Editorial: Highlights from the 12th plant growth-promoting rhizobacteria workshop

The term 'Plant Growth-Promoting Rhizobacteria (PGPR)' was coined by Joseph W. Kloepper in 1978 (Kloepper andSchroth, 1978). He defined PGPR as "Naturally-occurring, root-colonizing bacteria that benefit plants by growth promotion and biocontrol". Nine year later, he organized the first international PGPR workshop in 1987 in Canada. Subsequently, the Workshop has been held in Switzerland (1990), Australia (1994), Japan (1997), Argentina (2000), India (2003), The Netherlands (2006), Oregon (2009), Colombia (2012), Belgium (2015) and Canada (2018). The International PGPR Workshop series is the most comprehensive conference in the field of PGPR/ Plant Growth-Promoting Bacteria (PGPB). Professor Kloepper's philosophy for the workshops was as a venue to bring together young and established researchers and industry to increase our understanding of the fundamental and applied aspects of PGPR via formal presentations and informal discussion to advance the field. The 12th PGPR Workshop was held in Toulouse, France, on May 29th -June 2nd, 2023. This Frontiers Research Topic aims to collect selected contributions from this PGPR Workshop. This Research Topic aims to bring together new findings and applications of PGPR.Since Professor Kloepper's retirement, the chronicles of a representative PGPR strain were summarized by former students in his laboratory at Auburn University (Jang et al., 2023 https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2023.1279896/full). Out of many PGPR species, Bacillus spp. received much attention due to their ability to form endospores that are resistant to biotic and abiotic stresses such as limited nutrients, hydration, heat, and pH. Bacillus velenzensis (formerly Bacillus subtilis and B. amyloliqufaciens) strain GB03 was the first Bacillis product developed by long-term collaboration with Professor Kloepper and Gustafson LLC, Plano Texas, USA. In this review, authors describe the process from isolation to commercialization as well as the mechanisms of plant growth promotion and biological control against soil-borne pathogens. The status of PGPR study in central Africa including Burundi, Rwanda, and the Democratic Republic of Congo (Nihorimbere et al., 2023 https://www.frontiersin.org/journals/plantscience/articles/10.3389/fpls.2024.1349357/full). PGPR provide a greater potential than chemical pesticides in the tropical region to manage the plant diseases occurring because the European Union banned 30% of the pesticides used in this region The knowledge and know-how transfer from scientists in global north countries needs to come through scientific collaboration.As we already recognized in 2023, the hottest year in human history reported by European Union's Copernicus Climate Change Service (https://www.ncei.noaa.gov/access/monitoring/monthlyreport/global/202313), the crisis of climate change is upon us. Due to climate change, the cultivation of crop plants can be limited. To combat environmental stresses, we need silver bullets to overcome these constraints. Elicitation of induced systemic tolerance by PGPR can be a solution to abiotic stresses in plants (Yang et al., 2009). Through biochemical and molecular evaluation, three bacterial strains isolated from tomato soil and roots were selected for plant responses under water stresses. The tomato's physiological changes were characterized following treatment with PGPR (Zampieri et al., 2024 https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2023.1297090/full).Heavy metals can also suppress plant growth directly. A metal tolerant PGPR strain Paraburkholderia ultramafica STM0279 T elicited metal tolerance in a tropical hub plant Tetraria comosa (Bourles et al., 2024 https://www.frontiersin.org/journals/plantscience/articles/10.3389/fpls.2024.1349724/full). The experiments were conducted in New Caledonian ultramafic soil also known as serpentine soil containing high level of toxic metals, Ni, Co, Cr, and Mn and low levels of essential plant nutrients N, P, and K. T. comosa, belonging to the Cyperaccae family, is a pioneer and abundant herbaceous member in New Caledonian ultramafic soil. Abou Jaoude ́ et al. (2024) https://www.frontiersin.org/journals/plantscience/articles/10.3389/fpls.2023.1332864/full gave a new insight into PGPR and their metaboliteinduced changes to plant photosynthetic activity as a parameter of plant ecophysiology under nonstress and biotic and abiotic stress conditions through metanalyses of the topic-related references.Collectively, PGPR provide greater potential to protect plants from environmental stresses derived from climate change.PGPR utilize plant protectants as a biological control tool against foliar and soilborne pathogens. Due to the experimental difficulty of growing non-culturable pathogens, testing the biocontrol efficacy by PGPR was limited. Here, two new results with unculturable aboveground and belowground pathogens shed light on a new strategy to control non-culturable and economically important pathogens. First, the cell wall degrading enzyme LytD from B. subtills plays a pivotal role in biological control against grape downy mildew caused by Plasmopara viticola (Wang et al. 2024 https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2024.1381018/full). The loss of function of the autolysin N-acetylglucosaminidase LytD in B. subtills GLB191 disrupts biocontrol capacity, biofilm formation, and leaf colonization. The authors provide the first example of the role of bacterial autolysin in plant protection by PGPR. Secondly, a group of scientist in Colombia targeted the management of another non-culturable fungal pathogen Plasmodiopora barassicae, a causal factor of clubroot diseases in cruciferous plants. The PGPR consortium reduced the disease incidence up to 56% indicating an efficient and integrated disease management tool even in the highly P. barassicae-infested fields (Morendo-Velandia et al., 2024 https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2023.1323530/full).Tanney and coworkers demonstrated that application of Bacillus sp., Pseudomonas sp., and two bacterial combination treatments increased the cannabinoid contents of cannabis (Cannabis sativa). Cannabis has received increased attention recently, although heavy legalization limits its cultivation under greenhouse and field conditions. PGPR inoculation triggered trichome development and increased cannabinoid amount only under a low nutrient regime for six weeks (Tanney et al., 2023 https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2023.1131346/full). In addition to PGPR-mediated increase of plant secondary metabolites, it is also reported that mineral uptake into vegetables was promoted by B. subtilis inoculation (Oliveira et al., 2023 https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2023.1248044/full). B. subtilis inoculation increased Ca, Mg, S, K, and N in shoot and as well as enhanced photosynthesis, intracellular CO2 contents, and water use efficiency. Guardado-Fierros and colleagues compared a classic indole-3-acetic acid (IAA) quantification method, the Salkowski method to ultra-performance liquid chromatography coupled with a Mass Spectrometer (LC-MS/MS) using the supernatant culture of ten PGPR strains resulting on overestimation on the IAA concentration with a maximum of 1,042 fold without tryptophan and 16,330 fold with tryptophan due to the detection of other indole compounds (Guardado-Fierros et al, 2024 https://www.frontiersin.org/journals/plantscience/articles/10.3389/fpls.2024.1378079/full). The demands for high quality and functional food are increasing worldwide. PGPR bring a great potential to induce beneficial plant secondary metabolite production or mineral uptake to meet consumer demand.

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