High Temperature Acclimation of Leaf Gas Exchange, Photochemistry, and Metabolomic Profiles in Populus trichocarpa

TL;DR: Dewhirst et al. as discussed by the authors investigated the effect of methanol on the physiological and biochemical impacts of high growth temperature on poplar seedlings and highlighted the enhancement of the optimum temperature of ETR as a rapid thermal acclimation mechanism.

Abstract: Author(s): Dewhirst, RA; Handakumbura, P; Clendinen, CS; Arm, E; Tate, K; Wang, W; Washton, NM; Young, RP; Mortimer, JC; McDowell, NG; Jardine, KJ | Abstract: High temperatures alter the thermal sensitivities of numerous physiological and biochemical processes that impact tree growth and productivity. Foliar and root applications of methanol have been implicated in plant acclimation to high temperature via the C1 pathway. Here, we characterized temperature acclimation at 35 °C of leaf gas exchange, chlorophyll fluorescence, and extractable metabolites of potted Populus trichocarpa saplings and examined potential influences of mM concentrations of methanol added during soil watering over a two-month period. Relative to plants grown under the low growth temperature (LGT), high growth temperature (HGT) plants showed a suppression of leaf water use and carbon cycling including transpiration (E), net photosynthesis (Pn), an estimate of photorespiration (Rp), and dark respiration (Rd), attributed to reductions in stomatal conductance and direct negative effects on gas exchange and photosynthetic machinery. In contrast, HGT plants showed an upregulation of nonphotochemical quenching (NPQt), the optimum temperature for ETR, and leaf isoprene emissions at 40 °C. A large number of metabolites (867) were induced under HGT, many implicated in flavonoid biosynthesis highlighting a potentially protective role for these compounds. Methanol application did not significantly alter leaf gas exchange but slightly reduced the suppression of Rd and Rp by the high growth temperature while slightly impairing ETR, Fv′/Fm′, and qp. However, we were unable to determine if soil methanol was sufficiently taken up by the plant to have a direct effect on foliar processes. A small number of extracted leaf tissue metabolites (55 out of 10 015) showed significantly altered abundances under LGT and methanol treatments relative to water controls, and this increased in compound number (222) at the HGT. The results demonstrate the large physiological and biochemical impacts of high growth temperature on poplar seedlings and highlight the enhancement of the optimum temperature of ETR as a rapid thermal acclimation mechanism. Although no large effect on leaf physiology was observed, the results are consistent with methanol both impairing photochemistry of the light reactions via formaldehyde toxicity and stimulating photosynthesis and dark respiration through formate oxidation to CO2.

Summary

2.1 Plant materials and growth conditions

• 36 Populus trichocarpa trees in 1-gallon pots (6.5” diameter and 7” deep) with Sunshine MVP soil mix (Sun Gro Horticulture, MA, USA) and between 3.0-3.5’ in height were obtained from an established tree nursery in Eastern Washington, USA (Plants of the Wild).
• The trees were delivered to the Environmental Molecular Sciences Laboratory (EMSL) on 03 June 2019 and placed in two identical growth chambers (Percival Intellus Control System, Iowa, USA) maintained under a 16-hour photoperiod (6:00-22:00) with constant daytime photosynthetically active radiation intensity of (350 µmol m-2s-1), and day/night air temperatures of 25°C/20°C and a constant relative humidity set at 60%.
• Plants were watered with 50 mL of water or one of the three methanol solutions (0, 9 or 27 mM) for the first two days (06 June and 07 June), then watered with 200 mL water or methanol solutions every other day until the end of the experiment on 01 Aug 2019 (total experimental duration of 57 days).
• These methanol concentrations were chosen to be physiologically relevant, but low enough to avoid methanol toxicity.
• A summary timeline of the leaf gas exchange, chlorophyll fluorescence, and metabolomics data collected including biological replicates is shown in Table 1.

2.2 Leaf gas exchange

• A portable photosynthesis system (6800, LI-COR Biosciences, Nebraska, USA) was used to collect leaf gas exchange fluxes with gas flow rate entering the 6 cm 2 leaf chamber maintained at 500 µmol s-1, and a reference CO2 and H2O concentrations of 400 ppm and 25 mmol mol-1, respectively.
• Where possible, mature leaves near the top of each branch were studied.
• After placing the leaf in the chamber, gas exchange measurements were collected after an equilibration period of 3-10 minutes in the light and 2-3 minutes in the dark, in order to allow Pn and gs to reach steady state conditions.
• Dark measurements were collected after dark adapting the leaf for 20 minutes using aluminum foil just prior to measurements.

