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Plants that grow and thrive under abiotic stress often do so with the help of endophytic microorganisms. Although nitrogen-fixing (diazotrophic) endophytes colonize many wild plants, these natural relationships may be disrupted in cultivated crop species where breeding and genotype selection often occur under conditions of intensive fertilization and irrigation. Many energy crops including corn may still benefit from diazotrophic endophyte inoculations allowing for more efficient biomass production with less input of petroleum-derived fertilizer. A selection of diazotrophic endophytes isolated from willow (Salix sitchensis, Sitka willow) and poplar (Populus trichocarpa, black cottonwood) growing in nutrient-poor river sides were used as inoculum in three experiments testing the effect on plant growth and leaf level physiology of a sweet corn variety under various levels of applied nitrogen fertilizer. We report substantial growth promotion with improved leaf physiology of corn plants in response to diazotrophic endophyte inoculations. Significant gains of early biomass with a greater root : shoot ratio were found for plants receiving endophytic inocula over the uninoculated control groups regardless of the nitrogen level. Furthermore, inoculated plants exhibited consistently higher rates of net CO2 assimilation than did those without endophytic inoculation. These results have beneficial implications for enhanced plant growth in a low-input system on nutrient-poor sites. The immediate increase of root mass observed in endophyte inoculated plants has the potential to provide better establishment and early growth in resource-limited environments. The initial results of this study also indicate that the beneficial effect from endophytes isolated from poplar and willow species is not restricted to the species from which they were initially isolated.
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There is a growing emphasis on sustainable food and energy crop production that maintains high productivity while minimizing inputs such as fertilizer, herbicide, and insecticide that are both economically and ecologically costly. Major gains have been made by genetic selection of high-yielding varieties and matching genotypes to specific environmental conditions. However, much of this improvement has been made within the context of modern agricultural practices that involve the routine application of chemical fertilizers. The improvement of non-leguminous crop species to become self-sufficient in terms of nitrogen fertilizer is not a new concept (Cocking, 2005). Indeed, since the discovery of Rhizobium in the early 19th century, scientists have identified numerous bacterial and fungal species with growth-promoting effects captured by plants either through rhizospheric or endophytic relationships (Sturz & Nowak, 2000; Bhattacharjee et al., 2008; Hirsch, 2009). The term endophyte, meaning within plant, is given for fungal and bacterial microorganisms living within plant tissue without causing disease (Wilson, 1995). Symbiotic relationships between endophytes and their plant hosts are likely the result of selective pressure for plant association with specific symbionts, those which provide the maximum benefits to their hosts. The benefits to the plant might include growth stimulation through the production of growth hormones, nitrogen fixation, enhanced nutrient uptake, and protection from potential pathogenic colonizers (Latch, 1993; Malinowski et al., 2000; Khan et al., 2012a; Taule et al., 2012). Endophytic relationships also bestow both abiotic and biotic stress tolerance to plant hosts (Rodriguez et al., 2008; Redman et al., 2011). These interactions suggest that endophytes have co-evolved toward symbiosis with plant hosts. The ability of relatively fast adaptation and genetic evolution within microbial communities compared to that of the more complex host plants suggests that it is in part the microbial element that allows for timely adaptation to environmental challenges. This rapid adaptation of microbes hints at the potential broad applicability of endophytic relationships; recently reported cross species inoculations with Class 2 fungal endophytes demonstrated their ability to confer abiotic stress resistance to rice (Redman et al., 2011). The findings of Redman et al. (2011) highlight the role environmental pressures play in contributing to the endophytic biome of the host plant; e.g., endophytes that confer salt-stress tolerance are likely to be isolated from plants thriving in a high-salt environment. Logically, endophytes isolated from plants growing under nutrient limitation (e.g., nitrogen) are likely to contribute to the nutritional demands of the host. Moreover, with successful cross-host applications, endophyte inoculations can be a critical tool for adapting crops to climate change that can complement current efforts for crop genetic improvements, which often have longer time scales (Kim & Cregg, 2012).
Nitrogen often limits plant productivity and is made available to plants in the absence of chemical fertilizer by two main sources: (1) the recycling of biologically available nitrogen deposited in the soil through decomposition of organic matter or manure and/or (2) the fixation of atmospheric dinitrogen gas by diazotrophic (nitrogen fixing) microbes. Diazotrophic microbes have been isolated as free-living in the soil (Rozycki et al., 1999) and water (Short et al., 2004), in the rhizosphere, and as symbionts in both leguminous and non-leguminous plants (Bhattacharjee et al., 2008). Diazotrophic endophytes have been isolated from several non-leguminous energy crops including sugarcane (Dobereiner, 1992), poplar and willow (Doty et al., 2005, 2009), miscanthus (Davis et al., 2010), and corn (Montanez et al., 2009). Diazotrophs seem to be ubiquitous. However, endophytes differ in their colonization capacity for different hosts (Rosenblueth & Martinez-Romero, 2006) and plant-microbe relationships are affected by cultivation practices such as fertilization and irrigation (Seghers et al., 2004).
