Rhizobia are capable of promoting the growth of legumes through a number of mechanisms of action including phytohormone production. Auxin being a plant hormone is produced by rhizobia and this ability increases many folds in supplementation with some suitable physiological precursor like L-tryptophan. Substrate-derived auxins have the ability to promote plant growth by supplementing sub-optimal auxin levels in legume plants like mung bean. So, this project was designed to evaluate the effectiveness of rhizobia under axenic conditions in supplementation with L-tryptophan on the growth and yield of mung bean crop. To test this hypothesis, a series of laboratory, wire house and field experiments were conducted. Rhizobium phaseoli were isolated from different locations of Faisalabad from nodules of mung bean crop. Forty four rhizobial cultures were isolated from mung bean nodules and tested for auxins biosynthesis in vitro colorimetrically and confirmed by high performance liquid chromatography (HPLC). Rhizobial isolates varied in auxin biosynthesis ranging from 8.8 μg IAA equivalents mL -1 to 38.3 μg IAA equivalents mL -1 without L-tryptophan. Auxin biosynthesis by rhizobial isolates in supplementation with L-tyrptophan, increased many folds (1.1 to 7.9 folds) and N42 produced maximum auxins in L-tryptophan supplemented media. Different environmental factors like substrate (L-Trp) and C-source (glucose), pH, incubation temperature, incubation time and aeration (static vs. shaking) significantly affected auxin production. Growth pouch experiments were conducted in the laboratory using 34 rhizobial isolates for improving growth of mung bean seedlings using three mung bean cultivars i.e. NM-92, NM-98 and NM-2006 under axenic conditions. Fifteen isolates were selected through screening in jars using principal component scoring method. Three most efficient rhizobial isolates i. e. A23, N12 and N42 were tested alone and in combination with different L-TRP concentrations (10 -3 , 10 -4 and 10 -5 M) to test the hypothesis of substrate dependent auxin production for improving the growth of mung bean (var: NM-92, NM-98 and NM-2006) under axenic conditions. These isolates were further tested alone and in combination with 10 -3 , 10 -4 and 10 -5 M L-Trp for improving growth and nodulation of mung bean. Finally these three rhizobial isolates i.e. A23, N12 and N42 along with 10 -3 , 10 -4 , 10 -5 M L-Trp concentrations were further used in pot and field experiments using mung bean variety NM-2006. Rhizobial isolates A23, N12 and N42 alone and in combination with 10 -3 , 10 -4 and 10 -5 M L-Trp concentrations were tested in pots at the wirehouse, Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad. Results of pot experiments revealed that separate application of rhizobia significantly increased growth parameters but the combined application of rhizobia and L-tryptophan further significantly increased yield and nodulation. Field trials were conducted at two sites these rhizobial isolates A23, N12 and N42 alone and in combination with 10 -3 , 10 -4 and 10 -5 M L-Trp concentrations were tested at two N levels i. e. half and full recommended dose of N fertilizer. Results revealed that rhizobial inoculation alone significantly increased growth parameters (fresh biomass, grain yield, shoot length, root length, number of pods plant -1 , and number of grains pod -1 , 1000-grain weight, number of nodules plant -1 , fresh and dry weight of nodules, NPK contents in grain) of mung bean variety NM-2006. But the combined application of rhizobia along with L-Trp (10 -4 and 10 -5 M) further increased the mung bean yield and nodulation. This implies that substrate dependent microbial biosynthesis of auxins as a result of precursor (L-Trp)-inoculum (rhizobia) interactions can be successfully used for improving the growth and yield of legumes.