Plant Signals to Soil Microbes: Regulators of Rhizosphere Colonization

Abstract

It is now well-established that natural plant products induce gene transcription in plant- associated soil bacteria. For example, nodulation (nod, nol) genes in Rhizobium and Bradyrhizobium as well as virulence (vir) genes in Agrobacterium are controlled by plant compounds (Long, Staskawicz, 1993). The list of plant flavonoids that regulate nod genes in rhizobia has now been extended to include diverse groups of flavonoids and flavonoid glycosides (Hungria et al., 1991; Smit et al, 1992; Phillips et al., 1994). In the case of Rhizobium meliloti, two nod-gene-inducing betaines, stachydrine and trigonelline, also are released from many Medicago species (Phillips et al., 1995). The availability of nod-gene-inducers and other compounds in soil around alfalfa roots (Léon-Barrios et al., 1993) suggests that applied ecological studies should now consider whether molecular signals from plants can affect other important microbial functions, such as growth and root colonization in soil. Understanding signal-response events that actually occur in soil should make it easier to introduce genetically improved rhizobia to soil containing indigenous strains and may enhance efforts to use other beneficial microbes that control plant disease, degrade pesticides, or deter invertebrate pests.