Willow (Salix sitchensis) and poplar (Populus trichocarpa, black cottonwood) are both fast growing species with the ability to thrive in nutrient-limited environments. Interest in these species for efficient biomass production has led to the isolation and characterization of diazotrophic endophytic bacteria (Doty et al., 2005, 2009; Xin et al., 2009b) and yeast (Xin et al., 2009a) from these tree species. Furthermore, Xin et al. (2009b) demonstrated the colonization of Kentucky bluegrass by the poplar endophyte WPB with an overall increase in plant biomass of 42% over the uninoculated controls. Recently, Khan et al. (2012b) tested the effectiveness of several willow and poplar endophyte isolates for plant growth promotion of a variety of commercially important crops. Successful colonization and increased biomass were observed for nearly all the crop species tested, but with cultivar-specificity. These reports provide additional evidence for specific interactions between plant host cultivars and endophytic strains.
This study tested the hypothesis that diazotrophic endophytes isolated from poplar and willow can be employed as cross-species inoculum for growth promotion in corn; a bioenergy crop species typically grown with large inputs of fertilizer. Experiment 1 tested poplar and willow endophytes as single strain inoculants in a greenhouse trial with the objective of comparing the main effect of plant growth promotion under two nitrogen fertilizer regimes: low-N and high-N. The objective of the second greenhouse experiment was to compare the main effect of plant growth promotion between single strain endophyte inoculations and multi-strain consortia. The objective of Experiment 3 was to test whether the plant growth promoting effect of endophyte inoculations would transfer from the greenhouse to the field and if the level of growth promotion would differ at differing levels of applied nitrogen fertilizer.
Materials and methods
Eight bacterial strains (WPB, WP5, WP19, PTD-1, WW2, WW6, WW7, and WW11) and one yeast strain (WP1) were chosen from a collection of diazotrophic endophytes isolated from cottonwood and willow species native to the Snoqualmie River in Washington State (Doty et al., 2005, 2009; Xin et al., 2009a) for investigation of plant growth promotion in corn (Zea mays) as a novel host species. Each strain exhibited strong growth on nitrogen-free media and gave positive amplification of the nifH marker gene for nitrogenase reductase. Endophytes were tested for production of the growth hormone, indoleacetic acid (IAA) using the colorimetric method described by Gordon & Weber (1951). All endophytes, except for WP19, tested positive for IAA production.
The single strain endophyte inoculation treatments were as follows; WP1 (Rhodotorula graminis), WPB (Burkholderia vietnamiensis), and WW7 and WW11 (both Sphingomonas sp). The two multi-strain consortia were as follows: (1) Poplar Mix (WPB, PTD-1 (Rhizobium tropici bv populus), WP5 (Rahnella sp.), and WP19 (Acinetobacter sp.); and (2) Willow Mix [WW7, WW11, WW2 (Herbaspirillum sp), and WW6 (Pseudomonas sp)].
The effects of endophyte inoculations on a sweet corn cultivar (Honey & Cream, Territorial Seed Company, Cottage Grove, OR, USA) that is suitable for Pacific Northwest climate were tested. Seed kernels were surface sterilized by incubation in 1% sodium hypochlorite solution for 30 min followed by three washes in sterile distilled water. The sterility of water from the third rinse was verified on MG/L agar (g L−1: 5.0 tryptone, 2.5 yeast extract, 5.2 NaCl, 10.0 mannitol, 1.32 sodium glutamate, 0.50 KH2PO4, 0.2 MgSO4 × 7H2O, 2 μg biotin at pH 7.0, and 15 g agar). The kernels were then allowed to germinate in sterile, sealed containers on a thin layer of water agar. Just as root and shoot tissue emerged, within 4 days, the seedlings were divided equally into groups for inoculation. All endophytes were cultured overnight in nutrient-rich MG/L broth or yeast extract-peptone-dextrose (YPD) medium (Difco Laboratories, Detroit, MI, USA) from stock collections held at −80 °C. The endophytes were washed three times in nitrogen-free Murashige and Skoog (MS) (MSPOO7; Caisson Labs, North Logan, UT, USA) liquid broth, then suspended to an OD600 of 0.05 for the single-strain inocula and a total OD600 0.10 for the multi-strain consortia where each strain is added in equal proportion. The corn seedlings were incubated for 3 h on a gentle shaker (40–60 rpm) submerged in endophyte solution. The control group was incubated in the same manner with sterile nitrogen-free media (MS).
