Rhizobium infection threads in root hairs of Glycine max (L.) Merr., Glycine soja Sieb. & Zucc, and Vigna unguiculata (L.) Walp.

Abstract

Eight lines of soybean (G. max), 4 of wild soybean (G. soja) and 1 cowpea (V. unguiculata) cv. [Pink Eye Purple Hull] were inoculated with 18 Rhizobium strains. After 4 days, root hairs were examined for infection threads. Threads were produced by all hosts, but exclusively in nodulating combinations. Only Rhizobium sp. strains 3G4b9a and 3G4b19 were inconsistent; they nodulated soybean and G. soja in some experiments but rarely formed infection threads. Soybean and G. soja were indistinguishable in their interactions with rhizobia, as were lele soybean lines (generally lack soybean lectin) ''Hardee'' soybean (contains the nodulation-influencing genes Rj2 and Rj3) and several other soybean cultivars. Threads formed in cowpea with all of the R. japonicum strains and most Rhizobium sp., but not with R. lupini. Infection of all 3 host species occurred in portions of the primary root containing immature or no root hairs at the time of inoculation; proximal tissues having elongated root hairs lacked infection threads. Infected root hairs were short and commonly shaped like question marks. Threads usually branched and sometimes intertwined prior to elongation into the basal portion of root hairs.