Stomates are the main site of cellular respiration and photosynthetic gas exchange, as well as evaporation, and are regulated by the circadian rhythm. The stomates open to allow the plant to breathe or cool off. They close if threats from drought or the presence of plant pathogenic bacteria are detected.
Some pathogens, e.g. certain fungi can enter through closed stomates using brute force. However, bacteria do not have the enzymes necessary for this. Some plant pathogenic bacteria have found the way to reopen these closed stomates and enter the plant.
It has been known that opportunistic bacteria such as Salmonella, E. coli or Listeria interact with plants and cause ingression and contamination, which can cause foodborne illness through consumption or cross-contamination.
Several studies have already demonstrated that some (wild) strains of the human pathogen Salmonella are able to enter plants and use them as alternative hosts. A study published in 2020 also showed that the human pathogen Salmonella Typhimurium has also developed the ability to reopen closed stomas. The plant shows no symptoms of this invasion and once inside the plant, the pathogens cannot be simply washed off, thus the number of diseases caused by the consumption of “invaded” raw vegetables and fruits can increase.
The researchers have developed and patented a beneficial microbe – B. subtilis strain UD1022 – to protect and strengthen plant root systems. According to their results roots inoculated with UD1022 - through watering and irrigation - could provide protection from these opportunistic bacteria and the plants treated with UD1022 had significantly higher stomatal closure rates. The UD1022 microbe has been licensed by BASF and is incorporated into several applications. The patented microbe is a unique strain of Bacillus subtilis, a natural, beneficial bacterium that lives on the surface of roots and the surrounding soil, or rhizosphere.
Organisms within the rhizosphere improving plant health and resilience directly or indirectly are known as plant growth-promoting rhizobacteria (PGPR). These bacteria have enormous potential for solving some of the challenges facing our global agricultural system. Many bacteria have been identified as generalist PGPRs for their association with a broad range of plants, and their specific plant beneficial activities have been described. Bacillus species are highly researched generalist PGPRs known to promote plant growth through their ability to solubilize nutrients and produce phytohormones, antifungal secondary metabolites, and volatile organic compounds (VOCs).
In order to develop better crop solutions, producers often include multiple different species of PGPR in their formulations. Synergistic plant growth promotion by multiple PGPR species has been observed in certain plant–bacteria–bacteria interspecies systems, but has also failed to produce in others.
Several Bacillus species are also known to express quorum quenching (QQ) enzymes, which interfere with quorum sensing (QS) signalling from other bacteria, including pathogenic bacteria. QQ activities could be a promising tool for improving plant health and bypassing antibiotic resistance in the development of biological products against plant pathogens.
These activities are promising areas of research in the rhizomicrobiome, as they may influence complex interactions between species. As our knowledge on the mechanisms of PGPR species interactions is still limited, further research is needed to investigate the aspect of bacterial interactions.
A recent paper has shown that UD1022 can protect alfalfa plants (Medicago sativa) from fungal pathogens that cause plant diseases, such as spring black stem or root rot, that can limit crop yields.
Bacillus subtilis like UD1022 produce a molecule known as surfactin, which acts as an antibiotic, enhancing the plant’s fungal resistance. Further experiments showed that biofilms also play a role in UD1022’s ability to suppress alfalfa fungal pathogens in some species. This is good news and could pave the way for additional plant and field studies to understand this antagonistic behavior.
The research team theorized that since UD1022 was known to be so helpful in some lentil-rhizobium interactions, it might also benefit this association in alfalfa by promoting root growth and giving more surface area for the rhizobium to grow. Thus, another experiment was carried out, which resulted unfavourably. Instead of helping, UD1022 somehow harmed the relationship between the plant and the rhizobium, and the plants did not grow well.
This has prompted researchers to parse out these bacteria-bacteria interactions between rhizobium and UD1022. The research team knew that rhizobium bacteria secrete chemicals to encourage more bacteria to colonize the root. When UD1022 was directly added to the rhizobium bacteria without the plant, it was not able to grow. UD1022 was secreting a molecule that interrupted the messaging the rhizobium bacteria was sending to bring its own kind to colonize the plant roots to help the plant grow. This placed the plant at increased risk because it was not able to fix nitrogen properly.
These interactions between bacteria may be common and are critical to understand as new sustainable solutions are developed in the agriculture.
UD1022 has been licensed by BASF and incorporated into four commercial products already in the market and sold in the U.S. and Canada. More study is needed to understand specifics on how and when to apply these beneficial bacteria. It may be more appropriate to apply rhizobium first to promote growth and then UD1022 later to control pests. Also, product developers should be aware that bacteria that work in one system may not work in another.
However, understanding the ability of UD1022 to disrupt pathogen communication signals could also be valuable in the field of human health, as many human pathogens use the same strategy to cause virulence or infection. A better understanding of the mechanisms may provide clues for new treatment approaches to keep patients healthy.