To verify the effectiveness of the inoculation technique, two of the bacterial isolates were labeled with green fluorescent protein through electroporation with the broad-host-range plasmid pBHR1-GFP (Stevens et al., 2005). Inoculation occurred as described above with WPBgfp and PTD-1gfp as separate inocula along with an uninoculated control group. Colonization of root, stem, and leaf tissue was verified through fluorescent microscopy using a Zeiss Imager M2 equipped with an AxioCam MRM and recorded with Zeiss AxioVision software (Karl Zeiss, LLC, Thornwood, NY, USA). Colonization was followed from 1 week after inoculation for 2 months with samples visualized at 1 week, 1 month, and 2 months after inoculation.
Plant growth and physiology
Experiment 1: greenhouse single-strain inoculation trials in sand
Endophytes WP1, WPB, WPB-2, WW7, and WW11 were evaluated as single-strain inoculants for plant growth promotion under two nitrogen regimes through fertigation with a modified Hoagland's nutrient solution containing (g L−1): 0.22 CaCl2(2H2O), 0.17 K2SO4, 0.26 MgSO4(7H2O), 0.136 KH2PO4, 0.015 NaFeDTPA (10% Fe) with 1 mL −L-1 micronutrient solution containing [g L−1: 0.773 H3BO3, 0.169 MnSO4, 0.288 ZnSO4(7H2O), 0.062 CuSO4(5H2O), and 0.04 H2MoO4 (83% MoO3)]. The concentration of nitrogen was controlled through varied addition of ammonium nitrate [NH4(NO3)] with a concentration of 0.04 g L−1 (low-N) and 0.322 g L−1 (high-N) Inoculated seedlings were sown in a 1 : 1 sterilized sand and perlite mixture in four 6-well plastic seedling packs (1.13 L per 6-well pack; McConkey, Sumner, WA, USA) per endophyte treatment and maintained in separate drip trays to avoid cross-inoculation or contamination of the uninoculated control group. Three weeks after inoculation, six plants (n = 6) from each treatment (6 endophyte treatments × 2 nitrogen treatments) were transferred to individual 4 inch square pots (2.17 L “The Square One”; McConkey) containing the same sand : perlite mixture, separated by individual drip trays, and arranged in a complete randomized block design. Plants in the remaining three 6-well packs were harvested 25 days after inoculation (DAI), then pooled by 6-well pack (n = 3) and cleaned of the sand mixture. Roots and leaves were separated from the stalk. Plant tissue was placed in brown paper bags and oven dried at 70 °C for 3 days prior to being weighed. Transferred plants continued to receive 200 mL fertigation solution once weekly and distilled water was applied as needed. The corn plants were maintained in the greenhouse at an average temperature of 21 °C with a 14 h photoperiod for 90 days after inoculation. At harvest, the majority of corn plants completed tasseling and entered the silking stage; cob formation was nascent and is not reported separately than overall stem mass. Plants were harvested and processed for measurements described above for the 25 DAI.
Experiment 2: greenhouse single-strain endophyte and consortia inoculation trial
Growth and physiological responses to inoculation with Poplar Mix and Willow Mix consortia were compared to WP1, WW11 inoculations, and an uninoculated control group with or without nitrogen limitation. Square 2.17 L pots were filled with 0.35 kg Sunshine #2 mix (SunGro, Bellevue, WA, USA) and saturated with tap water. The sweet corn was prepared as described above and inoculated seedlings were sown 4 to a pot and then thinned to 2 per pot after 1 week. A total of 50 pots, five replications of each endophyte × nitrogen treatment, were arranged in a randomized complete block design on one greenhouse bench and separated by individual drip trays to avoid cross-inoculation. All plants received 200 mL of the modified Hoagland's solution once a week; half received no nitrogen (no-N) and the other half received 0.322 g L−1 added NH4(NO3) (high-N). Plants were irrigated with tap water as needed to avoid drought and maintained at 21 °C with a 14 h photoperiod. Instantaneous photosynthesis data were measured 32 DAI and 36 DAI from the second most developed leaf from the top of the plant using the LI-6400XT portable photosynthesis system (LI-COR, Lincoln, NE, USA) with block temperature set to 25 °C, CO2 concentration of the chamber was set to 1000 μmol CO2 mol−1air and the photosynthetically active radiation (PAR) set to 1500 μmol photons m−2leaf area s−1. Leaf net CO2 assimilation rate at saturating light (Amax) was recorded after 5 min stabilization period, and measurements occurred between 11:00 and 15:00 both days. Plants were harvested 39 DAI and cleaned of planting media before roots and leaves were removed from the stalk. Green leaf area was measured immediately using a LI-3100C leaf area meter (LI-COR). Dry plant tissue weight was measured after tissue was allowed to oven dry at 70 °C for 3 days. Specific leaf area (SLA) was calculated as the green leaf area per gram of leaf dry mass, cm2 g−1.