2.3 Chlorophyll fluorescence

• For each of the three gas exchange experiments using the Li6800 photosynthesis system (light, dark, and temperature responses curves), chlorophyll fluorescence measurements were simultaneously collected using a multiphase flash fluorometer (6800-01F, LI-COR Biosciences, USA).
• Samples were randomized and analyzed using an Ultimate 3000 HPLC system (Thermo Fisher Scientific/Dionex RSLC, Dionex, Massachusetts, USA) LC system coupled to an LTQ Orbitrap Velos high-resolution mass spectrometer (Thermo Fisher Scientific, Massachusetts, USA) equipped with a HESI (heated electrospray ionization, Thermo Fisher Scientific).
• Samples were analyzed in both positive (+ve) and negative (−ve) ionization modes.
• Mobile phase A (0.1% acetic acid) and mobile phase B were initially at a ratio of 90:10 respectively and maintained for 5 min.
• A comparable leaf with similar distance to the growth chamber light source was marked from each plant and the fluorescence was captured weekly during the same time of the diurnal cycle.

2.6 Statistical analysis

• ANOVA and Tukey post hoc tests were carried out in R version 4.0.0 to determine significant effects of temperature and methanol treatment on leaf gas exchange variables.
• The number of observations included in the statistical analysis differed with each variable and group (see Table 2).
• All statistically significant results (p ≤0.05) are reported with the exact p value, unless p< 0.0001 (Table 3), and originate from ANOVA with Tukey post hoc analysis.
• LC-MS data was analyzed and visualized with metaboanalyst and R version 4.0.0.
• Compounds were considered significantly altered by either temperature of methanol treatment if they showed a ∓ 2-fold change in abundance which was statistically significant (p≤0.05, Students t-test).

3.1 Impacts of HGT on leaf gas exchange and photochemical parameters of

• Photosynthesis After arrival at the Environmental Molecular Sciences Laboratory (EMSL) in eastern Washington, USA, the 36 poplar saplings were placed within plant growth chambers (18 in LGT and 18 in HGT) and allowed five days to acclimate prior to the commencement of leaf measurements, which started on day 6 and continued periodically until day 57 (Table 1).
• Moreover, despite showing lower magnitudes under standard environmental conditions, , the optimum temperature of ETR was found to be enhanced in HGT plants relative to LGT plants regardless of methanol treatment.

3.2 Impacts of HGT on extractable LC-MS/MS metabolites

• Non-targeted LC-MS/MS analysis of leaf metabolites extracted in methanol: water (80:20) detected 10,015 features from leaf samples collected weekly throughout the experiment from each individual (each feature represented by a single m/z value).
• Putative compound identification based on MS/MS mass spectral comparisons with known compounds were obtained for 110 of these 867 significant metabolites (supplementary table 1) and grouped according to their relevant metabolic activities as defined by the Kyoto Encyclopedia of Genes and Genomes (KEGG, https://www.genome.jp/kegg/pathway.html).
• The majority of the identified extractable metabolites are involved in secondary metabolism, predominantly the flavonoid and polyphenolic pathways (the latter included in ‘other secondary metabolites’ KEGG category; Table 4).
• This pattern was repeated with other secondary metabolites, with 30 compounds decreasing under HGT and 16 increasing.

3.3 Influence of methanol treatments on leaf physiology and LC-MS/MS

• Metabolites Similarly, changes due to the methanol treatments could not be detected gs and E.
• This analysis did not reveal any statistically significant trend in the HGT suppressions of Pn, E, and gs due to the methanol treatments .
• Methanol treatments appeared to reduce the suppression of the mean values of leaf dark respiration (Rd) by the high growth temperature (36.8% suppression in water controls compared with 30.8-35% suppression in the methanol treatments).
• A small number of extracted leaf tissue metabolites (55 out of 10,015) showed significantly altered abundances under LGT and methanol treatment relative to water controls, and this greatly increased in compound number (222) at the HGT.
• There were a greater number of compounds showing a significant methanol-induced change in the HGT , including 141 compounds that decreased with methanol and 81 compounds that increased.

4.1 Response of leaf gas exchange and photosynthesis to HGT

• Ongoing and predicted increases in surface temperatures beyond the optimal for plant metabolism can have strong negative impacts on plant productivity.
• Moreover, due to CO2 production via the C1 pathway, methanol-treated plants are anticipated to have suppressed rates of photorespiration (Rp) and enhanced dark respiration (RD).
• The results support a view that despite of a suppression of its magnitude relative to LGT plants, an enhancement of the optimum temperature of ETR and the activity of pathways that utilize the end products of ETR (ATP and NADPH) including isoprene biosynthesis, represents a rapid thermal acclimation mechanism for P. trichocarpa and potentially other poplar species.
• Nonetheless, this was associated with the upregulation of energy dissipating processes in HGT plants including nonphotochemical quenching (NPQt) and leaf isoprene emissions at 40°C.
• These average % changes were not statistically significant due to the high variability within the temperature and methanol experimental treatments.

