Examining the Effects of the Nitrogen Environment on Growth and N2-Fixation of Endophytic Herbaspirillum seropedicae in Maize Seedlings by Applying 11C Radiotracing

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.

Responses in Nodulated Bean (Phaseolus vulgaris L.) Plants Grown at Elevated Atmospheric CO2

“…In bean leaves, it was observed that the C:N ratio was higher in plants grown under eCO 2 and NN conditions; however, nodulation with R. leguminosarum ISP 14 in combinated with eCO 2 decreased the ratio significantly, showing that nodulation could reestablish the low nitrogen levels caused by eCO 2 (Table 3). In Pisum sativum L. grown using a Free-Air CO 2 Enrichment (FACE), it was found that under eCO 2 conditions, there was a higher proportion of total N in the plant which resulted from N 2 fixation and a small proportion of N taken up from the soil compared to plants grown under aCO 2 conditions [38,39]. The lower availability of nitrogen in the leaves of NN bean (P. vulgaris L.) plants grown under eCO 2 concentrations was related to significantly (p < 0.01) lower NR and GS activities than those grown under aCO 2 (Table 6).…”

A 16S rRNA Gene-Based Metabarcoding of Phosphate-Rich Deposits in Muierilor Cave, South-Western Carpathians

“…Higher amounts of Fe, N, and Mn were also found in the samples collected from the Yellow Chamber when compared to the PM7 sediments. Some genera, such as Rheinheimera (PMW) and Ralstonia (PM11 and PMW), are known to be involved in the iron cycle ( Swanner et al, 2011 ; Schröder et al, 2016 ), while Aphanizomenon NIES81 (PMW), Herbaspirillum, Methylocella , and Ralstonia (PM11 and PMW) are involved in the N cycle ( Ploug et al, 2010 ; Dalsing et al, 2015 ; Dedysh and Dunfield, 2016 ; Waller et al, 2021 ). Bacteria involved in the Mn cycle ( Yang et al, 2013 ; Bai et al, 2021 ; Lee et al, 2021 ) were identified as Escherichia/Shigella, Halomonas, Microbacterium (PMW), and Cupriavidus (PMW and PMB).…”

The Hidden World within Plants

“…Endophytic microorganisms (bacteria, fungi, algae) display multiple beneficial effects on their host plants such as an increase in the resistance to pathogens as well as tolerance to abiotic stresses (cold, dryness); the promotion of plant growth and increase in crop yields through the production of hormones; and an improvement in nutrients uptake, etc. These latter aspects and the appropriate characterization of endophytes in plants are described in the following five papers [ 2 , 3 , 4 , 5 , 6 ].…”

“…N 2 -fixation is reliant on nitrogenase activity, which is related to microbial growth, the latter being linked with plant-borne carbon and the level of photosynthate availability. In their study, Waller et al [ 4 ] reported on the influence of various nitrogen inputs on growth and nitrogen fixation capability of the endophytic rhizobacterium, Herbaspirillum seropedicae , in maize. When the plant was deprived of nitrates (1 m M ), there was a resultant increase in nitrogenase activity along with an enhanced demand for plant carbon as compared to high nitrogen concentrations (10 m M ).…”