Abstract
Herbaspirillum seropedicae, as an endophyte and prolific root colonizer of numerous cereal crops, occupies an important ecological niche in agriculture because of its ability to promote plant growth and potentially improve crop yield. More importantly, there exists the untapped potential to harness its ability, as a diazotroph, to fix atmospheric N2 as an alternative nitrogen resource to synthetic fertilizers. While mechanisms for plant growth promotion remain controversial, especially in cereal crops, one irrefutable fact is these microorganisms rely heavily on plant-borne carbon as their main energy source in support of their own growth and biological functions. Biological nitrogen fixation (BNF), a microbial function that is reliant on nitrogenase enzyme activity, is extremely sensitive to the localized nitrogen environment of the microorganism. However, whether internal root colonization can serve to shield the microorganisms and de-sensitize nitrogenase activity to changes in the soil nitrogen status remains unanswered. We used RAM10, a GFP-reporting strain of H. seropedicae, and administered radioactive 11CO2 tracer to intact 3-week-old maize leaves and followed 11C-photosynthates to sites within intact roots where actively fluorescing microbial colonies assimilated the tracer. We examined the influence of administering either 1 mM or 10 mM nitrate during plant growth on microbial demands for plant-borne 11C. Nitrogenase activity was also examined under the same growth conditions using the acetylene reduction assay. We found that plant growth under low nitrate resulted in higher nitrogenase activity as well as higher microbial demands for plant-borne carbon than plant growth under high nitrate. However, carbon availability was significantly diminished under low nitrate growth due to reduced host CO2 fixation and reduced allocation of carbon resources to the roots. This response of the host caused significant inhibition of microbial growth. In summary, internal root colonization did little to shield these endophytic microorganisms from the nitrogen environment.
Highlights
Introduction distributed under the terms andOne of the most promising agro-ecological approaches that could enable a reduction in use of synthetic N fertilizer is the exploitation of N2 -fixing (BNF) bacteria
To better understand how carbon resources derived from the host plant can influence microbial growth and their biological functions, such as Biological nitrogen fixation (BNF), we first examined how the status of N during plant growth affected carbon uptake and allocation of resources, reexamined the influence of H. seropedicae inoculation on these plant processes
“appetite” for carbon was high under these circumstances, less carbon10was host, causing less carbon to transport to the roots to satisfy the microbial carbon dem Results microbial growth performance on live maize roots by drop 5
Summary
Introduction distributed under the terms andOne of the most promising agro-ecological approaches that could enable a reduction in use of synthetic N fertilizer is the exploitation of N2 -fixing (BNF) bacteria. Previous studies have shown that PGPB commonly impact root architecture and plant health, attributing these effects to such things as BNF, production of phytohormones, and enhancement of nutrient acquisition, which can protect hosts against pests and pathogens and build tolerances to extreme climatic conditions [7,8]. BNF relies on nitrogenase to catalyze the energy-demanding reduction of N2 to ammonia coupled with H2 production [9]. This enzyme is composed of two protein components: one composed of a MoFe protein, and the other a Fe protein. Nitrogenase requires 16 Mg-ATP molecules for the reduction of 8H+ + N2 → 2 NH3 + H2.
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