Experiment 3: field site single-strain endophyte and consortia inoculation trial
A field trial was initiated with the sweet corn inoculations as described in Experiment 2. Inoculated seedlings were sown in 24-well seedling trays (4.52 L per tray, McConkey) containing 1 : 1 sterilized sand and perlite mix and maintained for 1 week at 21 °C with a photoperiod of 14 h, then relocated to a greenhouse at the Charles L. Pack Experimental Forest where they were allowed to acclimate to ambient temperature for 1 week prior to outplanting. Up to this point, the corn plants received only irrigation with tap water. This experiment tested five endophyte treatments, Poplar Mix, Willow Mix, WP1, WW11, and uninoculated control, with three levels of nitrogen fertilizer, and was replicated in three blocks (N = 45). The corn was planted in one meter square plots separated by one meter on all sides from any neighboring plot. Each block contained one row of each of the three nitrogen levels; each row contained each of the five endophyte treatments, endophyte treatments were randomized across the entire plot to account for variation in soil condition across the field.
The Willow Mix inoculated plants became contaminated with fungus during transportation and transplantation to the field site and suffered approximately 2/3 mortality. Therefore, only one full experimental block was planted with all endophyte treatments. The data acquired from Willow Mix inoculated plants had no statistical power and are not reported here. The open plots within blocks where Willow Mix plants died had no effect on neighboring plots and were excluded from contributing to any variation in the statistical model.
The experimental site is located in the Charles L. Pack Experimental Forest, Pierce County Washington. The soil is a sandy, glacial outwash of the Indianola series (mixed, mesic Dystric Xeropsamments) (Gaulke et al., 2006). This site is excessively drained, nutrient limited, and receives full sun. Site preparation included the removal of approximately 12 inches of the top soil along with any vegetation using a bulldozer, and the entire site was repeatedly disked to reduce variability. The one meter plots were then individually tilled to a depth of at least 24 inches. All plots received 24 g of granulated monopotassium phosphate (MPK). Three levels of nitrogen were applied as pelleted urea, 0 g m−2, 6 g m−2, and 24 g m−2, with half of the nitrogen applied along with the MPK prior to planting and the remaining nitrogen dose applied as a side dress 4 weeks after planting. The corn was planted with an initial density of 18 stalks per plot, and then thinned to 9 stalks per plot 4 weeks after planting.
Plants were harvested 87 days after inoculation. Stalks from each plot were bundled together and wrapped in plastic for transport from the field. All harvested plants were kept refrigerated until final measurements were taken. Leaves were separated from the stalks and total green leaf area was measured using the LI-3100C leaf area meter (LI-COR). Plant height was measured as the distance from the root collar to the top of the stalk. Leaves, stalks, and cobs were dried in a 70 °C oven 3 days prior to determination of dry mass. Leaf N content was obtained from the oven-dried leaf tissue using a PE 2400 Series II CHN elemental analyzer (CHN work carried out at the UW SEFS Chemical Analysis Center: Perkin Elmer, Waltham, MA, USA). Specific leaf area was calculated as the green leaf area per gram of leaf dry mass, cm2 g−1. Leaf chlorophyll was measured twice with a Konica Minolta SPAD 502 (Konica Minolta, Ramsey, NJ, USA) hand-held chlorophyll meter, 4 (August) and 8 (September) weeks after transplanting. Measurements were taken on the second full leaf from the top of the plant for consistency and an average of five measurements per plot was recorded. Instantaneous photosynthesis data were measured from the second most developed leaf from the top of the plant using the LI-6400XT portable photosynthesis system (LI-COR). Measurements were taken after plot thinning in the 4th week after transplanting under ambient humidity with a block temperature of 25 °C; reference CO2 concentration of the chamber was set to 440 μmol mol−1 and the photosynthetically active radiation (PAR) set to 2000 μmol photons m−2leaf area s−1. Leaf CO2 assimilation rate at saturating light (Amax), transpiration rate (E), and stomatal conductance (gs) were recorded after a stabilization of 10 min. Intrinsic water-use efficiency (iWUE) was calculated as net photosynthesis per unit water lost, Amax/gs. Photosystem II maximum quantum yield was measured through dark-adapted fluorescence, Fv/Fm, using the LI-6400XT after a period of 3 h of natural darkness. The leaf carbon : nitrogen ratio was calculated at the end of the study as the final leaf C (g m−2) per leaf N (g m−2).
All experiments were designed for contrast of treatment means to the corresponding uninoculated control group. Data were analyzed using the sas statistical software version 9.3 (SAS Institute Inc., Cary, NC, USA). Two-way anova was used to determine the significance of the effects of endophyte inoculation and nitrogen treatment levels. Duncan's multiple range test (DMRT) and the CONTRAST statement within PROC GLM were used to compare endophyte treatment means within nitrogen treatment levels.
