Nitric oxide signaling by plant-associated bacteria

Abstract

The primary source of biogenic NO in the biosphere is energy metabolism within nitrifying and denitrifying bacteria, which are common inhabitants of the rhizosphere. NO produced by these bacteria regulates their mobility and gene expression and can elicit responses in neighboring organisms, including plants. Many other bacteria employ bacterial nitric oxide synthase to generate NO. One clear function for bNOS is in oxidative defense signaling. An endophytic association of actinomycetes and other bNOS-containing bacteria, some of which may be vertically transmitted within seeds, has been demonstrated among diverse plant species. Actinomycetes possessing bNOS activity display increased capacity for rhizosphere colonization and two species have been shown to upregulate bNOS activity in response to plant-derived disaccharides. As yet, evidence of an effect of bNOS-derive NO on the plant host has been indirect. In contrast, there is strong evidence that denitrification-derived NO from Azospirillum brasilense can stimulate root branching in tomato, and this path of NO production is also responsive to plant host compounds. Further investigations will likely reveal other influences of bacterial NO in known plant NO-responsive pathways.