Comparison of hup trait and intrinsic antibiotic resistance for assessing rhizobial competitiveness axenically and in soil

Abstract

The competitiveness of dual-strain inocula of cowpea rhizobia for nodulation of Vigna unguiculata (L.) Walp. was studied axenically between one slow-growing strain (P132, HP147, 401, or 22A1) and one fast-growing strain (176A26 or 176A28) at logarithmic inoculum ratios ranging from 103 to 10-3. Nodule infectivity was determined by multiple intrinsic antibiotic resistance, since both fast-growing strains were sensitive. Different hydrogen uptake (Hup) efficiencies of dual-strain inocula allowed for the comparison of an indirect rapid method. Infectivity data based on antibiotic resistance and Hup efficiency were fit to linearized fractile plots of log-normal distributions to determine C(AB) (percent infectivity at a 1:1 inoculum density) or I50 (inoculum ratio at 50% infectivity). The slow growers were always better competitors and had I50 values which ranged from 7 to 160,000 and C(AB) values which ranged from 62 to 97%. P132 was the best competitor of all those tested. Antibiotic resistance and Hup efficiency methods were in agreement with 401 (Hup-) and 176A26 (Hup+), but the Hup efficiency method overestimated the I50 index with 22A1 (Hup-) and 176A28 (Hup+). The competition of each of the four slow-growing strains with indigenous rhizobia was examined in Cajanus cajan from three tropical soils. Nodule infectivity for all strains ranged from 42 to 96%, and P132 was the best competitor in all the soils. Hup efficiency overestimated infectivity by about 2-fold when Hup+ inocula (P132 and HP147) were used but underestimated infectivity by more than 100-fold when Hup- inocula (401 and 22A1) were used. Although the Hup trait has limited quantitative usage axenically, it is only qualitative in soil competition studies and can only be used with Hup+ inocula.