Host specificity of nodulation in Rhizobium (Cicer) as revealed by Tsr bioassay

Bioassay based on thick and short root (Tsr) and hair deformation (Had) phenotypes were used to test the activity of Nod factors produced by Rhizobium sp. (Cicer) strains HS-1, Rcd-301, IC-59, IC-76 and Ca-181 on chickpea (Cicer arietinum) cv. 'C-235'. Nod- mutants of Rhizobium sp. (Cicer) did not produce Tsr+ and Had+ phenotypes on chickpea, indicating the requirement of nodulation genes for their appearance. The strain HS-1 treated with root exudates of pea (Pisum sativum), berseem (Trifolium alexandrinum) and lucerne (Medicago sativa) failed to produce the Tsr+ and Had+ phenotypes on chickpea. Conversely R. leguminosarum bvs. viciae and trifolii, R. meliloti, Rhizobium sp. (Sesbania), and Rhizobium sp. (Cajanus) induced with chickpea root exudates did not show Tsr+ and Had+ phenotypes on chickpea. It appears that host specificity in Rhizobium sp. (Cicer)-chickpea symbiosis is regulated by the production of host-specific factors which are not active on heterologous hosts.