Which chemicals are responsible for host-specificity of Rhizobium?

Flavonoids and Isoflavonoids are the chemicals responsible for the host-specificity of Rhizobium.
Rhizobium is a genus of gram-negative soil bacteria that fix atmospheric nitrogen. The symbiotic association between rhizobium and legume is a highly specific interaction between the two partners. Host specificity is evident at the early stages of infection and results from multiple interactions involving signaling among bacteria and host plants.