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.”
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.
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