ABSTRACT Mesorhizobium ciceri USDA 3378 has a competitive advantage over the indigenous Mesorhizobium muleiense CCBAU 83963 in nodulating chickpea (Cicer arietinum L.) in newly introduced planting areas in China. The underlying mechanisms for this dominance remain unclear. A comparison of the genomes of USDA 3378 and CCBAU 83963 revealed significantly more genes involved in flagellum production and cell movement in USDA 3378. USDA 3378 produced flagella, but CCBAU 83963 did not and showed lower motility, biofilm production, and extracellular polysaccharide secretion than USDA 3378. Transcriptome analysis of USDA 3378 under simulated symbiotic versus non-symbiotic conditions showed strong induction of nodulation genes and a broader transcriptional response among genes assigned to quorum sensing, chemotaxis, and flagellar assembly, with flgL (encoding a flagellar hook-associated family protein) being the only upregulated flagellar structural gene detected. A flgL mutant strain based on USDA 3378 (ΔflgL-3378) showed similar growth to USDA 3378 but was unable to produce flagella and exhibited concomitant reductions in motility, biofilm production, and extracellular polysaccharide secretion. Nodule occupancy by USDA 3378 was 100% when co-inoculated with CCBAU 83963. In contrast, nodule occupancy by ΔflgL-3378 was significantly reduced to 39.88% when co-inoculated with the wild-type USDA 3378. However, when co-inoculated with the indigenous strain CCBAU 83963, ΔflgL-3378 still showed a dominant occupancy of 82.8%. Transcriptome analysis of ΔflgL-3378 under the same comparison showed continued induction of nodulation genes and several flagellar system genes, an altered quorum-sensing-associated response, and no detectable chemotaxis-related differentially expressed genes. We conclude that flgL and flagella act as important contributors to the superior competitive nodulation ability of M. ciceri USDA 3378 over M. muleiense in chickpea, although other intrinsic genomic advantages likely contribute to its basal competitivenessIMPORTANCEChickpea is an important legume crop that depends on symbiotic rhizobia for biological nitrogen fixation. In newly introduced chickpea-growing regions of China, Mesorhizobium ciceri USDA 3378 shows a strong competitive advantage in nodulating chickpea compared with the indigenous strain Mesorhizobium muleiense CCBAU 83963, but the mechanisms underlying this advantage remain unclear. This study identifies the flagellar hook-associated gene flgL as an important contributor to the competitive nodulation ability of USDA 3378. Deletion of flgL abolished flagellum formation and reduced motility, biofilm formation, extracellular polysaccharide production, and competitive nodulation ability. However, the ΔflgL mutant still retained higher competitiveness than CCBAU 83963, indicating that additional motility-independent traits also contribute to the basal competitiveness of USDA 3378. These findings improve our understanding of the bacterial traits that influence rhizobial competitiveness and may help guide the development of more effective chickpea inoculants for diverse agricultural environments.

The introduced rhizobial inoculum M. ciceri USDA 3378 demonstrates a significant competitive advantage over the indigenous M. muleiense CCBAU 83963 for nodulating chickpea in newly established planting areas in China. Previous genomic analyses revealed that USDA 3378 possesses a greater number of genes related to cell movement and flagella production compared to CCBAU 83963. Transcriptomic analysis indicated that the expression of the flagella-associated gene motA (flagellar motor protein) significantly changed under symbiotic conditions. Although the genome of M. ciceri USDA 3378 contains the motA gene, its biological function within this strain has not been previously reported. In this study, we constructed a motA mutant (ΔmotA-3378) in USDA 3378 using homologous recombination and biparental conjugation methods to assess the differences in bacterial structure, growth, motility, exopolysaccharide synthesis, biofilm formation, and competitive nodulation ability between the wild type and the mutant. Experimental results showed that the ΔmotA-3378 mutant was unable to produce flagella, leading to reduced motility, diminished biofilm formation, and lower exopolysaccharide production. In competitive nodulation with wild-type USDA 3378, the ΔmotA-3378 mutant's nodule occupancy was 40.43 %. Furthermore, its competitive nodulation advantage against CCBAU 83963 decreased from 100 % (achieved by wild-type USDA 3378) to 94.6 %. These findings indicate that the motA gene plays a crucial role in the motility, exopolysaccharide synthesis, biofilm formation, and competitive nodulation ability of M. ciceri USDA 3378.