Successful colonization by both WPBgfp and PTD-1gfp was evident in root and stem tissue when viewed 1 week following inoculation. The bacteria were also evident in root and stem tissue when viewed 1 and 2 months after inoculation (data not shown). A representative micrograph demonstrating the colonization of leaf tissue 2 months after inoculation by PTD-1gfp is shown in Fig. 1.
Significant early growth promotion was observed for each of the single-strain inoculants over the control group within both nitrogen application levels recorded 25 DAI (Table 1). Within the low-N treatment, the largest gain in total biomass of 61% occurred in the WP1-inoculated plants followed by WW7 with 52% gain, WPB with 35% gain and WW11 with 26% gain at 25 DAI. Biomass allocation in this low-N treatment group is pronounced in roots over above-ground biomass. Inoculated plants accumulated more root mass than the control group; with 84%, 47%, 41%, and 37% gains from WP1, WPB, WW7, and WW11 inoculations, respectively. Above-ground biomass gains occurred similarly for the inoculated plants with 41% (WP1), 35% (WW7), 23% (WPB), and 16% (WW11). Endophyte inoculations improved early total biomass within the high-N treatment as well; WPB inoculated plants gained 40%, WW11 gained 35%, WW7 gained 34%, and WP1 gained 11% at 25 DAI over the control group. While root mass was improved over the control group, more biomass was allocated to the above-ground tissue under the high-N regime (Table 1). Plants inoculated with WPB exhibited a 42% gain followed by WW11 (35% gain), WW7 (30% gain), and WP1 (10% gain) in above-ground biomass over the control group.
Table 1. Greenhouse experiment 1. Oven-dry biomass of endophyte-inoculated maize plants grown in sand under two nitrogen fertigation regimes. Means (standard error) of biomass for single-strain inoculation trial at 25 DAI (n = 3) and 90 DAI (n = 6). Means followed by different letters in each column within a nitrogen treatment are significantly different at alpha = 0.10 using Duncan's multiple range test. P-values are given where Endophyte treatment means differ from the Control group for a specific contrast of means within the Nitrogen treatment level
25 days after inoculation
90 days after inoculation
Above-ground dry weight (g per plant)
Root dry weight (g per plant)
Total biomass dry weight (g per plant)
Above-ground dry weight (g per plant)
Root dry weight (g per plant)
Total biomass dry weight (g per plant)
Treatment A Low-N 0.04 g L−1 NH4(NO3)
0.220 (0.04)aP = 0.09
0.257 (0.04)aP = 0.006
0.476 (0.02)aP = 0.003
0.206 (0.04)aP = 0.078
0.397 (0.01)abP = 0.053
0.449 (0.07)abP = 0.007
Treatment B High-N 0.322 g L−1 NH4(NO3)
0.274 (0.04)abP = 0.071
0.263 (0.01)aP = 0.04
0.256 (0.03)abP = 0.06
6.56 (0.59)abP = 0.06
0.276 (0.03)aP = 0.02
At 90 DAI, total dry plant weight and the above-ground biomass were no longer statistically different between the inoculated plants and the control group for either nitrogen application level. Within the high-nitrogen application group, the root mass gain was still present for plants inoculated with WW11 (24% gain), WPB (15% gain), and WP1 (13% gain) over the uninoculated control group.
Experiment 2: greenhouse single-strain isolate and consortia inoculation trial
Nitrogen application had a significant effect on plant growth and biomass (Table 2). Although not statistically significant, plants inoculated with the Poplar Mix, Willow Mix, and WW11 gained more total biomass than the uninoculated control group while plants inoculated with WP1 produced the least biomass in the no-added-nitrogen treatment group. Under added nitrogen, total biomass gain was observed for all inoculation types relative to the control group. Plants inoculated with the Willow Mix consortium gained the most with a 10% gain over the control group under both nitrogen regimes. Consistent with Experiment 1, root gain in inoculated plants was favored over above-ground biomass. Willow Mix inoculated plants gained 14% and 11% root mass over the control group under the no-N and high-N regimes, respectively.
Table 2. Greenhouse experiment 2. Endophyte-inoculated maize grown in the greenhouse under two nitrogen fertigation regimes. Means (standard error) of biomass and growth parameters 39 DAI (n = 5). Means followed by different letters in each column within a nitrogen treatment are significantly different at alpha = 0.10 using Duncan's multiple range test. P-value is given where specific contrast of means differ from the control group within nitrogen treatment
Root dry weight (g per plant)
Total above-ground dry weight (g per plant)
Total biomass dry weight (g per plant)
Leaf area (cm2 per plant)
Specific leaf area (cm2 g−1 leaf)
Nitrogen application is significant at P < 0.0001 for all parameters measured.
ns, Endophyte inoculation is not significant as a main effect at alpha = 0.10.