4.3 Metabolomic response of poplar to HGT and methanol

• In this study the authors collected both leaf physiological and metabolomic data in parallel over the two-month experimental period.
• Flavonoids serve as effective antioxidants and genes involved in the flavonoid biosynthesis pathway have been implicated in response to heat stress64.
• Importantly, their analysis revealed that high growth temperature and methanol treatments have largely distinct patterns of impacts on metabolites.
• Compounds that were altered in abundance in the methanol treatment were largely distinct from those that changed due to the HGT.
• Gene expression can be modified by DNA methylation.

Figures

Figure 6: Volcano plot showing growth temperature-induced differences in extracted leaf compound abundances (LC-MS positive mode). Compounds that have a greater than ± 2-fold abundance change (x-axis) that is statistically significant (p<0.05, t-test; y axis) between LGT and HGT plants at 0 mM methanol are shown in pink. The number of significant compounds that decreased in abundance in the HGT plants relative to LGT plants is indicated on the left side of the plot, and the number of compounds significantly increased in the HGT plants relative to LGT plants is indicated on the right side.

Table 4: Summary of HGT effect on metabolomic profiles. The number of samples that it was possible to obtain a compound ID for that decreased or increased in each metabolic pathway as defined by KEGG under HGT are indicated. All compounds included in this analysis were identified as having a significant change in abundance in HGT as in Figure 6.

Figure 7: Volcano plot showing growth methanol-induced differences in

Figure 4: Mean values +/- 1 standard deviation in the light of isoprene emissions at 40 ℃ leaf temperature determined by PTR-MS (LGT: n=3, HGT: n=3) and NPQt determined by a handheld fluorometer (LGT: n=49, HGT: n=45) under two growth

Table 2: Number of leaf observations for each variable shown used to calculate the mean values in Figures. 1-4.

Table 3: P values of ANOVA and Tukey post-hoc analysis. Statistically significant values are in bold. Data was compared between temperatures (at the same methanol concentration, the first three columns of values) and between methanol concentrations (at the same temperature, the last six columns of values).

Figure 5: Leaf temperature ETR response curves in the light during three different dates for a) LGT plants (n=12) and b) HGT plants (n=10). Note, select plants from each of the three methanol treatments were measured (0 mM, 9 mM, and 27 mM). Also shown c) are the mean optimum leaf temperature of ETR (ToptETR) +/- 1 standard deviation for the LGT and HGT plants shown in a. and b.

Table 1: Timeline of experimental treatments, observations, and biological replicates throughout the 2 month (57 day) experimental period on 36 poplar saplings. The numbers refer to the total number of individuals analyzed at each timepoint.

Figure 8: Heatmap of compounds that show a significant temperature or methanol effect. Magenta represents compounds that show a significant effect, a +/- 2-fold change in abundance, p<0.05 t-test, grey signifies no significant effect. The 1034 compounds identified in figure 6 are included in this plot, with 867 significant compounds represented for temperature effect, 222 for meOH effect at HGT and 55 for meOH effect at LGT. The m/z values range from 77 to 707. Compounds are grouped according to methanol or temperature effect.

Figure 1: Mean values +/- 1 standard deviation of leaf net photosynthesis (Pn), stomatal conductance (gs), and transpiration (E) determined over the two-month experimental period in 2019 by a Li6800 photosynthesis system under two growth temperatures (25 °C, LGT and 35 °C, HGT) and three methanol concentrations (0, 9, and 27 mM).

Full text

Lawrence Berkeley National Laboratory
Recent Work
Title
High Temperature Acclimation of Leaf Gas Exchange, Photochemistry, and Metabolomic
Profiles in Populus trichocarpa
Permalink
https://escholarship.org/uc/item/6sp1155f
Journal
ACS Earth and Space Chemistry, 5(8)
ISSN
2472-3452
Authors
Dewhirst, RA
Handakumbura, P
Clendinen, CS
et al.
Publication Date
2021-08-19
DOI
10.1021/acsearthspacechem.0c00299
Peer reviewed
eScholarship.org Powered by the California Digital Library
University of California

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Arm
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, Kylee Tate
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, Nancy M. Washton
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Jenny C. Mortimer
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, Nate G. McDowell
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 ?A
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?R
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A
B:6
1@ 4C?NPQ
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        ?73'A        1@  
>
:+
       R
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   
  @-;DE;Dq
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: 
  F"
        B     : $   
       ?00      ))0A     
1@ 
       ?(((A   1@: @

 
@-:
$B  
                
"
2
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1. Introduction
1                 
          44    
"?A
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:.?g
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A 
 g
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 P
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: @                 
      G          
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:  $
 %7(%%%  
 ?#,,A
   
    #,,
&
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Frequently asked questions

Q1. What have the authors contributed in "High temperature acclimation of leaf gas exchange, photochemistry, and metabolomic profiles in populus trichocarpa" ?

