2 Volatile Organic Compound (VOC) based Diagnosis

Transcript

Hello!! Welcome back to the second talk of this week, and today we’ll be talking about Volatile Organic Compounds (VOC) based diagnosis of plant pathogens. Volatile organic compounds are released by plants and the pattern of or the profile of volatile organic compounds get changed when the plant interacts with its surrounding, or its atmosphere in a comfortable situation or with a stress conditions. So, that is the principle behind the pattern of different volatile organic compounds being released or emitted by the plants into its surrounding.

So, volatile organic compounds emitted from leaf surfaces are terminal metabolites of the host plant and can indicate its physiological health status. Volatile organic profiling may describe ‘plant-to-plant’ and ‘plant-to- pest communication and therefore it’s gaining importance. VOC markers like Hexenols, hexenals, hexanyl esters and classes of terpenoids and indoles may help in rapid discrimination of fungal infection and insect vector feeding. Because the pattern of emission of this particular compounds get changed and it follows a definite pattern when it is infected by a fungal pathogen and when it is being feeded by a Herbivory Insect. So based on this whether the plant is being challenged by an insect or certain fungal pathogens then release of all VOCs get varied.

So applications of VOC profiling and there are certain success stories. Let us go into it. There was a classical example of peanut plants that is Arachis hypogaea were the volatile organic compound profile of a healthy control was significantly different than those of infected with the white mold that is Sclerotium rolfsii. The major differences were in concentrations of methyl salicylate and 3-octanone so the concentration of methyl salicylate and 3-octanone were very high in case of the infected plants with the white mold pathogen and it was comparatively much lower in the healthy control plants.

Then the feeding behavior of beet armyworms (BAW) on healthy and white mold-infected leaves showed that that beet armyworms preferentially consumed more on the white mold infected leaves. That means higher the concentration of these molecules in the infected leaves it attracted the insect that is beet armyworm to the plant but BAW that is Beet armyworm naturally emitted certain trace amount of hexenyl acetate, linalool, and methyl salicylate, which further retarded the growth of sclerotium rolfsii. So this is just a reverse situation that following white mold infection it invited the bet armyworm pest to attack the peanut plants but at the same time once the beet armyworm attacked the peanut plants it started repelling the white mold pathogen from the host plant. So in contrast the emission of induced volatiles of sclerotium rolf’sii infected plants specifically methyl salicylate and 3- octanone, attracted insect vectors such as beet armyworm. Further, VOCs profiling of potato tubers inoculated with the late blight Pathogen (Phytophthora infestans) and Dry rot pathogen (Fusarium coeruleum) after harvest identified 52 different volatiles.And the most abundant compounds were benzothiazole, 2-ethyl-1-hexanol, hexanal, 2-methyl propanoic acid and so on. The differences in the emitted VOCs profile of potato tubers inoculated with Phytophthora and Fusarium were evident and provide an early warning VOC system for postharvest disease management in potato.

So even if we don’t go for detection of the plant pathogen directly by simply going for VOC profiling one can have an idea whether the potato tubers were infected with a Phytophthora or Fusarium, and if it is infected then definitely one can go for adoption of post-harvest management technologies for protecting the potato tubers from Phytophthora and Fusarium. Similarly infection by Phytophthora cactorum that causes crown rot disease in strawberries results in release of p-ethylguaiacol and p-ethylphenol as characteristic VOCs from the infected portion of the strawberry plant Fruit. So this is another signature that the strawberry fruits are infected by the Phytophthora species because it is emitting this particular volatile organic compound and corrective measures or control measures can adopted or applied immediately if these molecules are detected.

Similarly, Cucumber mosaic virus (CMV) in cultivated squash plants. The CMB infected plants showed an overall net increase in quantities of VOCs like Hexenal, methyl, hepten and so many others In all the plants but no major qualitative difference in VOC profile could be identified in infected plants. Both insect vectors, Aphis gossypii and Myzus persicae, were preferentially attracted to CMV- infected plants, in a similar way with beet armyworm (BAW) to mouldy peanut plants. Despite the smaller size and inferior quality of CMV-infected plants. So, this is another example were Aphis gossypii and Myzus persicae these insects were attracted towards the Cucumber mosaic virus (CMV) plants when it was infected with the cucumber mosaic virus (CMV ). This demonstrate that the plant is inducing altered VOC profile in response to viral infection and this type of mechanism is popularly known as super normal stimulus. So this is super normal stimulus because the prior pathogen is infecting the host plants and modifying the VOC profiling and this modified VOC profiling is attracting certain insects and which is not a normal phenomenon and that is why it is 06:54 known as super normal stimulus.

The volatile signature of plants could be analyzed using gas-chromatography technique to analyze the presence of specific VOC that is indicative of a particular disease. To enhance the performance of compounds separation and analysis the gas chromatography is often combined with mass spectrometry to identify unknown compounds in the volatile sample. So this is normally deployed for edification of known compounds but when it is combined with mass spectrometry then even the unknown compounds can be identified in the VOC profile. GC/GC-MS can provide more accurate information about the plant disease due to its high specificity. It also allows the detection of diseases at different stages based on quantitative information collected from the VOC sample.

So gas chromatography coupled with mass spectrometry is a very good system to accurately profile the VOC of healthy and infected plants and based on these patterns one can very well identify the health status of the plant whether it is infected or it is not infected by any particular pathogen and then based on this signature molecules one can start protecting the plants through other corrective measures or protective measures if the profile indicates presence of a pathogen.

So with this we have seen that how Volatile Organic Compounds can help in diagnosis and detection of certain plant pathogens indirectly because if these pathogens are associated then definitely the VOC profile will be of a particular type in comparison to a healthy one. So that helps us to judge whether the seed lot or the plants are whether infected by any pathogen or not. And if it is infected we can very well take either appropriate control measures for protecting the seed lot or the plants from the suspected pathogen.

So with this we have come to an end of this today’s topic and in the next talk we will be talking about micro needle based diagnosis which is again a new dimension of plant disease diagnosis using very easy and rapid methods.

Thank you very much.