Here is an article about production of citric acid from the tuber that you asked: http://nopr.niscair.res.in/bitstream/123456789/7738/1/IJBT%204%282%29%20246-250.pdf
however, you need to be more specific. What is the main component of the plant that you are going to use? what actually you want to do? what is the final product that you aimed?, what is the microbe that you are going to use?, etc. But I hope the article can be a good start..
In that case, you need to use food borne microorganisms. There is a review of microorganism used in cassava fermentation, which you might be used in your study: http://www.tandfonline.com/doi/pdf/10.1080/10408699891274174. You can use similar microorganisms and see what happen to your fermented tuber. There is also a research that isolating lactic acid bacteria from fermented cassava, which also you can refer: http://researchspace.csir.co.za/dspace/bitstream/10204/1309/1/Kostinek_2007.pdf?pagewanted=all.
You don't need to get an article that use the same tuber that you going to use, as long as you understand the background behind it you will be fine.