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Teaching Resources

Our Plants (and Food Crop Farming)

Humans have significantly altered the planet, converting over a third of ice-free land to farmland and 1% to urban areas. Within cities, wild spaces have been transformed into sports fields, parks, and gardens, all carefully managed to ensure desired plants thrive. To achieve this, we use agrochemicals like fertilizers and pesticides, which come at a high environmental cost. Their production, transport, and use contribute to greenhouse gas emissions. Fertilizers can be converted into potent gases by soil microbes, while pesticides may degrade, preventing accumulation. However, both can contaminate rivers, lakes, oceans, and drinking water. In the future, microbes may offer sustainable, eco-friendly alternatives for plant health, reducing the environmental impact of agrochemical use and supporting sustainable development goals.

Agrochemicals, microbes and the environment

Sir: what is that stuff being sprayed on our sports field?

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Sustainable agriculture relies on protecting crops from insect pests to increase yield and food quality. While chemical insecticides have been widely used to control pests, they are toxic to beneficial organisms, animals, humans, and plants, and they persist in the environment. Therefore, alternative strategies to reduce chemical use are needed. Microbial insect pathogens, such as viruses, fungi, and bacteria, offer a promising solution. Bacillus thuringiensis (Bt) is the most effective insect pathogen, showing specific insect control properties. Additionally, Bt genes have been incorporated into crop plants to enhance their insect resistance. Using microbial pathogens or their genes can boost agricultural productivity while reducing reliance on harmful chemicals, supporting Sustainable Development Goals.

Biocontrol Microbes in Plant Protection

Sir: what is BT – I always thought it was British Telecom?

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Microorganisms play a vital role in the health of animals, plants, and soil ecosystems. They decompose organic matter, providing essential nutrients for plant growth, and are crucial to the productivity of agricultural systems. Microorganisms form the foundation of the global food web, supporting plant growth, which is the primary food source for humans and animals. While most microorganisms benefit plants by protecting against diseases and supplying nutrients, some have evolved strategies to acquire nutrients in ways that harm plants. These plant pathogens cause diseases that can significantly impact crop yield and quality, with consequences for food security and the economy. Plant pathogens spread through wind, rain, insects, animals, and contaminated equipment, with some using flagella to move actively. Plant diseases typically show visible symptoms, especially on above-ground parts. These diseases have wide-ranging effects on Sustainable Development Goals.

Plant Disease Transmission

How to recognize a diseased plant?

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Photo by Fred Springborn

Many animals consume plants, but most are visible to the naked eye and are considered pests due to the damage they cause. These pests include mammals, insects, arthropods, and mollusks. Unlike these visible pests, plant-parasitic nematodes are microscopic and cannot be seen without a microscope. These pathogens feed on plants, causing diseases, and have long-term associations with their hosts. Nematodes are the primary animals responsible for plant diseases, affecting nearly every plant species. They are often called "hidden enemies" because their presence is not noticed, yet they result in significant crop damage, leading to reduced profits for farmers, nurseries, and greenhouses. Nematodes are common in fields with a history of plant production, making it likely they are feeding on crops in your garden, farm, or greenhouse.

Nematodes that Eat Plants

George: why does the root of that sick celery look peculiar?

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Taken from the
slide collection of Dr. George Bird, Michigan State University.

Plants use various strategies to defend against natural enemies, such as insects and pathogens. Beyond their physical and chemical defenses, plants rely on a "microbial army" to protect them from herbivores and diseases. Disease-suppressive soils are a prime example of this partnership. In these soils, plants show little or no disease symptoms despite the presence of pathogens, thanks to microorganisms (bacteria and fungi) in the soil and roots that inhibit pathogen growth by producing compounds like antibiotics. These beneficial microorganisms can be harnessed in agriculture to control plant diseases and improve crop yields, offering a sustainable approach to enhancing agricultural productivity and supporting multiple Sustainable Development Goals (SDGs).

Disease suppressive soils: a battlefield beneath our feet

Mummy: How can plants protect themselves if they can’t run away from danger?

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Modified from starline and brgfx/Freepik

Communication, the exchange of information, is essential for interactions between living organisms. The universal language for communication is not speech, but chemical signals. Some of these, known as volatile compounds or "odours," are transmitted through the air. Bacteria play a key role in mutualistic relationships with plants, and their volatile compounds help strengthen this connection. When plants detect bacterial odours, they grow better, develop more roots, and become more resilient to environmental stresses like drought and heat. These odours also serve as warning signals, helping plants defend against harmful fungi. Using bacteria and their volatile compounds in agriculture offers a sustainable alternative to synthetic fertilizers and pesticides. These compounds are naturally degradable, unlike chemicals that can accumulate in soil or contaminate water sources.

