bacteria
Fungi and bacteria boost resistance to pests and diseases
More and more research is revealing that endophytes – microorganisms that live in the plant without harming it – can significantly boost a plant’s resistance to pests. These findings prompted researchers to investigate the potential of endophytes in pest control in greenhouse horticulture. Experiments in tomato, sweet pepper, bedding plants and chrysanthemum have shown that while the fungal isolates tested thus far don’t deliver complete pest control, they do significantly inhibit population growth.
Endophytic fungi are therefore of particular interest alongside other control measures, but there are still many unanswered questions about how they work and how to use them. A recently launched fundamental project on endophytes is expected to provide more basic knowledge to support applied research into endophytes.
Entomopathogenic fungi
The study of microbial ecosystems in humans, animals and plants has been the focus of much research in recent years. Just as gut flora is essential for good health in humans, microorganisms in plants also play an important role in resilience to pests and diseases. Precisely how this works is not always clear. Fungi can have a direct effect on pests by secreting certain metabolites, but it has also been shown that fungi and bacteria in plants can accelerate resilience in the plant (known as priming).
Some of these fungi are also known as entomopathogenic fungi, such as those of the Beauveria, Metarhizium, Lecanicillium and Isaria genera. Many of these fungi are used as a product in crops to control insects such as whitefly and thrips. We are increasingly discovering that most of these types of fungi can also grow in plants without harming the plant. Over the past two years, Wageningen University & Research in the Netherlands has been running a number of studies looking at the effects of endophytes on pests that are relevant to greenhouse horticulture.
Inhibiting aphid growth
Aphid control in greenhouse horticulture is still often based on the use of chemical crop protection products, but new regulations and customer requirements are making chemical control increasingly difficult. The alternative – biological control with natural predators – is not always as effective. Aphids are known for their ability to grow explosively on plants, making it difficult for natural predators to get the infestation under control fast enough. Measures that could inhibit the explosive growth of aphids would therefore be a welcome addition.
A study funded by the Dutch Horticulture Product Board was set up to investigate whether endophytic fungi could inhibit the growth of aphids in sweet pepper and the bedding plant Calibrachoa. Many isolates produced no effect at all, but the fungus Lecanicillium muscarium was able to inhibit the increase in aphids by 25-30% in both Calibrachoa and sweet pepper. This may not seem very much, but on a rapidly growing aphid population and in combination with other pest control methods, it can make a big difference. In further research, it is being investigated whether these effects actually result in better control when combined with natural predators.
Thrips
Another major problem in greenhouse horticulture is the western flower thrips, or Californian thrips, Frankliniella occidentalis. This pest is particularly difficult to control in ornamental crops, partly due to the ever diminishing package of chemicals available and growing resistance to pesticides. In a large public-private project (Masterplan Thrips), various endophytic fungal isolates are now being screened for their effectiveness against these thrips in ornamental crops.
In the fight against thrips, increased plant resilience would be a welcome addition alongside other measures. More than 40 fungal isolates have already been screened, some of which show significant inhibition of 40-50% in the population growth of thrips. Here too, therefore, what we are seeing is not complete control but inhibition of growth. However, the results are promising and the expectation is that they will provide a very useful addition to the various measures for keeping thrips in check. In research carried out in other countries, good results have also been achieved on the tobacco thrips in onion, with endophytes reducing thrips damage by 80%.
Further research
The next step is to take a look at potential applications in greenhouse horticulture. As with aphids, the question in terms of thrips is how to combine endophytes with biological pest control. A delayed development time could be beneficial for predatory mites, which would then have longer to work on the susceptible stages (predominantly L1), but there may also be effects that impact negatively on predatory mites. These and other predators could be affected by certain substances that enter their prey (the pest) via plants containing endophytes. So it is important to test the effects in the overall system of natural predators.
The initial results with endophytes in greenhouse horticulture indicate that there is definitely potential for using these fungi to increase resilience to pests. However, there are still a lot of questions to be answered in the research: for example, what spectrum of pests they are effective against and whether they have any adverse effects on some pests and plant pathogens. Other very practical questions concern the methods of inoculation and longevity during cultivation. It has been found that there are several types of microorganisms in plants (bacteria and fungi) that interact with each other but also with the host plant. These interactions are complex and as yet not fully understood, but they will certainly affect the behaviour of introduced endophytes.
Endophyte project
The basic questions surrounding the plant microbiome are being investigated in a new fundamental endophyte project led by Leo van Overbeek of Wageningen Plant Research. In this project, he initially wants to determine how endophytes can best be introduced into the plant. He is looking at methods of application in practice, such as via treatment of seed or young plant material, possibly in combination with irrigation water treatment.
The researchers are also taking a close look at how endophytes behave in a plant, for example which parts of the plant they spread to which also contain the various pests and plant pathogens.
Measurable effects
Using modern DNA sequencing and other so-called ‘omics’ technologies, including metabolomics, the research is also determining how and to what extent the microbiome of the plant is influenced, based on a) the presence of the causative agent, b) the presence of the endophyte applied, and c) the combination of both. This should shed light on how pests and endophytes respond to each other and what factors play an important role in that.
The hypothesis is that endophytes will affect the composition of the microbiome, which would be expected to have an effect on the metabolism of the plant and, in turn, on pests and diseases. The researchers hope to be able to correlate measurable effects with parameters that provide an indication of a plant’s resilience. This fundamental project is expected to culminate in an integrative model that indicates how to boost resilience in plants by applying endophytes. The model will then be used to develop new technologies to make plants more resilient to pests and diseases.