Changes in biomass allocation and leaf properties were evident for endophyte-inoculated plants within nitrogen treatment levels. Green leaf area was increased for plants inoculated with the Poplar Mix, WP1, and WW11 under both of the nitrogen regimes with marginally significant increases over the control group observed in plants inoculated with WP1 (P = 0.091) and Willow Mix (P = 0.19) within the high-N regime. However, specific leaf area was only marginally significant different between WP1 (P = 0.104)-inoculated plants under no-N and WW11 (P = 0.070) under the high-N regime.
Experiment 3: field site endophyte consortia inoculation trial
Overall, endophyte-inoculated plants produced more biomass and had a higher leaf net CO2 assimilation rate at saturating light (Amax) than the uninoculated control plants. Significant differences were achieved for above-ground biomass, average plant height, SLA, leaf N, and leaf C measurements between the nitrogen treatments (Table 3). There was no significant interaction between the endophyte inoculation and the applied nitrogen treatments. None of the single species or consortium endophyte inoculation treatments tested in this study were able to produce enough growth enhancement to overcome the nitrogen limitations in the low-N (0 g m−2) and mid-N (6 g m−2) treatments.
Table 3. Experiment 3: field site single-strain endophyte and consortia inoculation trial. Plot means (standard error) for biomass, stem height, specific leaf area (SLA), leaf nitrogen (N), and leaf carbon (C) measured at harvest 87 DAI endophyte-inoculated maize. Plot means (standard error) for SPAD taken twice during the growing season
Above-ground dry weight (g per plant)
Stem height (cm)
SLA (cm2 g−1)
Leaf N (g m−2)
Leaf C (g m−2)
Significant difference from control group within nitrogen treatment is indicated by; *alpha = 0.1, †alpha = 0.05.
Nitrogen level means followed by different letters are significantly different at alpha = 0.05 using Duncan's multiple range test.
The low-N (0 g m−2) treatment group displayed the most variability with 50% more above-ground biomass for the Poplar Mix-inoculated plants than the uninoculated controls. In addition, the leaf nitrogen content (N g m−2) measured from endophyte-inoculated plants increased over the uninoculated control plants (P = 0.038; Table 3). A similar pattern is visible in both the mid-N (6 g m−2) and high-N (24 g m−2) treatment groups. Plants inoculated with Poplar Mix had 24% (mid-N) and 22% (high-N) more biomass, a 15% (mid-N) and 7% (high-N) increase in stem height and a 9% (mid-N) and 3% (high-N) increase in leaf N over the uninoculated controls. Also trending higher in leaf N content, although not statistically significant, were plants inoculated with WW11 with a 14% increase over the uninoculated plants within both the mid-N and high-N regimes. Leaf SPAD was significantly affected by the applied N treatment (August SPAD P < 0.001 and September SPAD P < 0.001); however, endophyte inoculation did not significantly alter the August SPAD readings within N treatment levels. The September SPAD readings were similar between inoculation treatments within low-N and mid-N treatment levels with the exception of Poplar Mix and WW11-inoculated plants in the low-N treatment group. SPAD readings within the high-N treatment for all endophyte-inoculated plants were significantly lower than the control group (Table 3).
The photosynthetic rate (Amax), measured at saturating light and CO2 concentration, was significantly affected by nitrogen application (P = 0.0058). The average photosynthetic rate among all N treatments was measured at 35.83 μmol CO2 m−2 s−1, with the control = 31.25 μmol CO2 m−2 s−1, Poplar Mix = 36.89 μmol CO2 m−2 s−1, WW11 = 35.97 μmol CO2 m−2 s−1, and WP1 = 34.64 μmol CO2 m−2 s−1 (Table 4). Variation within the low-N plots was high and no statistical significance was achieved with the small sample size, however an 18% increase in CO2 assimilation is observed for endophyte-inoculated plants over the uninoculated controls; 31.74 and 26.9 μmol CO2 m−2 s−1, respectively (Fig. 2). Within the mid-N treatment, there was an increase of 19% CO2 assimilation measured from the inoculated plants over that measured from the control plants; 35.4 and 29.6 μmol CO2 m−2 s−1, respectively. The Poplar Mix-inoculated plants had the most significant increase in CO2 assimilation (38.0 μmol CO2 m−2 s−1, P = 0.028) followed by the WP1 (34.73 μmol CO2 m−2 s−1, P = 0.129)- and the WW11 (33.57 μmol CO2 m−2 s−1, P = 0.22)-inoculated plants (Table 4). Within the high-N treatment, there was a 13% increase in CO2 assimilation rate measured from inoculated plants over the uninoculated control plants; 40.32 and 35.8 μmol CO2 m−2 s−1, respectively. Again, the plants inoculated with the Poplar Mix recorded a significantly higher rate of CO2 assimilation with 42.16 CO2 m−2 s−1 (P = 0.007), followed by plants inoculated with WW11 (40.1 μmol CO2 m−2 s−1, P = 0.036) and WP1 (38.7 μmol CO2 m−2 s−1 (P = 0.120). The leaf carbon to leaf nitrogen ratio data were significantly affected by the nitrogen application regime (P < 0.001) and only significantly different between endophyte-inoculated treatments in the high-N treatment group (P = 0.0024).
