Plant life partners: Beneficial bacteria
The evolution of terrestrial plants from aquatic ecosystems came with challenges: adaptation to survive the harsh environments, and co-evolution with other life, such as bacteria. Bacteria are one of the most ancient forms of life on earth, and amount to about 13% of total biomass (measured as tonnes of carbon) seen on our planet. Land plants have evolved positive relationships with some bacteria, without whom they cannot thrive easily. Increasing crop yield through sustainable means such as beneficial bacteria, will have a positive impact for humanity.
No plant is an island. Plants live in communities, coexisting with other life such as microbes and animals. Communication between plants and microbes is invisible to us, but forms a key part of their sedentary survival strategies. While some of these interactions can be harmful to plant life, there are several positive effects of plant–microbe relations.
What lies beneath
Good soil is essential for a healthy plant. Soil is rich in microbes: a teaspoon of soil contains about a billion microbes. Just like the human gut microbiome (population of microbes), which enables a healthy immune system, the plant microbiome in the roots (rhizobiome) is unique to each plant and provides a healthy lifestyle.
Without soil microbes, plant life would be pretty much non-existent, with very weak and unhealthy plants. The rhizosphere surrounding plant roots contains bacteria, fungi, and nematodes, and here we highlight how bacteria benefit plants.
Probiotics for plants
Plant-friendly bacteria in the soil are grouped into plant growth-promoting rhizobacteria (PGPR) that live freely in the soil, or endophytes that colonise and live either in between or inside plant cells.
PGPR, as the name suggests, benefit plant growth by colonising roots. Plants choose their microbial partners by secreting sugars, hormones, and other compounds into the soil; specific PGPR in turn get attracted to the plant rhizosphere and secrete their own hormones. Communication between bacterial and plant hormones regulates effective root growth and protects the plant from pathogenic microbes.
PGPR secrete antibiotics that kill pathogenic bacteria, and cell wall-degrading enzymes that inhibit growth of fungal pathogens. PGPR can induce plant resistance to herbivores by producing specific toxins or volatiles that ward off attacking pests. PGPR can also act as a vaccine by protecting plants from future herbivore damage, and even across generations, by acting on seeds. In addition, PGPR release chemicals and make soil minerals more available to the plant; this enhances the plant’s fitness towards environmental stresses such as drought, salinity, heat, and heavy metals.
Accessible nitrogen
Plant probiotic bacteria also include endophytes, that live symbiotically within plants. Plants need nitrogen for the formation of chlorophyll as well as DNA and proteins, yet it is not in a biologically available form on earth. Some bacteria living in soils use a special enzyme (nitrogenase) to change (or ‘fix’) atmospheric nitrogen into ammonia under anaerobic conditions; plants then use the ammonia to form essential molecules. Rhizobial bacteria do this in close collaboration with legume plants, (eg, bean, pea) inside special anaerobic structures formed in root hairs, called nodules.
Other bacterial species like diazotrophs live in the rhizosphere of cereal crops and fix nitrogen without producing nodules. Rice fields are rich in cyanobacteria (bacteria which photosynthesise) that can fix nitrogen. Some species of maize produce sticky mucilage in their aerial roots that enhances the growth of nitrogen-fixing bacteria. However, for cereals, natural nitrogen fixation isn’t sufficient to support an increased plant yield. Efforts are under way to enhance the ability of cereals to interact with these microbes, reducing the use of polluting nitrogen fertilisers, enabling sustainable food production.
The hidden world aboveground
Equally important as the rhizosphere is the phyllosphere, the aerial regions of the plant colonised by microbes arising from soil, seeds, and air. Just like animals have microbes on their surfaces to help maintain a healthy lifestyle, the phyllosphere helps to maintain a healthy plant by preventing overgrowth of pathogens. The phyllosphere affects nectar quality in flowers, thereby manipulating pollinator behaviour, leaf longevity, and fruit development. Bacterial composition in the phyllosphere is affected by the changing climate, and more research is needed to understand how this affects plant productivity. The future survival of crops depends on protecting and enriching the plant microbiome.
Dr Radhika Desikan.