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BackgroundAtmospheric carbon-dioxide concentration ([CO2]) is increasing rapidly, but its interactions with potassium (K) fertiliser on wheat growth, grain yield and quality are not well understood.AimWe investigated the effects of ambient CO2 (aCO2, approx. 415 ppm) and elevated CO2 (eCO2, 760 ppm) on these growth parameters under optimum (2.01 mol m-3) and growth limiting (50 mmol m-3) K supply in controlled environment chambers.ResultsPotassium limitation decreased total biomass at anthesis and maturity by approx. 13% and grain yield by 7.4%. The decreased grain yield was linked to decreased grain number. Grain K, P and S concentrations decreased by 6.5%-20.6%, under K deficiency, whereas Ca concentration increased by 8.0% and N and Mg concentrations were unaffected. These changes were closely correlated with changes in total aboveground nutrient accumulation, which were interpreted as changes in nutrient uptake. However, nutrient harvest index (NuHI) changed little with K supply. Under e[CO2], plant dry weight at anthesis, root + straw dry weight at maturity and grain yield were respectively 33.1%, 23.9% and 9.7% greater than at a[CO2]. The increase in yield was linked to an increase in thousand grain weight. Grain macronutrient concentrations (except P) decreased by 6.38%-16.0% with e[CO2]. Total aboveground macronutrient accumulation and NuHIs were unaffected by CO2 supply, except for KHI, which decreased with increasing [CO2].ConclusionIt is concluded that uptake of nutrients and their translocation within the plant were not inhibited by eCO2, and decreased grain macronutrient concentrations were attributed to nutrient dilution due to increased C fixation relative to nutrient uptake.

Atmospheric carbon-dioxide concentration ([CO2]) is increasing rapidly, but its interactions with nitrogen (N) and phosphorus (P) fertiliser on wheat grain quality are not well understood. We investigated the effects of ambient CO2 (aCO(2); similar to 410 ppm) and elevated CO2 (eCO(2); 760 ppm) on crop harvest index (CHI), nutrient harvest index (NuHI), shoot macro-nutrient content and grain macro-nutrient concentration of wheat grown under two contrasting amounts of N (0.5 and 6 mol m(-3) NO3- N) and P (10 and 250 mmol P m(-3)) fertiliser supply (low and optimum, respectively). Our results highlighted interactions between [CO2] and N and P fertiliser supply for the shoot biomass at anthesis and straw biomass at harvest maturity. This was because biomass yield did not respond to CO2 level when fertiliser was deficient. However, shoot and straw yield increased (10.0--34.0%) with increasing [CO2] at optimum fertiliser rates. Across experiments, grain yield increased (15.6%) with increasing [CO2], which resulted in grain nutrient concentration decreasing (3.0--13.0%) with increasing [CO2]. This was attributed to nutrient 'dilution' due to increased carbohydrate content in the grain. Overall, fertiliser supply impacted crop responses more than CO2 treatments, and the impact was greater under N than P deficiency. This was reflected through conservative values for CHI, thousand grain weight and NuHIs suggesting plants allocated biomass and nutrients at similar rates for vegetative and reproductive organs independent of [CO2].

Three slow-growing rhizobial strains (WYCCWR 12678, WYCCWR 12774 and WYCCWR 13023T) isolated from effective nodules of Arachis hypogaea L. (cultivated peanut) sampled in Zhengyang County, Henan Province, central China were characterized using a polyphasic approach. The three strains were assigned to the genus Bradyrhizobium based on phylogenetic analysis of their 16S rRNA sequences. Phylogenetic analysis based on their concatenated recA-glnII-gyrB-dnaK-rpoB gene sequences placed the strains into a distinct lineage. Whole-genome average nucleotide identity (ANI) values between WYCCWR 13023T and WYCCWR 12774 and WYCCWR 12678 were 99.43 and 99.31% respectively. ANI values between WYCCWR 13023T and the most closely related strains were all below 93 %. The digital DNA–DNA hybridization (dDDH) values between WYCCWR 13023T and ‘ B. guangzhouense ’ CCBAU 51670T, B. manausense BR 3351T and B. guangdongense CCBAU 51649T, the three most closely related type strains, were 50.40, 43.50 and 39.20% respectively. Phenotypic characterization also allowed the differentiation of the novel strains from their most closely related type strains. Based on the genotypic and phenotypic features, we conclude that the three strains represent a novel species for which the name Bradyrhizobium zhengyangense sp. nov. is proposed, with WYCCWR 13023T (=GDMCC 1.3180T=HAMBI 3760T) as the type strain.

