Cry-for-Help: when plants ask soil microorganisms for help

Plants are much more intelligent beings than we are willing to believe, and they demonstrate their intelligence with the ability to solve problems. Thank to this capacity, plants today represent together almost all of the living biomass found on Earth.

This is often and strongly stated by Professor Stefano Mancuso, professor of Arboriculture at the University of Florence, but also an established essayist and author of best sellers such as “Plant Revolution” and many more.

It is precisely thanks to this ‘soft skill’, or ‘problem solving’, that plants are able to manage difficult and adverse conditions, adapting or turning the situation in their favor. The latter is the case of the so-called rhizoremediation, a technique with which plants are essentially “helped” by the rhizosphere microbiome in restoring soils contaminated with phytotoxic molecules.

How? Talking with microorganisms.

When living conditions in the soil are hostile to plant roots, the latter are able to send chemical messages to the rhizosphere microbiome that translate into a real “cry for help”, an invitation to collaborate to re-establish a more favorable situation.

It is discussed in several more or less recent scientific publications, including that of 2021 by a research group from the University of Milan, led by Eleonora Rolli.

According to the researchers, authors of this study, there is an open question in environmental ecology, concerning the mechanisms triggered in root biochemistry to influence the composition and functionality of a beneficial microbiome that helps it adapt quickly to specific stress conditions.

The model defined as “cry-for-help” by Rolfe et al., in 2019, predicts that under conditions of stress induced by phytopathogens or parasites, the pattern of radical exudates changes, to release chemicals capable of “recruiting” beneficial microorganisms capable of counteracting the growth of pathogens/parasites.

For example, cucumber roots infected with Fusarium oxysporum have been shown to increase tryptophan production and decrease affinose exudation, increasing the colonization capacity of the beneficial microorganism Bacillus amyloliquefaciens. The ‘cry-for-help’ hypothesis also explains the development of disease-suppressing properties in infested land and the maintenance of these properties, which are somehow ‘inherited’, for long periods. To this end, when studying the microbiome of the rhizosphere of sugar beet cultivated in a suppressive medium for Rhizoctonia, there were bacteria identified belonging to various families, capable of counteracting the development of the pathogen thanks to the overexpression of specific genes and favored in their proliferation by the production of specific radical exudates.

Based on this evidence, Eleonora Rolli’s research group studied the adaptation of some tree species to soils polluted by PCBs, polychlorinated biphenyls, substances that are highly toxic to plants, to verify the possibility that a “cry-for-help” mechanism would also intervene in this situation.

It revealed that the phytotoxic effects induced by PCBs have a negative impact on the development and health of plants that grow in polluted soils, which therefore modify their radical chemistry in a “cry-for-help” strategy to recruit, nourish and support microorganisms capable of degrading PCBs in the rhizosphere. Thanks to the exudation of primary and secondary metabolites from the roots, different groups of microbial populations are involved in the degradation of PCBs, acting in a metabolic network and in different aerobic and anaerobic micro-niches that are established in the rhizosphere, and that change in response to the gradually different conditions that are created as a result of the metabolism of said PCBs.

The ability of plants to interact with microorganisms in the rhizosphere by means of specific chemical signals has been known for some time, but research is highlighting new traits, which reveal interesting prospects also for agriculture.

In a current overview of the concept of crop fertilization, the objective of satisfying the plant’s nutritional needs is associated with that of revitalizing and regenerating tired, unbalanced and contaminated soils. This can also be achieved thanks to the enhancement of the activity of the useful fraction of the microbiome already present in the soil and/or the addition of useful microorganisms by means of inoculated products.