- Fungi form a unique kingdom of organisms and are fundamental for life on this planet.
- Plants encounter many fungi, and adapt to survive or perish when fungal pathogens attack them.
- Several types of fungi also help plants to absorb soil nutrients better, ward off pathogens, survive environmental stresses, and decompose plants to provide nutrients for more plant growth.
- Fungi are present in every aspect of a plant’s life.
Plants have close encounters with fungi throughout their lives. Fungi need nourishment from plants or algae as they cannot make their own food. While fungal diseases cause huge devastations to crops, some fungi are beneficial and essential for plant life. Fungi can occur as mycorrhizae, living in or around plant roots, as endophytes inside plant cells, and as saprotrophs, which decompose plants once they die. In plant life and death, fungi play an intricate role.
Colonising collaborators
Fungi existed on land before plants. The evolution of plants from an aquatic to terrestrial lifestyle would not have occurred without soil fungi. Arbuscular mycorrhizal fungi (AMF) colonised early plants (bryophytes) that lacked roots, allowing these plants to absorb surrounding nutrients better, and therefore survive on land. This then led to the evolution of vascular plants with roots.
Plant host roots produce chemicals that attract AMF; the AMF enter host cells by digesting the plant cell wall and forming a fan-like structure (arbuscule). AMF increase mineral absorption from the soil for the plant, which in turn shares its photosynthesis products with AMF. More than 70% of plants depend on AMF for mineral nutrition. AMF also protect plants from environmental stresses, pests or pathogen attack, and enhance plant growth.
Another group of mycorrhizal fungi, called ectomycorrhiza (ECM), remain outside the root cells and form a net-like structure. This allows better nutrient absorption from the soil by root cells in exchange for plant carbohydrates provided to the ECM. ECM closely associate with trees and include the species forming truffles. Both ECM and AMF protect and strengthen plants, and are used as biofertilisers. Either of these underground fungi can form common mycorrhizal networks between connecting plants.
Co-existing in peace
Fungi other than AMF can also live inside plant cells. These non-AMF are termed endophytic fungi (EF) and can survive specifically inside leaves, stems, seeds, or roots. They are unique to their tissue location, as well as type of plant. Like AMF, EF break down plant cell walls and form structures inside membrane-bound compartments, without being exposed to the plant cell cytoplasm. Initially, the plant does not distinguish between pathogenic fungi and EF, but subsequently, proteins secreted by the EF are recognised by the plant, and host defences are suppressed. This allows the EF to establish, obtain lipids and sugars from the host, and in turn produce compounds that protect plants. Some orchids rely on EF to provide nutrients for their seeds to germinate and establish themselves atop trees.
In plant life and death, fungi play an intricate role.
Moreover, EF inside plants are essential to boost plant immunity, just like the human gut microbiome keeps us healthy. EF can trigger induced systemic resistance, whereby root-associated EF induce defence responses in aerial plant parts to ward off herbivores and pathogens. This has applications in biocontrol of crop pathogens. However, the climate influences the lifestyle of EF, and circumstances might force them to become pathogenic.
Dining on dead
Fungi can also live off dead plant material. Saprophytic fungi are major decomposers in the environment; without them there would be no organic matter available for new plant growth. Saprophytic fungi are the only organisms that can break down wood, which is the secondary cell wall of plant cells.
By producing specific enzymes that can digest plant cell walls, these fungi thrive. Saprophytic fungi act to recycle nutrients in the environment, thereby providing resources for new life.
Fungi that are friendly to the Earth’s flora are therefore vital components in our ecosystem.
Dr Radhika Desikan