Table 4. Experiment 3: field site single-strain endophyte and consortia inoculation trial. Leaf level physiology treatment means (standard error) for leaf CO2 assimilation rate at saturating light (Amax), transpiration rate (E), stomatal conductance (gs), leaf carbon to nitrogen ratio (C : N), intrinsic water-use efficiency, and the dark-adapted quantum yield of photosystem II (Fv/Fm)
Amax μmol CO2 m−2 s−1
E mmol H2O m−2 s−1
gs mol H2O m−2 s−1
Leaf C : N
Significant difference from control group within nitrogen treatment is indicated by; *alpha = 0.1, †alpha = 0.05, and ‡0.01. Differences between the nitrogen levels are indicated by different letters.
Nitrogen level means followed by different letters are significantly different at alpha = 0.05 using Duncan's multiple range test.
Transpiration rate (E) increased with endophyte inoculation in all nitrogen application levels and was significantly higher in the mid-N (P = 0.029) and high-N (P = 0.0024) treatment groups for the endophyte-inoculated plants (Table 4). Stomatal conductance was not significantly affected by endophyte inoculation in the low-N treatment, although inoculated plants recorded lower gs than the control group. Conductance was increased compared with the control group for endophyte-inoculated plants in both the mid-N and high-N treatments; Poplar Mix and WW11 showed the highest increases. The calculated iWUE for the inoculated plants within N application treatment groups was highly variable and no significant differences were found between the control plants and endophyte-inoculated plants in both the low- and mid-N treatments. Poplar Mix-inoculated plants recorded significantly lowered iWUE in the high-N treatment. Changes in maximum quantum yield, dark-adapted Fv/Fm, is an indicator of stress caused to photosystem II (Lambers et al., 2008). Nitrogen treatment had a significant effect on Fv/Fm (P < 0.01), whereas the endophyte inoculations did not (P = 0.39), suggesting no biological stress resulted from inoculations.
Colonization of the monocotyledonous annual grass (Zea mays) with diazotrophic endophytes isolated from dicotyledonous perennial trees (Populus trichocarpa and Salix sitchensis) was verified through fluorescent microscopy. The poplar endophytes PTD-1 and WPB were visualized colonizing root, stem, and leaf tissue and appeared to be primarily located in vascular tissue. Inoculations of the sweet corn variety ‘Honey and Cream’ by both single-isolate endophytes and multi-strain consortia had an overall positive growth affect with no indication of pathogenesis. To our knowledge, this report is the first of its kind to describe a significant impact on leaf level physiology attributable to endophytic colonization. This is in contrast to an earlier study where colonization of poplar with the single endophyte, Enterobacter sp 638, had no significant impact on leaf physiology (Rogers et al., 2012).
The early significant increase in root formation observed in the single-isolate greenhouse trial could help seedlings be more robust against early environmental challenge. For example, longer and more abundant roots will establish a solid foundation for resistance to drought and physical challenge. This early burst of growth did not appear to have an overall negative effect on inoculated plants at harvest. Endophyte inoculation did not significantly decrease biomass nor did endophyte inoculation significantly affect the dark-adapted quantum yield, suggesting there is little or no metabolic cost for the plant host. Similar early growth response with endophyte inoculation is reported by Redman et al. (2011) where more rapid root development was observed in symbiotic rice seedlings over non-symbionts. Although the mechanism for abiotic stress protection from such symbiosis is not clear, the early and robust root growth is likely to play an important role.
Leaf physiology was significantly affected by diazotrophic endophyte inoculation where a higher rate of light saturated net CO2 assimilation (Amax) was consistently recorded for inoculated plants in the field. Amax values recorded within the high-N treatment are close to other reported Amax of corn leaves grown in high nutrients (Kim et al., 2007). Increased CO2 assimilation rate was greatest for the plants inoculated with the Poplar Mix consortium. These plants also showed the greatest increase in above-ground biomass, plant height, and leaf N. The increased Amax could be a result of increased leaf N through nitrogen fixation by the endophytes that make up the Poplar Mix consortium. Amax is influenced by the allocation of N within the leaf to proteins involved in the photosynthetic apparatus. Carbon assimilation occurs in Zea mays through the C4 photosynthetic pathway where gaseous CO2 is fixed in mesophyll cells by phosphoenolpyruvate (PEP) carboxylase to form oxaloacetate and is eventually transported into bundle sheath cells as malate. The CO2 is then released within the bundle sheath cell chloroplast to be refixed by the enzyme ribulose-1,5,-bisphosphate (RuBP) carboxylase (Rubisco) and assimilated to sucrose or starch through the C3 photosynthetic carbon reduction cycle. Thus, the rate of CO2 assimilation in maize is regulated by the rate of C4 enzyme activities including PEP carboxylase and NADP-malic enzyme as well as RuBP regeneration; all three require energy derived from the electron transport chain located on the thylakoid membrane (Sage & Monson, 1999). Mathematical modeling of the C4 pathway by von Caemmerer and Furbank (Von Caemmerer & Furbank, 1999) illustrate the relationship between light saturated PEP regeneration rate and the overall CO2 assimilation rate by Rubisco. Gas exchange measurements of C4 leaves taken under both saturating light and CO2 should be a reflection of Rubisco capacity and RuBP regeneration rate (Von Caemmerer & Furbank, 1999, 2003). PEP regeneration rate also affects Amax and it is difficult to distinguish between these limiting factors. Increased leaf nitrogen is expected to be correlated with a higher percentage of leaf N allocated to the thylakoid membrane proteins of C4 plants (Makino et al., 2003).
