Rice is a very important food source for human population. Typically attributed to the diet of Asian people, rice is consumed all over the world and has some very beneficial effects for human health. However, growing rice is not so easy and crops may get damaged by some fungus.
Now scientists at the RIKEN Center for Sustainable Resource Sciences have found an enzyme, which is needed for synthesis of tenuazonic acid. It is a well-known toxin that is produced by multiple types of fungus and affects fruits, vegetables, rice, and other crops.
It turns out this enzyme is rather unique among known enzymes because of its structure. Toxic secondary metabolite production of fungi is called mycotoxins. They colonize crops and quickly become a real economic problem for the farmer. It is known to science that at least three different plant pathogenic fungi produce tenuazonic acid, which causes rotting in fruits vegetables, and food-crops, as well as post-harvest decay. This is why fungi are a real burden for farmers and, if not controlled, may even cause shortages of certain foods. Needless to say, scientists are rather happy about their discovery as it may help fight this problem in the future.
Takayuki Motoyama, co-lead author of the study, explained: “Now that we know the gene responsible for biosynthesis of this harmful toxin after further testing we might be able to devise a way to regulate its expression and prevent destruction of important crops”. Research was full of surprises from the very beginning. When scientists were researching microorganisms like fungus they found that genes for many secondary metabolites are silent under laboratory conditions. This means that finding these genes is a hard task for scientists.
Scientists assumed that the gene associated with responses to environmental stress, called OSM1, may be related to production of tenuazonic acid in a pathogenic rice fungus Magnaporthe oryzae. Even though wild-type of this fungus did not yield any of the acid scientists have been looking for, scientists managed to produce it from OSM1 knockout strains. They were also succeeded in producing the tenuazonic acid from cultured wild-type M. oryzae. These two methods to get the acid proved to be very useful in attempts to identify the genes behind this biosynthesis.
Team of researchers performed a DNA microarray analysis using the total RNA, which was extracted using both methods. They discovered only one gene, which was expressed significantly more in these conditions than when no toxin was produced. Scientists continued analysis trying to confirm whether this gene really is responsible for biosynthesis of the tenuazonic acid. They knocked out this gene and this yielded a strain, named by scientists TAS1, which could not produce the toxin. Then scientists created an M. oryzae strain that overexpressed TAS1. This experiment provided unsurprising results – this strain with overexpressed TAS1 produced the toxin under normal conditions.
In the next step of the research scientists analysed the structure of the TAS1 and to their surprise they found that it is a hybrid enzyme containing an NRPS region followed by a PKS region. Scientists expected structure to be PKS-NRPS, rather than NRPS-PKS, which has been found in bacteria before, but never in fungi.
Scientists also looked for homologues in other organisms. Although they did find several other species of fungi that have genes for homologues sharing the same domain structure, bacterial sources with similar amino acid sequences did not share the same characteristic domain structure. This means that research will need to continue in order to understand if particular enzyme has homologues that biosynthesize other compounds with useful biological functions.
This research may have several positive implications. The most obvious is crop preservation – preventing synthesis of the tenuazonic acid would save a lot of rice, vegetables and fruit that are crucial sources of nutrition. But this acid also has antitumor, antibacterial, and antiviral properties, which are very useful. This is why synthesis has to be researched further. Scientists will perform X-ray crystallographic analysis, which should reveal how TAS1 synthesizes the tenuazonic acid. This may lead to controlled biosynthesis when needed as well as preservation of farming goods.