cellular levels,” Morris said. “It simply means that we cannot function as individuals without other individuals inside of us, where all participants in the symbiosis are termed bionts. For humans, millions of beneficial bacteria forming the microbiome in our guts is essential for our survival. In plants, this role is mostly carried out by fungi called beneficial endophytes.”

Endophytes are microscopic fungi that live within plants. Morris said that while endophytes might look like an infection, they differ from pathogens in their relationship with the plant. “These endophytes have a close evolutionary symbiotic coupling with plants, unlike decay fungi. I must be clear here, many decay fungi are also essential and are always found on old veteran trees, where they perform important ecological functions, such as nutrient recycling. However, the tree still recognizes them as a threat, unlike beneficial endophytes. A holobiont functions similarly to a superorganism, such as bees or ants, in that survival is better together than alone.”

As the endophytes need the tree to survive, for their own wellbeing, they too can be called upon to help fight pathogen attacks. An example given in the paper is Taxomyces andreanae. “This is a particularly interesting fungus,” Morris said. “For years, we have been extracting a secondary metabolite called taxol from yew trees (Taxus spp.) for the treatment of ovarian, breast and lung cancer, but this chemical is actually produced by a fungus (e.g. Taxomyces andreanae) in the tree and not the tree itself. So in this case, you could say that the tree is using the fungus in the defence against decay fungi, a remarkable example of symbiosis that probably contributes to the great life spans of these conifers.”

Where next for CODIT?

In the paper Morris and colleagues say CODIT’s “…focus on wood decay has prevented its usefulness be-yond forestry.” I wondered if that meant there could be a use for CODIT as a model for herbaceous plants too. “I think the CODIT model is quite unique to trees,” Morris replied, “as trees are highly compartmented organisms unlike herbaceous plants and the “T” in CODIT stands for trees. However, all plants compartmentalize and the fundamental principles are the same. Living cells react and form boundaries in all plants. I think a key difference might be that living cells in xylem appear to always die in response to a threat due to the accumulation of toxin polyphenols.”

“However, there is ample evidence that cells in herbaceous plants can be triggered into a heightened awareness of a threat through chemical signalling from the roots without resulting in cell death. This is a kind of ‘priming’ which can be triggered by Trichoderma spp. for example. Perhaps, due to plentiful xylem space in trees, cell sacrifice to prevent decay spread is an acceptable response, but not so in herbaceous plants. Also, reaction zones in trees are rough necrotic antimicrobial barriers, which is needed to resist the powerful hyphae from decay fungi. Much more research is required in this area, especially to find evidence for immune system priming in woody xylem tissue.”

Morris and colleagues have produced their review as CODIT approaches a half-century as a model. Since then, there has been a massive change in how we can examine cells, but Morris said that this doesn’t mean that CODIT is now out-of-date. “I think that more research into tree defence, especially at the cellular level, is only helping to bolster the concept and further its relevance for understanding plant defence processes.”

However, after almost 50 years there is new data that can inform new models, and this is where Morris sees his work going. “There are many interesting areas of research in plant defence but one of my aims is to construct a new defence system for understanding plants, integrating all components/organs of the plant, the roots, stem, branches, bark and leaves. The new defence model would incorporate elements of hydraulic models and other defence models and be done with the aid of sophisticated 3-Dimensional reconstruction techniques using high-resolution computed tomography. One key area related to these techniques is by observing the connectivity between living cells and vessels in wood. This is an important avenue to learn more about the interplay between hydraulics and defence in trees.”

Whatever the new model brings, compartmentalzation will be part of it. Trees use compartmentalization thoughout their bodies Morris said. “A nice example of this is when main branches are shed (cladoptosis) when energy demands by the branch exceed energy supply, becoming a drain on the tree’s resources. The wound compartmentalizes (seals) after branch abscission. Leaf drop in Autumn is the most well-known phenomenon with a similar process to cladoptosis. Trees are giant shedding organisms that readily dispose organs as they grow to maintain function.”