Root clusters (also known as proteoid or cluster roots) are bunches of hairy rootlets recorded in > 1800 species from nine families so far. The possible involvement of microorganisms in root cluster formation has produced conflicting results over the last 40 years.
Lamont et al. show that there are circumstances where soil bacteria can promote or inhibit root-cluster formation in their study of the effects of inoculation with seven different bacterial strains and nitrogen treatments on three different genera: Leucadendron (protea) from the Cape of South Africa, Viminaria (legume) from Australia and Lupinus (legume) from Europe. The outcome, whether promotion or inhibition of root cluster formation, depends on the identity of bacterium used, the identity of the host species and the nutritional environment, specifically nitrogen status. Thus, growth promoting bacteria can be described as having a facultatively beneficial influence on plants.
This paper is part of the Root Biology Special Issue.