Downy Mildew ROUND TWO

Another week gone....time really does speed up as you get older but at least it is finally staring to get a bit warmer here in Sheffield. Although I'm not quite ready to break out into the short sleeves and shorts like all the undergraduates, who celebrated end of exams today. I remember THAT feeling!

This morning marked the critical stage of my second attempt to investigate whether infection by the root parasitic weed Striga gesnerioides makes the host plant more susceptible to above ground pests, in this case Downy Mildew ( Peranospora parasitica). You may remember how my first experiment was turned upside down when my growth cabinet broke, sending the temperature skyrocketing and putting my plants into a 'stress response'. My supervisors advised me to finish that assay anyway, as good practice, and to score the leaves. The data didn't suggest that there was a below- above ground interaction but we decided it was worth checking once more. Last week, I infected half of my host plants with Striga and today was the day to challenge my plants with the downy mildew pathogen. Next week, I will score the leaves to see if there is any difference in the susceptibility of Striga infected and non-infected Arabidopsis plants. I am not looking forward to that bit - many hours of staring down a microscope, struggling to discern the microscopic pathogen. And there will be a lot of leaves : 8 leaves per plant, forty plants in total... No, I am going to STOP that calculation before I become depressed....

We use some very sophisticated equipment for infecting...

But how DO you challenge a plant with downy mildew? This part is beautifully simple but involves a bit of advance planning. First, you make sure that there will be a culture of downy mildew available on the day in question, growing on a susceptible host ( an Arabidopsis genetic mutant that has reduced salicyclic acid regulated defences). Then, you harvest leaves from said host into a falcon tube, shake well with distilled water, then drain through a fine cloth into a new tube to remove the leaves. This gives a cloudy solution containing ( hopefully!) oodles of downy mildew spores. This has to be quantified under the microscope by counting the number of spores in a single droplet using a counting chamber. If necessary, the solution can then be diluted to the correct concentration. Now the fun part: fill a perfume spray bottle with the solution and dowse your plants with 'eau de Peranospora'. I can get carried away sometimes, imagining that I am a perfume trader...

Infecting my plants with downy mildew. Red labelled ones have the Striga parasite

Job done, my plants are now residing in 'quarantine' - Cabinet 511, especially reserved for downy mildew experiments to avoid cross contamination. With so many growth cabinets in the annexe ( Ok, the 'Sir David Read Controlled Enviornment Facility' to be absolutely precise), there must be a fascinating array of work quietly being carried out, covering everything from soil dynamics, plant defences against disease, boosting the efficiency of photosynthesis .... None of which would want my downy mildew near them! 

I am slowly making progress on getting more sleep and have been making fewer mistakes this week - I did check the microtome properly this time! But right now, my eyes are dropping and my bed beckons. Hopefully, whilst I sleep, those downy mildew spores will start to awaken and make themselves at home on their new hosts...