Plant breeders regularly claim to have developed a new potato variety that is resistant to the harmful micro-organism Phytophthora infestans (see inset). By cross-breeding they have introduced a resistance gene that they think will keep the little fungus-like pathogen out. But Francine Govers, personal professor in Phytopathology and a leading expert on Phytophthora, never makes these kinds of claims.She knows that the stubborn pathogen cannot be stopped with a single resistance gene and will get around this new defence barrier sooner or later.
So WUR is looking for heavier weaponry with which to protect potatoes from Phytophthora infection. Firstly, Govers and colleagues at the Laboratory for Plant Breeding are looking into how they can bolster the potato’s defences using new techniques. Secondly, they are looking at how they can deactivate Phytophthora’s weapons, the so-called effectors. These proteins force their way into the potato plant’s cells while suppressing the plant’s immune system. And thirdly, the group looks at how you can ‘blind’ Phytophthora so that it can no longer find its way to the potato.
Scorched earth
Phytophthora has to get past two defence barriers to damage the plant. The first is the cell membrane, where receptors recognize invaders. About three years ago, Plant Breeding found a receptor that recognized all Phytophthora infestans strains. PhD candidate Manos Domazakis did further research on this, but could not find any leads for strengthening resistance.
Phytophthora infestans mutates easily and can make hundreds of different attacker proteins.
The second line of defence lies inside the plant cell. Cells of potato varieties with resistance genes that have been introduced through cross-breeding make proteins that cause local cell death around the locus of infection. This scorched earth policy works well, but there is a problem. ‘Because Phytophthora mutates easily or throws away pieces of DNA, and can make about 300 different effectors, there are always strains that are not recognized by the new varieties. For a watertight defence system you need multiple resistance genes, each of which recognizes a different effector,’ says Govers.
Gunpowder
So it turns out to be no easy matter to hermetically seal the potato’s defence barriers against Phytophthora. So Govers’ group wants to find out more about the attacker’s weaponry. There are very many species of Phytophthora and each species makes its own arsenal of effectors. The only thing these different effectors have in common is a small motif of four amino acids. But this is a crucial detail, because all effectors need that motif in order to be able to penetrate the plant cell. By way of analogy: if gunpowder is used in all attacks, you only need to make gunpowder harmless. So the researchers are hoping to find something that can disrupt the activity of this motif of four amino acids.
Govers’ PhD candidate Chara Schoina found 3 genes in Phytophthora that code for enzymes that may contribute to the functioning of the motif. Let’s call them the ‘gunpowder genes’. Further research on two of these genes revealed that they make an enzyme that cuts effectors into pieces, and this cutting could be essential for getting the effectors into the plant cell.. Govers: ‘If you can put the brakes on this enzyme, Phytophthora finds it more difficult or even impossible to penetrate the plant cell.’ Further research is needed, but this may be the beginnings of a form of biological pest control.’
Switching off antennae
Meanwhile, Govers’ group is digging deeper into the secrets of Phytophthora, taking a leaf out of the medical sciences’ book. ‘Medical scientists look at very specific processes in pathogens, for example a unique biochemical reaction,’ says Govers. ‘If you bring a process like that to a standstill, you’ve conquered the disease – that’s how a lot of medicines work.’
PhD candidate Johan van der Hoogen looked at the GPCRs, a group of receptors in cells that signal what is going on outside the cell, such as the presence of pheromones or a physical barrier. Humans have about 1000 GPCRs, Phytophthora has 132. Among the pathogen’s GPCRs some probably ‘feel’ the proximity of a potato plant and then activate the attacker proteins. If you switch off these antennae, Phytophthora loses its capacity to locate potential victims.
Alternative disease control
‘In one case we already knew that without a particular GPCR, Phytophthora loses all its sense of its surroundings,’ says Govers. But Van der Hoogen discovered that this receptor was present in 21 species of watery microbes that live in the same habitat as Phytophthora. It remains to be seen whether he can deactivate this GPCR without damaging an entire ecosystem. Four other types of GPCRs are unique to Phytophthora, Van de Hoogen discovered. Such unique receptors offer starting points for the development of alternative pesticides that do not affect other organisms. One possible ingredient in such a pesticide is a ligand, a molecule that specifically activates a GPCR. So you can use it to overwhelm and disable a receptor, in just the same way as a DDOS attack disables a computer with a barrage of messages. Research is ongoing on whether a flavonoid could serve as a ligand and could disable Phytophthora’s antenna for its environment.
Weak points
‘We are looking for Phytophthora’s weak points,’ is Govers’ summary of the research strategy. And although the search is far from over, slowly but surely the contours of an alternative approach are beginning to emerge. One which will hopefully make the chemical pesticides currently in use superfluous in the long term, and which can limit the economic damage caused by potato late blight.
Macro damage by micro-organism
Phytophthora infestans is the cause of the potato late blight disease. The oomycete (water mould or pseudo fungus) damages the leaves and stems of potato plants. Currently, the disease is mainly controlled by spraying the crop with chemicals, which costs Dutch arable farmers between 80 and 120 million euros per year, according to a WUR estimate. The farmers also lose about 11 million euros a year in harvest losses caused by the disease. Worldwide, the costs mount up to billions. The pesticides also cause serious environmental harm.