Summary
Fungi and bacteria that live in plants can help make the plants more resilient to pests and diseases. It has been shown in a range of crops that population growth in aphids and thrips can be inhibited by certain endophytic fungi. Further research is looking at developing practical inoculation methods for different crops, integration with natural predators, endophytic behaviour in plants and the underlying mechanisms that increase plant resilience with endophytes.
Text and images: Gerben Messelink and Leo Overbeek, Wageningen University & Research.
Only a systematic approach can limit excessive root growth
Virtually nothing can be done about excessive root growth. However, this troublesome bacterial disease doesn’t always get the chance to develop. Other microorganisms that live in the root environment fight against its rapid spread. The search for underlying mechanisms is gradually progressing. It’s a brainteaser for the researchers.
Genetic changes have always occurred and they take place in nature all around us. Rhizobium rhizogenes for example, is a bacterium that very successfully infects plants with pieces of its own DNA. These pieces (T-DNA) merge with the plant’s DNA, which as a result behaves somewhat differently. For example, it can become more sensitive to the hormone auxin. This sets off explosive root growth. The roots then excrete substances that create a favourable environment for the bacteria to thrive, which perpetuates the situation further.
These ideal living conditions for the bacteria have large consequences for the host. Infected crops such as tomato, aubergine and cucumber show more vegetative growth and production lags behind. At least 400 plant types are susceptible to these bacteria and it can cause big problems for the greenhouse crops mentioned.
Difficult to grasp
Over the last decade a lot of research has been carried out into the mechanism that causes this abnormality and into methods (hygiene measures) to prevent the spread of the bacteria. Marta Streminska and Ineke Stijger, both researchers at Wageningen University & Research Greenhouse Horticulture, are researching a new strategy to prevent excessive root growth in collaboration with the Dutch growers’ association LTO Glaskracht Nederland, Rijk Zwaan and Koppert Biological Systems. This is within the project ‘Next Generation Plant Health’.
“It is still very difficult to get a grip on this disease,” says Streminska. “One year we see a lot of infection, another year hardly any, sometimes even on the same nursery and under exactly the same circumstances. We still don’t know why that happens.” There is no chemical cure. Once the plant is infected, the DNA changes forever. Since no solution can be found through chemical means, except the thorough disinfection of the watering system, a biological approach offers the best potential. In this respect the activity of microorganisms around the plant’s roots plays a central role.
Fathom out the system
Within the plant health project the two researchers are looking into all the fundamental principles that could set off or stop the excessive growth of roots. Until now this research has mostly been carried out within the four walls of their laboratory. The results and next steps are regularly discussed with the project’s committee of growers from the national committees for tomato and eggplant. “We are not looking for an antibiotic as this would increase the risk of resistance build-up. We are studying the entire microbial system,” explains Streminska.
Previous research showed that the symptoms of excessive root growth are not always observed in old slabs. These old slabs contain an established, stable microbial environment that it is not going to be pushed around by a wrongdoer.
Used substrate slabs
Therefore the researchers are now studying fungi and bacteria that were isolated from used substrate slabs to try and fathom out the microbial mechanisms involved. In addition they are testing a range of biological products that could offer solutions. In one tomato trial for example slabs were used that were treated with useful microorganisms. Here too excessive root growth reduced.
“The systematic assessment of all microorganisms is a huge undertaking,” says Stijger. “We want to know exactly what happens in the slabs. If there is a substance or microorganism that can slow down the explosive growth of this bacteria you could add this to new slabs.”
Disrupt communication
From research into human medicine we know that bacteria don’t strike without warning. Instead they wait until an ‘army’ of bacteria has built up and then launch a joint attack. Bacteria communicate with each other via the production of signal substances. Different bacterial types produce different signal substances. For example, Bacillus makes different signal substances than Dickeya or Rhizobium. Streminska and Stijger are looking at ways of disrupting this communication system to prevent a joint attack from occurring.
There are therefore several ways to approach the problem. The researchers are cautious about drawing conclusions too early. Stijger: “Even if we start to understand why excessive growth reduces in one crop, it does not automatically mean than it will be the same for another crop. Besides the root environment contains many organisms that work together.”
More than 90% of all the bacteria present live in symbiosis with the plant’s roots and actually stimulate growth. If you add organisms or substances that reduce the excessive root growth the rest still need to remain alive. If that doesn’t happen other undesirable changes might occur.
Disinfect and protect
In anticipation of the results the researchers sketch a scenario that could be used in greenhouse vegetable production. Firstly before the crop is planted the slabs and watering system need to be scrupulously clean. Opinions are divided on the cleaning effect of hydrogen peroxide and some people have preferences for certain types of brands or compositions. Stijger is not sure if this is a valid case. She does know that some strains of bacteria are more sensitive to peroxide than another. “The problem is that bacteria spread rapidly throughout the system if just a tiny little bit is left behind. The bacteria appear to protect themselves with biofilm that is very difficult to remove.”
Then, immediately after disinfection has taken place, the new substrate slabs have to be injected at the start of the cultivation with substances or microorganisms that will create a stable living environment for the plant’s roots to grow. As a result pathogenic bacteria have no chance to multiply and launch an attack.
Streminska and Stijger finished most of their laboratory research in 2016. They are now starting to test the substances on young plants.
Summary
Two years ago researchers in the Netherlands began a systematic approach to solving excessive root growth. Ineke Stijger and Marta Streminska virtuously studied the natural substances and organisms that could slow down the explosive development of Rhizobium bacteria. It is gradually becoming clear that a great number of different organisms are needed to develop a stable and resilient system.
Text and images: Pieternel van Velden