Atmospheric carbon dioxide concentrations ([CO2]) are increasing, but little is known about how this will affect macronutrient (nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg)) accumulation and partitioning in the aboveground biomass (AGB) for different hard spring wheat genotypes. We examined the responses of six spring wheat genotypes ('Discovery', 'Duchess', 'Reliance', PFR-3026, PFR-3019, PFR-2021) to two CO2 levels (ambient [aCO(2)] and elevated [eCO(2)]) and six nitrogen rates (N; 1-10 mM), at the stem elongation growth stage of wheat grown in controlled environment chambers. The AGB yield increased by 35.2% with increasing [CO2] when N rate was >2 mM. Increasing N supply also increased AGB by up to 3.2-fold over the entire N range applied. The AGB responses to N differed among the genotypes, being lowest for PFR-3019 (7.71 +/- 0.11 g/pot) and highest for PFR-2021, PFR-3026 and Duchess at 8.84 +/- 0.11 g/pot at both CO2 levels. Macronutrient concentrations decreased with eCO(2) by 28.0% for Ca to 17.4% for P and K. Nevertheless, absolute nutrient uptake was higher for eCO(2) treatments, because the AGB increase (20.0-52.0%) was proportionally higher than the 4.0-28.0% increase in nutrient uptake. The AGB non-response to [CO2] at N rates <2mM indicates that this nutrient deficiency was more limiting than the effects of CO2 level(.) Therefore, the impact of eCO(2) in the future will depend on N fertilizer management. These results suggest that critical nutrient concentrations used to diagnose the nutrient status of wheat crops will need to be reassessed for eCO(2) conditions.

A total of 219 rhizobial strains isolated from peanut grown in soils from six peanut croplands in Zhengyang county, Henan Province, were typed by PCR-RFLP of IGS sequences. Their phylogenetic relationships were refined on representative strains using sequence analyses of 16S rRNA genes, housekeeping genes (atpD, recA, glnII) and symbiosis genes (nodA, nodC and nifH). The 219 rhizobial isolates were classified into 13 IGS types, and twenty representatives were defined within eight Bradyrhizobium genospecies: B. guangdongense covering 5 IGS types (75.2% of total isolates), B. guangzhouense (2 IGS types, 2.7% total isolates), B. zhengyangense (1 IGS type, 11.3% total isolates) and five novel genospecies (5 IGS types, 0.9 to 3.2% total isolates). All representative strains had identical nodA, nodC and nifH sequences except for one nifH sequence. With this one exception, these sequences were identical to those of the type strains of Bradyrhizobium species and several Bradyrhizobium genospecies isolated from peanut in different regions of China. The nodC sequences of all strains showed < 67% similarity to the closest strains on the Genbank database indicating that they are representative of a novel Bradyrhiobium symbiovar. This study has shown that (1) diverse Bradyrhizobium spp. with similar symbiosis genes nodulate peanut in different regions of China. (2) Horizontal transfer of genes involved in nodulating peanut is common between Bradyrhizobium species in soils used to grow the crop in China. (3) The strains studied here are representative of a novel Bradyrhizobium symbiovar that nodulates peanut in China. We propose the name sv. arachis for this novel symbiovar indicating that the strains were isolated from Arachis hypogaea. Results here have practical implications in relation to the selection of rhizobial inoculants for peanut in China.

Faba bean (Vicia faba L.) is a major introduced grain-legume crop cultivated in China. In this study, rhizobia that nodulated faba bean grown in soils from three sites in North China (Hebei Province) were isolated and characterized. Firstly, isolates were categorized into genotypes by ribosomal IGS PCR-RFLP analysis, then representatives of the different IGS genotypes were further identified by phylogenetic analyses of 16S rRNA, housekeeping (atpD, recA) and nodulation (nodC) gene sequences. Rhizobial distribution based on the IGS genotype was related to the different soil physicochemical features by redundancy analysis. IGS typing and phylogenetic analyses of 16S rRNA and concatenated housekeeping gene sequences affiliated the 103 rhizobial strains isolated into four Rhizobium species/genospecies. A total of 69 strains of 3 IGS types were assigned to R. sophorae, 20 isolates of 5 IGS types to R. changzhiense and 9 isolates of 3 IGS types to R. indicum. The representative strain of the five remaining isolates (1 IGS type) was clearly separated from all Rhizobium type strains and was most closely related to defined genospecies according to the recently described R. leguminosarum species complex. Rhizobium sophorae strains (67% of total isolates) were common in all sites and shared an identical nodC sequence typical of faba bean symbionts belonging to symbiovar viciae. In this first study of rhizobia nodulating faba bean in Hebei Province, China, R. sophorae was found to be the dominant symbiont in contrast to other countries.

