The influence of toxicity on plant growth dynamics


For as long as people have farmed, it has been known that planting the same crop in the same field without change over the years leads to reduced crop yield. Decaying plant material from past harvests leaves extracellular, or free-drifting, plant DNA in the soil, that prohibits the growth of new plants of the same species. This phenomenon, known as toxicity, is responsible for a plant–soil negative feedback mechanism


Dr Annalisa Iuorio from the University of Vienna, a specialist in the mathematical modelling of dynamic systems in ecological settings, researched the effects of toxicity on vegetation distribution. They hope that their research can be applied to  other fields in biology, even including  cancer research. This research supported by the Austrian Science Fund (FWF).


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Hello and welcome to Research Pod. Thank you for listening and joining us today.


In this podcast we are exploring the research of Dr Annalisa Iuorio from the University of Vienna, a specialist in the mathematical modelling of dynamic systems in ecological settings. She and her colleagues have researched the effects of toxicity on vegetation distribution, and they  hope that their research can be applied to  other fields in biology, even including  cancer research.


Climate change is a well-known factor in putting increasing pressure on ecosystems around the world; too much stress can lead to ecosystem collapse. However, resilient ecosystems are more able to withstand these additional stresses.


The variety and distribution of plant life is critical in determining the resilience of an ecosystem, and also in developing  effective strategies to protect it. The places that plants grow –  self-organised plant distribution patterns –  are important indicators of potentially catastrophic shifts in ecosystem dynamics, but also show how resilient the ecosystem is.. Understanding the underlying mechanism is important for conservation efforts.


Plant distribution throughout an ecosystem is ‘self-organised’, as species compete with one another for resources and space. Previous studies of arid and semi-arid environments suggest that non-biological factors in the landscape, such as water sources, play a prominent role in species distribution. These factors were thought to be scale dependent, or affect the distance between plants. For example, plants would congregate heavily around a water source and become more sparse further away from the source.


In one theory, plants are considered to be ‘ecosystem engineers’: plant life draws water up from deeper soil layers and redistributes it to drier areas. This theory has its limitations though, and cannot explain vegetation distribution patterns in ecosystems where water is more abundant.


It was previously thought that plant distribution was heavily influenced by the location of water sources, but now we understand that plants play a more dynamic role in water transportation. However, a growing body of research has demonstrated that toxicity plays an equally important role in plant distribution dynamics, and a crucial one in biodiversity.


Since the dawn of agriculture, humanity has been aware that planting the same crop in the same place, year after year, produces diminishing returns, if the crops grow at all. Several factors are believed to be at play in this phenomenon, starting with plant pathogens. These are organisms that cause disease in plants which can build up in the soil if the same crop is grown for multiple years in the same location. The soil microbiome, usually a complex community of healthy microbes, may change or become depleted. Most importantly, decaying plant material releases toxins in the soil,  leading to an inhospitable habitat for the same crop species to grow again but potentially inducing a positive feedback on other species which genetically differ from the one producing toxins.


Dr Iuorio’s collaborators from the Agricultural Sciences department in Naples have discovered that decaying plant material leaves extracellular, or free-drifting, plant DNA in the soil, and it is this that prohibits the growth of new plants of the same species. This phenomenon, known as toxicity, is responsible for a plant–soil negative feedback mechanism, and its effect on plant distribution was the main focus of Dr Iuorio’s mathematical models.


Toxicity has been found to have a substantial effect on plant life dynamics, dictating which species can coexist and how they organise themselves throughout the landscape. Toxicity also explains the diversity of plant species in an ecosystem, and their distribution on a larger scale. Predictive models have so far shown how  toxicity encourages dynamic plant distribution, and creates non-linear patterns as plants try to avoid the areas where harmful compounds accumulate. This new theory of plant distribution that considers toxicity as a key factor provides valuable insight into habitat conservation at different latitudes. However, the mathematical foundation behind the results (particularly their spatio-temporal dynamics) had not been fully explored. Dr Iuorio’s group explored two models to mathematically explain how toxicity affects plant dynamics. The first model, created in 2014, examines the effects of both  toxicity and water on plant life. Water is an important source of nutrients for plants, playing a crucial role in arid and semi-arid environments, and therefore its location must be considered as well.


The 2014 model used three equations to successfully predict the formation of asymmetric patterns, qualitatively observed in the experimental data gathered over a long period of time. The model analyses the interaction between biomass, available water, and plant toxicity to predict a vegetation pattern. In other words, using the data gathered by Dr Iuorio’s team on environmental aspects such as rainfall, the number of plants in the ecosystem, and other factors, the model is able to produce an image which maps out the most likely areas of plant dispersal. The model illustrates toxicity-induced negative feedback by inducing – when precipitation is low and the plant species is highly sensitive to toxicity – the emergence of dynamic patterns, since biomass will always try to grow in areas of low toxicity. Mathematically, these asymmetric, dynamic patterns can be investigated using techniques from dynamical systems known as geometric singular perturbation theory. Dr Iuorio published this theory in a 2021 paper, linked to in the show notes for this episode.


Precipitation is necessary for plant growth, and the model shows that as rainfall decreases, plant distribution goes from uniform cover, to more spotty distribution, and then bare soil, regardless of potential, and promising, areas for plant growth. This result mirrors how plants are dispersed in a lush, wet climate versus a desert. The model was able to successfully integrate the complex interaction between positive and negative feedback. Plants increase the availability of water locally by diverting water sources in their vicinity, but increased plant growth leads to higher competition for scarce resources, and increased soil toxicity. The 2014 model was able to qualitatively explain the plant distribution of real vegetation patterns in California and Sudan. Despite this success, the lack of effective data describing the time-evolution of real-life patterns (occurring on a very long time scale) implies that a thorough quantitative analysis will require further work before being able to link model predictions with real-life data.


Currently, Dr Iuorio is working on an extended version of this 2014 model , which build upon the original biomass-water-toxicity model and include the effects of toxicity on a community of diverse plant species. The inter-species dynamics and interactions with  toxicity have not been fully explored yet, and the addition of spatial effects may reveal patterns of coexistence between the different species. Through this upcoming model, it may well be possible to see how the impact of water and plant–soil interactions on emerging multispecies vegetation patterns varies in strength, depending on the environmental conditions (from arid to humid).


On how she sees her work fitting into the twin fields of mathematics and biology, Dr Iuorio says:


‘I always believed that mathematics provides a great help in advancing our understanding of the world we live in. That’s actually what made me want to pursue this career in the first place. In my opinion, investigating the mechanisms at the core of many relevant ecological and biological phenomena can not only help us tackling societal problems of utmost importance, but also allow us to expand the available mathematical tools along the way.’


These models, and the methods used to develop them, lay the foundation for studying similar phenomenon in biology. The same processes which lie beneath the ‘toxicity theory’ of plant distribution are also relevant for other areas of biology. For example, this could lead to a better modelling and understanding of phenomenon which involve similar mechanisms of localised growth, expansion, and direction of nutrient flow – namely, the cell dynamics of tumour growth and metastasis.


That’s all for now, thanks for listening. Be sure to check out the episode description for links to Dr Iuorio’s research, and stay subscribed to ResearchPod for more of the latest science. See you again soon.

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