Bacterial odours: a way of communication

Mum, why does the fridge smell so bad?

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Picture by Viviane Perraudin

Plants perform photosynthesis, using sunlight to convert carbon dioxide and water into food for growth, flowers, seeds, and new plants, while releasing oxygen essential for life. While most life depends on plant photosynthesis, plants also need minerals from the soil for healthy growth. Soils must contain adequate nutrients, which plants absorb through water. These nutrients, sourced from decaying organic matter, the atmosphere, and rocks, are vital for all living organisms, as animals feed on plants and we rely on both plants and animals for food. Minerals such as nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur are present in low quantities in soil, often leaving plants nutrient-starved. Fortunately, plants receive help from microorganisms like rhizobia, which fix nitrogen, and fungi that form symbiotic relationships with plant roots. Over 90% of land plants, including crops like rice, wheat, tomatoes, and fruits like apples, host arbuscular mycorrhizal fungi (AM fungi), while trees like pines and oaks associate with ectomycorrhizal fungi (ECM fungi). These fungi expand the plant's root system, acquiring minerals from the soil and providing them to the plant. In return, the plant supplies organic carbon from photosynthesis to the fungi. This mutual relationship also protects plants from harmful microbes, demonstrating a powerful example of cooperation among organisms.

Mycorrhizal fungi: the symbiotic friends of plants

Sir: I overheard some biologists mumbling about AM and P: were they talking about morning and afternoon?

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Nitrogen is vital for all life forms, essential for producing proteins, nucleic acids, and chlorophyll. Although nitrogen gas (N2) makes up 78% of the atmosphere, most organisms cannot use it directly, except for some nitrogen-fixing bacteria. These bacteria break the nitrogen bond to produce ammonia (NH3), which is incorporated into amino acids, proteins, nucleic acids, and chlorophyll. A subgroup of these bacteria, rhizobia, forms a symbiosis with legume plants, such as peas and beans. Rhizobia infect the plant's roots, forming nodules where nitrogen is fixed into ammonia, which the plant uses. In exchange, the plant supplies the bacteria with carbon and energy. This symbiosis benefits both parties: the plant receives nitrogen, and the bacteria get the nutrients they need. Additional benefits include: - Legume grains (peas, lentils, soybeans) are high in protein, benefiting human and animal diets. - Nitrogen from legume roots enriches the soil. - Nitrogen leaching is minimized, reducing environmental impact. - Legume root growth fosters diverse microorganisms, enhancing soil fertility. Beyond legumes, other nitrogen-fixing bacteria also contribute to plant growth through similar symbiotic relationships, playing a key role in the nitrogen cycle. This process reduces the need for chemical fertilizers, helping to prevent environmental problems like global warming and eutrophication. It also promotes the production of high-protein grains essential for food security in impoverished regions.

Microbial nitrogen Fixation

Pam: What are those strange pink bumps on the roots of our pea plants? Do all plants have them?

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The global population is nearing 8 billion and continues to grow, putting immense pressure on the world’s food supply. Nearly 1 billion people are malnourished, contributing to 3 million child deaths annually. To meet the food demands of this growing population, we must find sustainable solutions. However, environmental pollution—air, water, and soil—further complicates crop production. There is no single solution, but several steps can help increase the global food supply: First, reduce environmental pollution to preserve farmland. Second, improve food distribution to minimize spoilage and waste. Third, in wealthier countries, reduce restaurant portion sizes and overconsumption of meat, which uses more resources than plant-based proteins. Fourth, enhance agricultural productivity on marginal land. Fifth, promote the use of transgenic plants with higher yields, better nutrition, and greater pest resistance. Finally, reduce reliance on agricultural chemicals and support the use of natural plant growth-promoting bacteria.

Plant Growth-Promoting Bacteria

Daddy: why are the roots of plants coated with bacteria?

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Plants are vital for humans, animals, and insects, providing food, medicines, cosmetics, biofuels, and clothing. They also regulate Earth's climate by removing carbon dioxide and converting it into oxygen and cellular material through photosynthesis. Their extensive root systems reduce soil erosion and provide nutrients to many soil organisms, which in turn support nutrient cycling and supply plants with essential nutrients. A large proportion of these organisms are microorganisms such as bacteria, fungi, protists, and viruses, found not only in soil but also in plant tissues like leaves, flowers, stems, roots, fruits, and seeds. Together, these microbial communities form the plant microbiome. Plant microorganisms can positively impact plant health and productivity, benefiting agricultural production and contributing to the achievement of the United Nations Sustainable Development Goals (SDGs).

The Plant Microbiome

Miss: We cannot live without plants, but can the plant live without microbes?

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