Most terrestrial vascular plants can assimilate soil obtained NO3- in their root and shoot. Data from the literature are collated and analysed with respect to genotype and environmental effects on the partitioning of NO3- assimilation between root and shoot of terrestrial vascular plants. Temperate evergreen woody species in the Ericaceae and Pinaceae carry out most of their NO3- assimilation in the root when growing in low (0.5 mM) up to at least 5 mM soil NO3-. The root is the main site of NO3- assimilation for temperate deciduous woody species and perennial and annual herbaceous legume species at 0.5–1 mM NO3- but for many, shoot assimilation increases in importance with increased NO3- supply. Temperate perennial grasses and annual non-legume species and tropical/ sub-tropical species regardless of life-form, carry out a substantial, usually major proportion of their NO3- assimilation in shoots at NO3- concentrations above 0.5 mM. Furthermore, high NH4+ supply, mycorrhizal infection and infection by parasitic plants can increase the proportion of total plant NO3- assimilation carried out in the shoot while abiotic stress and elevated atmospheric [CO2] can cause this to decrease. Shoot NO3- assimilation is an advantage under non-stress conditions due to its positive effect on leaf expansion but can be a disadvantage under freezing and chilling stress conditions. Increased reliance on root NO3- assimilation at elevated CO2 was associated with increased and conversely decreased plant growth and NO3- assimilation depending on study. Resolution of these different findings across studies is an important area for further research.

The effects of NO3– supply (0–500 kg N/ha) on total plant dry weight (DW), shoot N content and nutritional quality, and the proportion of plant N derived from the atmosphere (%Ndfa) were determined for lucerne and perennial lupin using 15NO3– under glasshouse conditions. Fodder beet was used as a non-legume control plant. The experiment was repeated. In both the initial and repeat experiments, total plant DW, shoot N% and shoot nutritional quality for lucerne and perennial lupin were not affected by NO3– supply. Total plant DW increased 10-fold and shoot N% tripled for fodder beet with increased N supply. In the initial experiment, the %Ndfa for lucerne decreased from 89 to 37% with increased N supply from 0 to 500 kg N/ha: comparable values for perennial lupin were 96 to 64%. In the repeat experiment, %Ndfa decreased from 90 to 49% and 93 to 65% for lucerne and perennial lupin, respectively, with increased NO3– supply from 0 to 500 kg N/ha. Both legumes showed an increased reliance on NO3– with increased soil NO3– level, but even at 500 kg N/ha (similar to N in sheep urine patch) perennial lupin obtained much of its N from N2 fixation.

The photon costs of photoreduction/assimilation of nitrate (NO3-) into organic nitrogen in shoots and respiratory driven NO3- and NH4+ assimilation in roots are compared for terrestrial vascular plants, considering associated pH regulation, osmotic and ontogenetic effects. Different mechanisms of neutralisation of the hydroxyl (OH-) ion necessarily generated in shoot NO3- assimilation are considered. Photoreduction/assimilation of NO3- in shoots with malic acid synthesis and either accumulation of malate in leaf vacuoles or transport of malate to roots and catabolism there have a similar cost which is around 35% less than that for root NO3- assimilation and around 20% less than that for photoreduction/assimilation of NO3-, oxalate production and storage of Ca oxalate in leaf vacuoles. The photon cost of root NH4+ assimilation with H+ efflux to the root medium is around 70% less than that of root NO3- assimilation. These differences in photon cost must be considered in the context of the use of a combination of locations of NO3- assimilation and mechanisms of acid-base regulation, and a maximum of 4.9-9.1% of total photon absorption needed for growth and maintenance that is devoted to NO3- assimilation and acid-base regulation.

