Dear Kelli,

I think that you have asked a really interesting question which reflects the way in which we view the 'role' of bacteria we isolate from a variety of environments. Obviously, you might isolate a bacterium from the infected tissue of a plant and go on to demonstrate that it can re-infect the plant causing the same symptoms, but it is more often that you isolate a bacterium from an environment where it may be performing a number of different roles - so how do you categorise that isolate?

A role can only be tested and interpreted in a particular environment; change the environment and you may find that the role is changed too! Similarly, the presence of a gene or genes for some type of response may be of value in one environment, but may remain unused in another. Furthermore, the phenotype you determine in laboratory conditions may not reflect the nuanced responses you might see in the environment in which it was isolated.

Andrew

Nice question. In my opinion there is no differentiation between PGPR and pathogen based on hormone production. And mechanisms of hormone production in these two groups are also the same in most of the time. for example Fusarium is a plant pathogenic fungi and produce IAA and of course induce plant height but with the other pathogenic aspects cause plant disease. In the other hand Azospirillum is a PGPR bacterium and produce IAA also and cause plant growth. So hormone production is not a key role on diagnostic pathogenic or PGPR microorganism. You should consider a set of features for this.

Best,

Negar

Kelli Thorup Standard procedures for many plant pathogenic bacteria testing include hypersensitivity reaction test (HR, 12-24 h) on tobacco plants (I prefer Plectranthus verticillatus), pectolitic activity test (potato slices, 28oC 48 h), and seed (brassicas or solanum are the most sensitive) germinatation test for toxins. For xylem - infecting bacteriia - excised-cotyledon assay (3-5 days). For tumor-inducing bacteria (Agrobacterium) - 4-weeks test of carrot slices or some other plants. You can do it before plant growth experiment.

Dear Kelli,

it's a good question to have a broad discussion over this, for most of the PGPR researchers this scenario has to be faced most of the time. For example, the Pseudomonas group produces a broad range of metabolites/hormones/volatile compounds that have a positive effect on plants. But in this group if you see Pseudomonas aeruginosa is a very good plant growth promoting agent but it is a human pathogen. So sometimes being bad also has some good nature. I also wonder sometimes whether these pathogenic microbes of the same species share some antibiotic resistance or hormone production genes through horizontal gene transfer. We see this similar method in most pathogenic Xanthomonas species.

Practically, i think after characterization of isolates and molecularly identified the strains with best potential plant growth promoting (PGP) activities by16S rDNA sequence analysis you should eliminate the known pathogenic strains.

Kelli Thorup Current experience show that some emerging plant pathogenic bacteria (and fungi) evolved from the applied PGPR (biocontrol agents).

Kelli Thorup I am also curious about this selection criteria. Yeah, after isolation we can proceed with the 16S rRNA sequencing to identify the strain. If it's pathogenic, we try to avoid that organism. But, really sometimes we find out that pathogenic strains do have better phytohormone-producing abilities than most of the isolated PGPR strains. If you want to screen it before molecular identification, then I think that you can check whether the organism is capable of producing ACC deaminase enzyme, because most of the phytopathogens try to increase internal ethylene levels in plants to speed up the senescence process, whereas PGPR try to decrease internal ethylene levels by producing ACC deaminase enzyme that breaks down the immediate precursor of ethylene (ACC). But there is a problem that not all PGPRs are capable of producing ACC deaminase, so it's better to screen for the hypersensitive reactions.

Dear,

To differentiate a potential Plant Growth-Promoting Rhizobacteria (PGPR) from a potential pathogen based on hormone production before conducting a plant-growth experiment, you can employ several techniques and considerations. Here's a step-by-step guide:

1. Hormone Profiling:

- Begin by isolating and culturing the microorganism of interest.

- Conduct hormone profiling by analyzing the types and quantities of phytohormones it produces. PGPR typically produce growth-promoting hormones such as auxins (e.g., indole-3-acetic acid or IAA), cytokinins, and gibberellins.

2. Pathogenicity Assessment:

- Evaluate the potential pathogenicity of the microorganism. Look for any evidence in the existing literature or through preliminary tests that suggest it may have pathogenic characteristics. Pathogens often contribute to plant diseases.

3. Functional Assays:

- Perform functional assays by applying the microorganism to plants and assessing their growth responses. PGPR strains should enhance plant growth, while potential pathogens may lead to stunted growth or disease symptoms. Monitor plant health and any visible signs of disease.

4. Antagonistic Activities:

- Test for antagonistic activities. PGPR strains often exhibit antagonism against phytopathogens, which can help protect the plant. Evaluate if the microorganism has antimicrobial properties or can inhibit the growth of known pathogens.

5. Genetic and Molecular Analysis:

- Conduct genetic and molecular analysis to identify specific genes associated with hormone production. PGPR strains often have genes responsible for the biosynthesis of plant growth-promoting hormones. Analyzing the genetic makeup can provide insights into the microorganism's potential role.

6. Consultation with Experts:

- Seek advice and consultation from experts in the field of microbiology, plant pathology, or plant-microbe interactions. They can offer guidance on differentiating PGPR from potential pathogens based on their experience and knowledge.

7. Pathogen-Specific Markers:

- Utilize molecular markers or tests specific to known plant pathogens to check for the presence of pathogenic genes or characteristics. This can help rule out the potential for pathogenicity.

8. Host Range Testing:

- Assess the microorganism's host range. Some pathogens have a narrow host range and may only infect specific plant species, whereas PGPR strains often exhibit a broad host range. Conduct host specificity experiments to gauge this.

9. Long-Term Observations:

- Extend the duration of your experiments to observe the effects over a longer period. Pathogens may take longer to exhibit their detrimental effects, while PGPR typically show more immediate and sustained growth promotion.

By combining these techniques and considerations, you can differentiate potential PGPR from potential pathogens based on hormone production. This comprehensive approach helps ensure that you select beneficial microorganisms for your plant-growth experiments while minimizing the risk of introducing pathogens that could harm your plants.

Good luck.