Plant roots are surrounded by tens of thousands of species of microbes that collectively make up what is called the rhizosphere. Some of these microbes are phytopathogens that have damaging effects on the plant. Others benefit the plant through promoting nutrient uptake, nitrogen fixation, and pathogenic defense systems. Of the beneficial microbes in the rhizosphere that aid in immunity, some function by outcompeting pathogens for nutrients while others actively secrete antibiotics or induce an immune response in the plant.
It has been recently documented that maize is capable of attracting beneficial bacteria to its rhizosphere to promote health. Early stage maize seedlings are especially vulnerable to harm from phytopathogens. They have long been known to secrete a chemical called DIMBOA (2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one) from their roots which is a potent biocide for eliminating harmful bacteria, insects, and other plants. Remarkably, a plant-beneficial bacterium called Pseudomonas putida is attracted to DIMBOA by chemotaxis and can detoxify it. Once within the rhizosphere or the maize seedling, P. putida is often succesful at outcompeting phytopathogens for the limited nutrient supply. In this way, maize plants are able to recruit microbial allies to their roots in order to prevent the growth of phytopathogens.
An illustration of the rhizosphere. A=Amoeba consuming bacteria BL=Energy limited bacteria BU=Non-energy limited bacteria RC=Root derived carbon SR=Sloughed root hair cells F=Fungal hyphae N=Nematode worm. See Gallery for source of this illustration.
“[P]lants interact with a wide range of rhizosphere-colonising bacteria. These are attracted to root surfaces by chemical components in root exudates…This so-called rhizosphere effect supports bacterial cell densities in the root vicinity up to 100-fold greater than in surrounding soil…the plant-microbe interaction can range from deleterious, in the case of phytopathogens, to beneficial, where rhizobacteria can promote plant growth and resistance to plant stress…rhizobacteria can promote growth indirectly by protecting the host plant against pests and diseases. This protection can be based on direct antibiosis or competition for nutrients, but can also result from induced systemic resistance (ISR). Evidence suggests that plant-associating bacteria have evolved the ability to metabolise plant-derived aromatic compounds…Benzoxazinoids (BXs), such as 2,4-dihydroxy-7-methoxy-2H- 1,4-benzoxazin-3(4H)-one (DIMBOA), are heteroaromatic metabolites with benzoic acid moieties…their role in plant defence against above-ground pests and pathogens. BXs are typically produced during relatively early, vulnerable plant growth stages.” (Neal et al. 2012:1)
“BXs have…been implicated in plant defence below-ground. BXs are exuded in relatively large quantities from cereal roots, where they can act as allelochemicals against microbes, insects or competing plants…Plant-derived aromatic metabolites can act as chemo-attractants for Pseudomonas putida…DIMBOA as the dominant BX species in maize root exudates and found that exposure of P. putida to DIMBOA induces bacterial genes with putative functions in chemotactic responses…P. putida KT2440 displays taxis towards DIMBOA…root exudation of DIMBOA during the vulnerable growth stages of maize promotes colonization by plant-beneficial rhizosphere bacteria.” (Neal et al. 2012:2)
“P. putida KT2440 displayed similar growth rates up to 0.5 mM DIMBOA. By contrast, DIMBOA strongly affected growth rates of the ubiquitous soil bacterium Agrobacterium tumefaciens…Hence, P. putida KT2440 appears relatively tolerant to DIMBOA in comparison to other soil bacteria…P. putida KT2440 accelerates breakdown of DIMBOA.” (Neal et al. 2012:3)
“To our knowledge, DIMBOA is the first allelochemical shown to act as a chemo-attractant for beneficial rhizobacteria, and may explain why P. putida KT2440 is such a successful coloniser of the maize rhizosphere. Our discovery also strengthens the notion that certain bacteria have acquired the ability to detoxify aromatic plant compounds, allowing them to exploit the energy-rich rhizosphere of plant roots exuding allelochemical compounds.” (Neal et al. 2012:7)