Biotin and other water-soluble vitamins are key growth factors for alfalfa root colonization by Rhizobium meliloti 1021.

Rhizosphere growth limitations imposed on Rhizobium meliloti by availability of biotin, thiamine, and riboflavin were overcome by adding nanomolar amounts of these vitamins. Studies done with R. meliloti 1021 showed that both synthesis and uptake of biotin promote colonization of alfalfa roots. Two lines of evidence indicated that plant-derived biotin normally promotes root colonization: (i) adding avidin significantly (P < or = 0.01) reduced rhizosphere growth of R meliloti 1021, and (ii) growth of Tn5-induced biotin auxotrophs still increased 10-fold in the rhizosphere. Synthesis, however, is the more important source of biotin for R. meliloti 1021 because in root colonization tests biotin auxotrophs competed very poorly with the parent strain. Mutations conferring biotin auxotrophy were closely linked on a single restriction fragment, and one was complemented with the Escherichia coli bio operon. Initial nucleotide sequencing and DNA-DNA hybridization tests showed the biotin synthesis genes in R. meliloti are quite different from those in E. coli.