The more beneficial the bacteria and fungi are, the more “fertile” the soil is. These microorganisms break down organic matter in the soil into small, usable parts that plants can uptake through their roots. The healthier the soil, the lower the need for synthetic herb/pesticides and fertilisers. The concept that certain microorganisms ‘probiotics’ may confer direct benefits to the plant acting as biocontrol agents for plants. The plant probiotic bacteria have been isolated and commercially developed for use in the biological control of plant diseases or biofertilisation.

These microorganisms have fulfilled important functions for plant as they antagonise various plant pathogens, induce immunity, or promote growth. The interaction between bacteria and fungi with their host plants has shown their ability to promote plant growth and to suppress plant pathogens in several studies.

The concept of probiotic was first described by Elie Metchnikoff in the early 20th century, in an attempt to identify some beneficial bacteria that could colonise the human gut, probiotics are still associated with gut microbiota, although the FAO/WHO Expert Consultation Report defines them as “live microorganisms which when administered in adequate amounts confer a health benefit on the host”. This definition is perfectly applicable to microorganisms responsible for improving plant development or protection against pathogens, but it has not been used in this sense until recently. The microorganisms able to live inside healthy plant tissues are called endophytes, they have a strong relationship with their host and, in most cases, this relationship is the response of millions of years of co-evolution.

Plants are thought to rely on their microbiomes for faster adaptations to sudden environmental changes. While plants are quite limited in terms of adaptation (due to their inability to move and their slow mutation rate), microorganisms can compensate by evolving functionality more quickly with their short life cycles.

The plant probiotics concept includes all the microorganisms, specially fungi and bacteria—known as plant growth promoters (PGP)—due to their beneficial role in the general growth of plants and their faster adaptation to environmental changes, such as drought, heat or salinity. These microorganisms encompass the well-studied nitrogen suppliers (rhizobia strains or Frankia), other nutrient suppliers (Pseudomonas, which supply phosphorus), those that induce systemic resistance (Trichoderma) and those which directly protect the plants against pathogens (such as Bacillus spp. which produce fungicides). Focus on plant probiotic bacteria and their taxonomy.