Publication: Genome sequence of the metazoan plant-parasitic nematode Meloidogyne incognita

Published on
July 29, 2008

Root-knot nematode revealed; DNA-analysis shows enormous variety of ‘attacking enzymes’

One of the champions among plant parasites, a miniscule worm which penetrates the roots of numerous crops, has finally been forced to reveal some of its secrets. Scientists at Wageningen University have partnered with an international team to unravel the entire DNA-sequence of the root-knot nematode (Meloidogyne incognita). The results will soon be published in Nature Biotechnology. Their research shows that the root-knot nematode has a large number of enzymes which are deployed to ‘attack’ plants - probably the reason why the nematodes can attack so many different crops. These new findings will be used to develop fresh and much-needed strategies for dealing with these tiny worms.

Root-knot nematodes are, like the well-known cyst nematodes, worms of around one millimetre in length that feed on plant roots. The nematode larvae penetrate plant roots and drill their way through the root tissue. Thanks to specific compounds in their saliva, they change some cells near the vascular cylinder in what could be called a ‘food factory’. The nutrients transported via the vascular cylinder end up in these feeding cells and are absorbed by the larvae, causing the plant to lose a large amount of nutrients. In addition, the feeding cells block the water transportation from the roots, as a result of which the plant cannot transport sufficient water to its leaves.

On a global scale, the crop damage caused by these nematodes is estimated at more than 100 billion euros a year. Although the nematodes are currently controlled mainly by means of chemicals, many of these are harmful to the environment and increasingly being banned. The need to find other methods is pressing.

Nematode scientists around the world have been mystified for decades. Which chemicals in the secretion lead to the creation of feeding cells? And, especially in the case of the root-knot nematode, how do the nematodes succeed in evading the defence system of so many different plant species? While other plant parasites are specialised in a certain plant family or even one plant species, the root-knot nematode can grow on virtually every species of plant, making it extremely difficult to control the problems caused.

Now that the entire DNA-sequence has been revealed, it will become much easier to see which weapons the root-knot nematode uses to attack so many plant species. This will allow scientists to develop new ways to battle these nematodes.

In their publication, the scientists show that root-knot nematodes have an enormous cocktail of enzymes degrading plant cell walls. They found five ‘enzyme families’, each with a large number of family members. Oddly enough, these enzymes look suspiciously like the enzymes used by fungi to degrade cell walls. The scientists suspect that genes from the fungi have been transferred to the nematodes during evolution. This horizontal gene-transfer phenomenon often occurs in bacteria, but is virtually unknown in higher organisms.

How root-knot nematodes evade plant defence mechanisms will remain a mystery for a while longer. Many of the proteins suppressing the plant defence system that were identified by the Wageningen scientists in potato cyst nematodes appear to be completely absent in root-knot nematodes. The major difference between gene copies is also remarkable. Normally an organism has two practically identical copies of each gene on their chromosomes. In addition to the fact that the root-knot nematode sometimes has three copies, these copies are also abnormally different from each other. It seems that during evolution this nematode has succeeded in creating such a wide variation in its genes that it is capable of infecting a great diversity of plants.

In addition to identifying these cell wall degrading enzymes, DNA-coding also allowed the scientists to identify secreted proteins that are probably involved in transforming root cells in ‘food factories’. The further analysis hereof and the comparison with secreted proteins found previously by Wageningen scientists in the beet cyst nematode will shed more light on the exact function of the enzymes. With this knowledge, the scientists aim to develop new strategies to deal with root-knot nematode problems.