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Microbial breakdown of Bt maize plants

Maize litter and micro-organisms

It is the middle of April and there is stubble as far as the eye can see. There are still a few weeks left until the next crop of maize will be sown and the trial field lies abandoned. Scientists from Müncheberg have arrived to collect samples of the maize litter that was left on the field after the harvest last autumn. They are investigating whether the cultivation of Bt maize has an impact on soil-dwelling bacteria and fungi that break down maize litter.

After havesting, the plant residue remaining on the field is collected at different time points to investigate the decomposition of the mulch and the microorganisms involved.

Ben Bubner and Regina Becker from ZALF take samples three times a year from each plot.

Dr Andreas Ulrich is head of the working group at ZALF Müncheberg that is investigating the breakdown of Bt maize litter as part of a BMBF-funded joint research project.

Maize stubbles

The harvested maize trial field.

Maize roots

Some of the maize roots burrowed down into the soil in stages.

Martina Wiemer uses an infrared gas analyser to measure how well the maize litter is broken down by micro-organisms.

Level of CO2 released during litter breakdown by micro-organisms over 100 hours. In the first year of the trial there were no significant differences between Bt maize and the isogenic parent variety, but there were differences between these two varieties and the other two conventional maize varieties being cultivated on the trial field.

Ben Bubner grinds the maize litter that had previously been flash-frozen with liquid nitrogen.

Finely ground maize litter, the starting material for extracting DNA and RNA.

Genetic profiles of the bacterial communities that break down the litter of the different maize varieties. Each peak represents a different group of bacteria. In the first year of the trial there were no significant differences between the Bt maize, the isogenic parent variety and the other two conventional maize varieties.

Regina Becker and Ben Bubner are moving over the trial field equipped with plastic bags and latex gloves. They take samples three times a year during the trial: four and eight weeks after harvest and shortly before the next crop is sown in the spring. A strip runs straight through all the plots. It contains the remains of whole maize plants lying on the surface. On this strip, unlike the rest of the field, only the grains were harvested. The plants are by no means fully decomposed; remains of maize ears, tassels, stems and roots can be seen all along the strip. Regina Becker and Ben Bubner collect a handful of maize litter from each plot. After an hour and a half they have all their samples and return to Brandenburg.

Müncheberg is a small town on the edge of Märkische Schweiz, 50 kilometres east of Berlin. Since 1928 this site, now the Leibniz Centre for Agricultural Landscape Research (ZALF), has been used for research in the areas of plant breeding, crop farming and soil fertility. Today, a large number of research groups work here, studying various agricultural and forestry topics. Within ZALF’s Institute of Landscape Matter Dynamics, Andreas Ulrich’s team is studying genetically modified Bt maize.

Over the past few years, some scientific studies have found differences in microbial breakdown between Bt maize varieties and their conventional parent varieties. One possible cause being debated is differences in the composition of the plants, e.g. their lignin content, which researchers believe could be the indirect result of transgene integration in the genome. Slower litter breakdown could lead to problems with crop rotation, e.g. a build-up of crop moulds (fusaria). Andreas Ulrich and his team want to find out whether there are differences in litter breakdown on the maize trial field in Braunschweig. They are comparing the different varieties to see how fast the litter is broken down and which bacteria and fungi are involved.

Varietal effects in litter breakdown Martina Wiemer’s workplace consists of glass tubes, hoses and pumps. The equipment used to measure the rate of litter breakdown fills a small room. Martina Wiemer mixes a precise amount of ground maize litter and soil from each of the plots. She pours the mixture into glass tubes, through which air is passed continually for 100 hours. When the micro-organisms in the plant remains and soil samples break down the ground maize litter, they use up oxygen and produce carbon dioxide. An infrared gas analyser measures the CO2 concentration in the air before and after it is passed through the samples. The faster the ground maize litter is broken down by the micro-organisms, the more CO2 is produced.

In the first year of the trial, there were no significant differences in maize litter breakdown between the Bt maize and the isogenic parent variety. However, the researchers found that the litter from the other two conventional varieties grown on the same trial field is broken down at a significantly slower rate than the litter from the Bt maize variety and its parent variety.

Genetic profiles: no differences White fumes waft over the work surfaces in the molecular biology laboratory. Just for a moment, as Ben Bubner pours liquid nitrogen over the maize litter, it is like standing in an alchemist’s kitchen. Flash-freezing the maize litter in this manner enables it to be ground into a very fine powder with a mortar. DNA and RNA are then extracted from the ground litter.

PCR, a molecular biological method, is then used to amplify gene segments that occur in all the bacteria and fungi under investigation and which are known to display group-specific sequence differences. These sequence differences cause the recognition sequences of a restriction enzyme to appear or disappear. This means that treating the PCR products with restriction enzymes results in DNA fragments of different sizes. It is possible to sort these by size using electrophoresis, which produces a characteristic peak pattern, or genetic profile, for each maize variety, with each peak representing a particular group of bacteria or fungi.

In the first year of the trial there were no significant differences between the Bt maize, the isogenic parent variety and the other two conventional maize varieties in terms of the composition of the bacterial and fungal communities. However, soil composition was found to have a significant influence. To obtain conclusive results, Andreas Ulrich and his team will have to make a few more trips to Braunschweig to collect maize litter.