, clearly indicating that the quality of root exudate determines the level of nod gene transcription. In fact, legumes which are inhibited in the biosyn- thesis and exudation of phenolic nod-gene inducers, exhibit reduced nodulation and N2 fixation for their N nutrition (Phillips et al., 1994). However, because high soil N inhibits nodulation and N2 fixation, and low concentrations of certain nutrients stimulate the biosynthesis of isoflavone nod gene inducers in symbi- otic legumes (Dakora and Phillips, 1996), low-nutrient soils would seem to be the ideal for stimulating N2 fix- ation and increased N nutrition in nodulated legumes. Whatever the case, root exudates contain the signals for transcription of nod genes in symbiotic rhizobia, and therefore control the extent of legume dependence on N2 fixation for its N nutrition.