http://www.nature.com/nature/journal/vaop/ncurrent/full/nature21417.html
It’s a rather dense abstract, be forewarned.
Basically, they’re outlining one of the genetic components by which a mutualism for plants and microbes forms in the roots. Plants have their own form of immune system, which helps control the microbes they form symbioses with. This paper demonstrates that an absence of phosphorus actually weakens their immune defense, allowing microbes and fungi to form closely with the plant roots and develop mutualisms.
It actually makes a lot of sense, in my mind at least. If a plant is doing well nutritionally, why bother forming a symbiosis that carries risks with it? Conversely, if you’re in dire straits, you may need to get in bed with the devil to survive.