Here, the authors characterized temperature acclimation at 35°C of leaf gas exchange, chlorophyll fluorescence and extractable metabolites of potted Populus trichocarpa saplings and examined potential influences of mM concentrations of methanol added during soil watering over a two-month period. A large number of metabolites ( 867 ) were induced under HGT, many implicated in flavonoid biosynthesis highlighting a potentially protective role for these compounds. 

Q2. What are the future works in "High temperature acclimation of leaf gas exchange, photochemistry, and metabolomic profiles in populus trichocarpa" ?

Plants 2021, 10. ( 42 ) Baker, N. R. ; Rosenqvist, E. Applications of chlorophyll fluorescence can improve crop production strategies: an examination of future possibilities. 2013, 60, 764–769. ( 64 ) Su, P. ; Jiang, C. ; Qin, H. ; Hu, R. ; Feng, J. ; Chang, J. ; Yang, G. ; He, G. Identification of Potential Genes Responsible for Thermotolerance in Wheat under High Temperature Stress. 

Q3. What is the way to reduce the suppression of Rd and Rp in tobacco plants?

Given that methanol supplied to the transpiration stream in plants can lead to enhanced CO2 production via formate oxidation36, a reduced suppression of Rd and Rp in HGT plants relative to LGT plants could be expected. 

Q4. Why did the observed decrease in abundance of lipid compounds and metabolism occur at HGT?

The observed decrease in abundance of lipid compounds and metabolism at HGT could be due in part to the increase in lipid peroxidation which occurs at high temperatures69, as well as changes in the expression of genes relating to lipid metabolism. 

Q5. What is the effect of heat stress on leaf metabolites?

In addition to changes in leaf gas exchange parameters, leaf metaboliteprofiles are also altered during heat stress, in part controlled by heat stress transcription factors22. 

Q6. What is the role of methanol in the acclimation of poplar trees?

Temperature acclimation of poplar trees is important to understand in the face of climate change-induced increase of surface temperatures and its corresponding impact on tree productivity for biofuel and bioproducts. 

Q7. What is the phenotypic plasticity of plants grown at higher temperatures?

With changes in growth temperature, many plants show phenotypic plasticity in their photosynthetic properties with plants grown at higher temperatures shifting their optimal temperature of Pn to higher values13. 

Q8. What is the role of methanol in enhancing biomass accumulation rates in crops under a?

there may be an important role of methanol in enhancing biomass accumulation rates in crops under abiotic stress through a proposed mechanism involving photorespiratory production of serine and CO2 production in chloroplasts37. 

Q9. What is the link between isoprene emissions and leaf temperature?

Isoprene emissions have been demonstrated to positively correlate with leaf temperature52 and are directly linked to photosynthesis through both carbon skeletons and the consumption of photosynthetic ATP/NADPH, which can be in“excess” during partial stomatal closure at high temperatures. 

Q10. What is the relationship between the temperature of the leaves and the photorespiration rate?

As instantaneous leaf temperatures increase in C3 plants, photorespirationrates rise faster than photosynthetic rates leading to an increase in the ratio of photorespiratory production of CO2 to photosynthetic CO2 assimilation9. 

Q11. What is the explanation for the increase in isoprene emissions in HGT plants?

One potential explanation for their observation of increased isoprene emissions at high leaf temperature (40°C) in the present study on P. trichocarpa is the stimulation of the optimum leaf temperature of ETR in HGT plants relative to LGT plants. 

Q12. What is the role of the tree in predicting the properties of natural and managed ecosystems exposed?

understanding the underlying biochemical and physiological processes involved in tree response and acclimation to elevated surface temperatures are critical for accurately predicting functions, properties, and structures of natural and managed ecosystems exposed to surface warming. 

Q13. What is the role of the methanol-induced C1 pathway in plants?

The authors also hypothesized (H2) that an important metabolic role of the methanol-initiated C1 pathway occurs during high temperature stress in C3 plants.