Microbials for Agriculture: Why Do They Call Them Biostimulants When They Mean Probiotics?

Abstract

There is growing interest in using plant-beneficial microorganisms to partially replace chemicals and help reduce the environmental impact of agriculture. Formulated microbial products or inoculants for agriculture contain single strains or a consortium of live microbes, well characterized and biosafe, which can contribute to the growth, health, and development of a plant host. This concept conforms to the definition of probiotics. However, some plant-growth-promoting microorganisms (PGPMs) have been considered a category of biostimulants since some years ago, despite the traditional concept of biostimulants involves substances or materials with no fertilizer value, which in minute amounts promote plant growth. The inclusion of PGPMs together with substances has also involved a significant distortion of the classical concept of biostimulants. Regulations such as the recent EU Fertilizing Products Regulation (EU No. 2019/1009) have incorporated the new definition of biostimulants and included microbials as a subcategory of biostimulants. We discuss that this regulation and the forthcoming European harmonized standards disregard some key features of microbial products, such as the live, true biological nature of their active principles. The factors that determine the complex functional compatibility of plant–microbe associations, and important biosafety issues that concern the intentional release of microbes into the environment, seem to be also ignored. We anticipate that by equating microbials to chemicals, the biological nature of microbial products and their specific requirements will be underestimated, with pernicious consequences for their future development and success.

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“…Two aspects of bioinoculants are crucial: they must contain viable microbes in their composition and exert a beneficial effect on their target host plants. Therefore, as Sanjuán et al (2023) noticed, they fall in the probiotics category.…”

“…Moreover, to profit from the ubiquity and good perception of many commercial products already in use to treat human gastrointestinal disorders, companies manufacturing 'bioinoculants' advertise them as 'probiotics', as a very effective marketing strategy. Nevertheless, as recently noticed by Sanjuán et al (2023), inoculants must share at least three features to be considered probiotics: (i) they must contain viable (active or dormant cells), safe and identified cells; (ii) they must follow strict rules to be applied, in sufficient amounts, to their target plants; and, (iii) they must have a positive impact on the overall health of their plant hosts. These characteristics are paramount in distinguishing between 'probiotics' and other inoculants, like 'biostimulants'.…”

“…These characteristics are paramount in distinguishing between 'probiotics' and other inoculants, like 'biostimulants'. Since the controversy on this subject has been addressed in depth by Sanjuán et al (2023) and Yakhin et al (2017), we will not discuss it further.…”

“…Once isolated, characterized, and identified, PGPM can be used to develop bioinoculants (or microbial inoculants). These commercial products contain live microorganisms as the active principles, aimed at improving plant health, nutrition, or development when applied to seeds or soil (Sanjuán et al., 2023 ). Bioinoculants are produced worldwide and commercially exploited by many transnational corporations (like the company formerly known as Monsanto, today Bayer), and other national enterprises or local initiatives.…”

“…However, when applied in the field, many plant probiotics are adversely affected by environmental factors that limit their universal and widespread use. Because they are alive, once introduced into agroecosystems, the microbes included in plant probiotics must reactivate and multiply actively to function efficiently (Sanjuán et al., 2023 ). Therefore, any biotic or abiotic factor that limits beneficial microbe's establishment and inhibits their active multiplication becomes a barrier to their utilization.…”