As Christmas draws near, growers are enjoying the time off to relax with family before planning for next year’s growing season. Some of you may ponder while drinking your hot cocoa and worry about your soil nutrient levels. You may also have heard about PGPR while considering your fertilizer options. So you may be wondering: what does PGPR stand for or what are they and how do they work? In this week’s edition of Growing Possibilities, we will be discussing what PGPR are and how they work.
Although we typically view plants as individuals in the field, they actually are part of a complex community. This community is comprised of the plant and the phytomicrobiome, which is the microbial community in the soil. The phytomicrobiome is elaborate and diverse, and a portion of it is comprised of PGPR (Plant Growth Promoting Rhizobacteria). The PGPR refers to bacteria that are part of the phytomicrobiome whose activity has a beneficial effect on the growth or health & performance of that plant. Legume farmers are generally familiar with the most common form of PGPR utilised in the crop production: nitrogen (N)-fixing bacteria, most commonly found as the active ingredient in inoculants. Legume farmers are used to inoculating their crops with rhizobia every year but many still do not know about the plant health benefits provided by other forms of PGPR. For example, the PGPR found in XiteBio® Yield+ has several modes of action including solubilising soil-fixed phosphorous (P) to increase phosphate availability to plants or by encouraging early root development and increasing root hair growth. These bacteria do not form nodules like the N-fixing species do, but colonise the surface of a plant’s roots and promote growth and enhance performance by other means (1).
As we are well aware of, P is of finite supply and we are slowly running out of it. Phosphorous fertilizer is extracted from rock phosphate, which is of limited supply. After P is applied to the ground, it can become fixed to minerals like calcium (Ca) and aluminum (Al) or precipitate as phosphate minerals. Both of these forms of P are unavailable to plants and are therefore “lost” in the field. Due to P-fixation and loss, farmers need to apply up to double what the crop requires in order to combat this issue (2). Fortunately, several members of PGPR, called phosphorous solubilizing bacteria (PSB) or phosphorus solubilizing microorganisms (PSM) can free these bound P and make it available for the plants again. These PSM can release compounds like acids into their surroundings, which then causes the phosphate to be released from the Ca and Al. Some PSM can also produces siderophores, which make iron more accessible to the plant, and phytohormones, which can help plant growth by inducing root growth, shoot growth, and increased tolerance to stress (1).
So you may be thinking the more the merrier, but that isn’t the case. A higher bacterial count is not always better. Having too high of a bacterial count will lead to competition for space and nutrient resources. If bacteria are competing for resources with each other, they will not be able to help the plant grow. In the case of N-fixing bacteria, nodulation will be negatively affected and N-fixation will suffer as a result. Whereas with PSM, P solubilisation will suffer and there will be a reduced amount of phytohormones produced. Therefore, providing the optimum number of healthy & active bacteria while preventing resource competition is best (3). An easy way to ensure you are getting the benefits of PGPR is to apply XiteBio® Yield+ into your fields next spring. XiteBio® Yield+ contains an optimum number of the PSB Bacillus, which maximizes the possible PGPR benefits while ensuring the competition for resources is minimal.
References
- https://www.frontiersin.org/articles/10.3389/fpls.2018.01473/full
- https://www.frontiersin.org/articles/10.3389/fmicb.2018.02054/full
- https://www.frontiersin.org/articles/10.3389/fpls.2020.00006/full