The leaf carbon to leaf nitrogen ratio (C : N) provides an indirect measurement of photosynthetic use efficiency (PNUE). The results of Experiment 3 are consistent with expected higher PNUE for plants under nitrogen limitation. Endophyte-inoculated plants grown in the high-N treatment had a significantly higher C : N ratio (higher PNUE) and higher leaf N than the uninoculated control plants. However, the SPAD measurements were higher for the uninoculated control plants suggesting a difference in nitrogen allocation. Although the SPAD measurements are effective non-destructive representation of leaf N status in corn and other field crops, a considerable variability can be introduced in correlating the SPAD readings with leaf N or chlorophyll content (Yang et al., 2012). Further study specifically aimed at measuring Rubisco activity and the efficiency of the electron transport chain is needed to better determine the possible contributions of diazotrophic endophyte colonization to N allocation to photosynthetic apparatuses and the regulation of C4 photosynthesis.
Intrinsic water-use efficiency (iWUE) was not significantly different between endophyte treatments within applied nitrogen levels. However, the data imply a slight interaction between the nitrogen application rate and endophyte inoculation, where iWUE is lowered in both the low-N and high-N treatment groups and raised within the mid-N for the inoculated plants in comparison to the control plants. Transpiration rate is raised by endophyte inoculation and becomes significantly different from the control group within the mid-N and high-N treatments. The higher transpiration rate is in line with the higher Amax for inoculated plants reflecting more open stomata to meet the increased demand for CO2. This rise in transpiration may also be a reflection of increased water availability through increased root mass.
The results of these trials are consistent with those reported by Mehnaz et al. (2010) where a measurable increase in biomass is observed in endophyte-inoculated corn after 30 days growth in the greenhouse. Likewise, Mehnaz et al. (2010) report that endophyte inoculation increases biomass in field trials, however, without strong statistical significance citing large variation in field trials. Where the common garden is more readily achieved in the greenhouse, field sites are ripe with natural variations. The native soil microbial community cannot be controlled and cannot be assumed to be evenly distributed within each test plot. Diazotrophic microbes isolated from soil have also been shown to colonize sweet corn and promote growth (Mehnaz & Lazarovits, 2006). In addition, Montanez et al. (2009) present evidence for biological nitrogen fixation capacity of endophytes native to maize cultivars. Their report demonstrates that variation in capacity for biological nitrogen fixation is affected through fertilization, that is, more available nitrogen in the soil leads to less biologically fixed nitrogen. Variation in biological nitrogen fixation is also attributable to variation of colonizing microbes.
This series of trials demonstrated an overall increase in biomass with significant early gains in both root and shoot production for a monocotyledonous grass inoculated with diazotrophic endophytes previously isolated from a dicotyledonous tree species. Furthermore, endophyte colonization increased leaf nitrogen content (g m−2) and CO2 assimilation rate of sweet corn in a field trial under three levels of nitrogen application. Variation observed in the current report could be an effect of multiple, unintended infections, or variation of the soil microbial communities between test plots. The larger variation within endophyte treatments, especially within the consortia treatments, may be due to variation in the colonization of the individual species within the host plant or microbial interactions. Additional investigation is required to assess the location, relative abundance, and relationship of these endophytes with the host plant's native microbial community as well as the artificial community used as inoculum.
Field site preparation and maintenance was conducted by Alvin Sharpe, Trevor Walter, Andrea Blin, Brendan Boyer, Matthew Grund, Robin McArdel, and Eric Snoozey. We are also grateful to Wade C. Boyd, Adriel Hutchinson, Conner Knoth, and Drew Zwart for helping with greenhouse study maintenance and take down and Zareen Khan for helping with the IAA assay. This research was funded in parts by the NSF ARRA grant 0930909 and NSF IGERT grant 0654252.