Three fast-growing rhizobial strains isolated from effective nodules of common vetch (Vicia sativa L.) were characterized using a polyphasic approach. All three strains were assigned to the genus Rhizobium on the basis of the results of 16S rRNA gene sequence analysis. Phylogenetic analysis based on concatenated atpD-recA genes separated the strains into a distinct lineage represented by WYCCWR 11279T, which showed average nucleotide identity values of 95.40 and 93.61 % with the most similar phylogenetic type strains of Rhizobium sophorae CCBAU 03386T and Rhizobium laguerreae FB TT, respectively. The digital DNA-DNA hybridization relatedness values between WYCCWR 11279T and the closest related type strains were less than 70 %. Therefore, a novel rhizobial species is proposed, Rhizobium changzhiense sp. nov., and strain WYCCWR 11279T (=HAMBI 3709T=LMG 31534T) is designated as the type strain for the novel species.

Henan Province is a major area of peanut production in China but the rhizobia nodulating the crop in this region have not been described. A collection of 217 strains of peanut rhizobia was obtained from six field sites across four soil types in Henan Province, North China, by using peanut as a trap host under glasshouse conditions. The 217 strains separated into 8 distinct types on PCR–RFLP analysis of their IGS sequences. Phylogenetic analysis of the 16S rRNA, recA, atpD, and glnII genes of 11 representative strains of the 8 IGS types identified Bradyrhizobium guangdongense, B. ottawaense and three novel Bradyrhizobium genospecies. Bradyrhizobium guangdongense was dominant, accounting for 75.0% of the total isolates across the field sites while B. ottawaense covered 5.1% and the three novel Bradyrhizobium genospecies 4.1 to 8.8% of the total. The symbiosis-related nodA and nifH gene sequences were not congruent with the core genes on phylogenetic analysis and separated into three groups, two of which were similar to sequences of Bradyrhizobium spp. isolated from peanut in south-east China and the third identical to that of B. yuanmingense isolated from Lespedeza cuneata in northern China. A canonical correlation analysis between the distribution of IGS genotypes and soil physicochemical characteristics and climatic factors indicated that the occurrence of IGS types/species was mainly associated with soil pH and available phosphorus.

Hexaploid Caucasian clover was grown in soil sampled at three New Zealand South Island high country sites to which specific rhizobium inoculum had been added with sowing of the legume in 1975, 1992 and 1997; two sites on the Lincoln University farm sown with inoculated Caucasian clover in 2012 and 2013; and six sites not sown with the crop. Caucasian clover nodulated in soil from all sites sown with inoculated Caucasian clover but did not nodulate in soils from the other sites. Rhizobial isolates from plants in each soil where nodulation occurred showed the same genetic profile, and gave a similar increase in growth of Caucasian clover in low nitrogen soil, as the recommended inoculum for hexaploid Caucasian clover. Caucasian clover specific rhizobia can persist and retain their effectiveness for at least 42 years in New Zealand South Island low fertility, high country soils and 5 years in high fertility soils.

Bloom et al. proposed that rising atmospheric CO2 concentrations 'inhibit malate production in chloroplasts and thus impede assimilation of nitrate into protein of C3 plants, a phenomenon that will strongly influence primary productivity and food security under the environmental conditions anticipated during the next few decades'. Previously we argued that the weight of evidence in the literature indicated that elevated atmospheric [CO2 ] does not inhibit NO3 - assimilation in C3 plants. New data for common bean (Phaseolus vulgaris) and wheat (Triticum aestivum) were presented that supported this view and indicated that the effects of elevated atmospheric [CO2 ] on nitrogen (N) assimilation and growth of C3 vascular plants were similar regardless of the form of N assimilated. Bloom et al. strongly criticised the arguments presented in Andrews et al. Here we respond to these criticisms and again conclude that the available data indicate that elevated atmospheric [CO2 ] does not inhibit NO3 - assimilation of C3 plants. Measurement of the partitioning of NO3 - assimilation between root and shoot of C3 species under different NO3 - supply, at ambient and elevated CO2 would determine if their NO3 - assimilation is inhibited in shoots but enhanced in roots at elevated atmospheric CO2 .

