Salix is our most important biofuel crop, grown on more than 10 000 hectares arable land in Skåne, Västergötland and Mälardalen. This figure may well increase in the future as the demand for renewable energy is rising both in Sweden and abroad. The system is however sensitive to the fungal pathogen Melampsora larici-epitea. Breeding programs have resulted in more resistant tree types, but the fungus is prone to change and an established salix field is expected to stand for 25 years, which gives the pathogen ample time to adapt to the resistance.
Important gene for resistance to rust fungi attacks discovered in willow
A gene thought to be of great importance to the ability of willow to identify an attacker has been identified by researchers from SLU. Two versions of the gene have been discovered, and one of these appears to make individuals more resistant to rust fungus infections.
Over the past decades, the importance of willow has grown steadily. The crop is used exclusively for bio fuel, and is as such a great substitute to fossil fuels like coal and oil, due to its fast growth, ability to grow on soil not suitable for food production, and the fact that it is considered to not add to the levels of atmospheric carbon. Nevertheless, like all industrially grown crops it is subjected to pathogen attacks. The direst threat comes from a rust fungus called Melampsora larici-epitea. This fungus causes damage on the leaves, which reduces their photosynthetic activity and may cause them to be shed early. Thus, the most pressing need for willow breeders has been to develop rust resistant tree types.
Abusing host metabolism
Melampsora is, like all rust fungi, a biotroph. This means that its primary goal is not to kill and degrade cell tissue, like necrotrophs do. Instead, biotrophs seek to avoid being detected and stealthily colonize the cells of its host, in order to hitch-hike on host metabolism for its own sustenance. As soon as the host recognizes the presence of the pathogen, the infected cells are closed down and destroyed, preventing further spread of the parasite. This means that the biotroph’s relative success as a pathogen is primarily decided by how adept it is at concealing itself to the host. The plant’s ability to recognize an intruder is regulated by resistance genes, of which the most prevalent class is called NBS-LRR. LRR is the part that governs pathogen identification, reacting to molecular patterns in factors being secreted by the microbe or anchored in its cell membrane. The intruder may then counter by concealing these factors, or by blocking the host's receptors. The host may in turn adapt a sensitivity towards the blocking molecules, and so forth. This system is highly prone to change and for obvious reasons subject to a high evolutionary pressure. Breeding programs have on several occasions successfully developed willow types resistant to the rust fungus, but the high level of plasticity allows the attacker to in due time overcome this resistance and again cause disease.
An identified resistance gene
Thus, it is important for breeders to identify new and more efficient resistance genes in willow. A research team at the Department of Forest Mycology and Plant Pathology at the Swedish University of Agricultural Sciences has previously identified an area on the willow genome that appears to control much of the resistance against Melampsora infection. In a new study, this area is being investigated. Berit Samils is one of the authors of the study.
"The genomic area of interest represents up to half of the variation in resistance in our population of study, which of course makes it highly interesting for breeding purposes. Using DNA analyses, we have now investigated this region and discovered a complete NBS-LRR gene, or a resistance gene", Berit says.
To investigate whether the identified gene really is the one causing the variation in resistance, the researchers compared its sequence in a willow known to be resistant, and in one known to be susceptible to rust disease. It turned out that there are at least five differences between the sequences, which would all cause alterations in the resulting protein. Three of these differences were located in the pathogen recognizing LRR part of the gene.
Renders the host blind
"The next step was to elucidate whether these different gene types behaved differently during fungal attack", Berit Samils says. "We infected willow trees carrying the respective type of the resistance gene with Melampsora, and subsequently measured gene expression, that is, the activity of the gene. We saw that the more sensitive tree had a lower gene expression after 24 hours of infection than it had before being infected, and it was always lower than in the resistant tree, no matter whether infected or not."
Jan Stenlid is another of the authors of the study. "The sinking gene expression during infection indicates that the invasive fungus is causing this in order to avoid detection, and that a critical level of expression is needed for such detection to occur", Jan says. "In effect, the fungus attempts to render the willow blind, or at least impair its vision. At the same time, infection of the resistant individual does not affect expression levels. This tree discovers the parasite immediately and is able to assemble its full responsive repertoire. These results suggest that the function of our new resistance gene is to identify the attacker, and has no role to play once this has happened."
"Unfortunately, a working transformation system is not yet developed for willow", Jan continues. ”Such a system is necessary to test the function of the resistance gene, by knocking the gene out or drastically boost its expression levels and then study the effect on resistance. Once such a system is online, it will give us critical information as to how this gene truly affects the ability of willow trees to defend themselves against Melampsora infections.
Read the full story here: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0168776