The most thoroughly studied of the known bacteria-plant interactions is the :-

Agrobacterium tumefaciens infects wounds of many dicotyledonous plants and causes crown gall disease, a tumour-like growth.
It does this by transferring a piece of DNA called T-DNA from its tumour-inducing (Ti) plasmid into the plant genome. The T-DNA genes make the plant cells produce growth hormones (causing the gall) and opines, which the bacterium uses as food.
Because this natural gene transfer is the basis of plant genetic engineering (Agrobacterium is called a natural genetic engineer), the Agrobacterium–plant interaction has been studied in great molecular detail.
Stem nodulation of Sesbania, phosphate-solubilising bacteria and the Anabaena–Azolla symbiosis are important but have been studied much less thoroughly.
So the most thoroughly studied bacteria-plant interaction is gall formation on certain angiosperms by Agrobacterium.