By modifying two genes involved in lipid biosynthesis and storage [cysteine oleosin (cys-OLE)/diacylglycerol O-acyltransferase (DGAT)], the accumulation of stable lipid droplets in perennial ryegrass (Lolium perenne) leaves was achieved. Growth, biomass allocation, leaf structure, gas exchange parameters, fatty acids, and water-soluble carbohydrates were quantified for a high-expressing cys-OLE/DGAT ryegrass transformant (HL) and a wild-type (WT) control grown under controlled conditions with 1-10 mM nitrogen (N) supply at ambient and elevated atmospheric CO2. A dramatic shift in leaf carbon (C) storage occurred in HL leaves, away from readily mobilizable carbohydrates and towards stable lipid droplets. HL exhibited an increased growth rate, mainly in non-photosynthetic organs, leading to a decreased leaf mass fraction. HL leaves, however, displayed an increased specific leaf area and photosynthetic rate per unit leaf area, delivering greater overall C capture and leaf growth at high N supply. HL also exhibited a greater photosynthesis response to elevated atmospheric CO2. We speculate that by behaving as uniquely stable microsinks for C, cys-OLE-encapsulated lipid droplets can reduce feedback inhibition of photosynthesis and drive greater C capture. Manipulation of many genes and gene combinations has been used to increase non-seed lipid content. However, the cys-OLE/DGAT technology remains the only reported case that increases plant biomass. We contrast cys-OLE/DGAT with other lipid accumulation strategies and discuss the implications of introducing lipid sinks into non-seed organs for plant energy homeostasis and growth.

Herein the members of the Subcommittee on Taxonomy of Rhizobia and Agrobacteria of the International Committee on Systematics of Prokaryotes review recent developments in rhizobial and agrobacterial taxonomy and propose updated minimal standards for the description of new species (and genera) in these groups. The essential requirements (minimal standards) for description of a new species are (1) a genome sequence of at least the proposed type strain and (2) evidence for differentiation from other species based on genome sequence comparisons. It is also recommended that (3) genetic variation within the species is documented with sequence data from several clearly different strains and (4) phenotypic features are described, and their variation documented with data from a relevant set of representative strains. Furthermore, it is encouraged that information is provided on (5) nodulation or pathogenicity phenotypes, as appropriate, with relevant gene sequences. These guidelines supplement the current rules of general bacterial taxonomy, which require (6) a name that conforms to the International Code of Nomenclature of Prokaryotes, (7) validation of the name by publication either directly in the International Journal of Systematic and Evolutionary Microbiology or in a validation list when published elsewhere, and (8) deposition of the type strain in two international culture collections in separate countries.

Five strains of Gram-stain-negative, rod-shaped bacteria were isolated from Carmichaelia and Montigena root nodules. Based on 16S rRNA gene phylogeny, they were shown to belong to the genus Mesorhizobium, and to be most closely related to Mesorhizobium jarvisii ATCC 33669T (100-99.6 % sequence similarity), Mesorhizobium huakuii IAM 14158T (99.9-99.6 %), Mesorhizobium japonicum MAFF303099T (99.8-99.6 %) and Mesorhizobium erdmanii USDA 3471T (99.8-99.5 %). Additionally, the strains formed distinct groups based on housekeeping gene analysis and were most closely related to M. jarvisii ATCC 33669T (89.6-89.5 and 97.6-97.3 % sequence similarity for glnII and recA, respectively), M. erdmanii USDA 3471T (94.3-94.0 and 94.9-94.1 %), M. japonicum MAFF303099T (90.0-89.9 and 96.7-96.2 %) and M. huakuii IAM 14158T (89.9-90.0 and 95.4-94.9 %). Chemotaxonomic data supported the assignment of the strains to the genus Mesorhizobium and DNA-DNA hybridizations, average nucleotide identity analysis, matrix-assisted laser desorption ionization time-of-flight MS analysis, physiological and biochemical tests differentiated them genotypically and phenotypically from their nearest neighbouring species. Therefore, these strains are considered to represent a novel species, for which the name Mesorhizobium carmichaelinearum sp. nov. is proposed. The type strain is ICMP 18942T (=MonP1N1T=LMG 28414T).

Atmospheric carbon dioxide concentration ([CO2]) increased from around 280 ppm in 1750 to 400 ppm in 2016 and is likely to continue to increase throughout this century. It has been argued that wheat, Arabidopsis, and C3 plants in general respond more positively to elevated atmospheric [CO2] under ammonium (NH4+) nutrition than under nitrate (NO3-) nutrition because elevated CO2 inhibits their photoreduction of NO3- and hence reduces their total plant nitrogen (N) assimilation and ultimately growth. Here, it is argued that the weight of evidence in the literature indicates that elevated atmospheric [CO2] does not inhibit NO3- assimilation and growth of C3 vascular plants. New data for common bean and wheat support this view and indicate that the effects of elevated atmospheric [CO2] on N assimilation and growth of C3 vascular plants will be similar regardless of the form of N assimilated.