Cyanobacteria in plant health: Biological strategy against abiotic and biotic stresses

Abiotic and biotic stresses can dramatically reduce the productivity of agricultural crops, causing serious economic and social problems. Cyanobacteria are photosynthetic prokaryotes, responsible for a large proportion of global CO2 and N-2 fixation, with numerous environmental, pharmaceutical, and agricultural applications. In symbiosis with plants, cyanobacteria are able to increase the supply of nutrients, not only nitrogen but also others such as phosphorus, and increase the availability of water in the soil, improving its physicochemical conditions and promoting plant growth because of the release of different hormones. Faced with different abiotic stresses, the ability of various species of cyanobacteria to increase plant tolerance has been described because of their direct action in soils or the activation of plant responses. They reduce the effect of salinity by producing extracellular polysaccharide or compatible solutions, increase germination in drought situations, and remove pollutants (heavy metals and pesticides) from soil and water. Faced with biotic stresses, cyanobacteria are capable of producing a great diversity of different chemical compounds with great effectiveness, in addition to releasing various enzymes, competing for the rhizosphere space and activating plant defense responses by interacting with their roots, representing an effective strategy against bacteria, fungi, oomycetes, nematodes, and harmful insects in agriculture.