Recipient Organization
VIRGINIA POLYTECHNIC INSTITUTE
(N/A)
BLACKSBURG,VA 24061
Performing Department
(N/A)
Non Technical Summary
The unprecedented rise in atmospheric CO2 concentration ([CO2]) is expected to increase the yields of C3 crops but at the cost of important mineral nutrients. Despite the large body of literature documenting the experimental effects of elevated [CO2] on biomass and nutrient accumulation, the physiological mechanisms that link the increased biomass response with alterations in mineral nutritional content have not been empirically tested. In soybean (Glycine max L. Merr.), there is significant phenotypic variation for biomass and mineral content responses to elevated [CO2] that can be used to dissect the underlying mechanisms of trait responses to rising [CO2]. Eight cultivars of soybean will be utilized in this proposal to answer three research aims: (1) empirically test the hypothesized physiological and molecular mechanisms of mineral composition response to elevated [CO2]; (2) build mathematical models of the underlying physiology of elemental accumulation to test how elevated [CO2] alters elemental accumulation in plants; and (3) validate outputs of these models in a field setting to determine the most likely mechanism(s) associated with lower mineral nutrients in plants in elevated [CO2]. Outputs from growth chamber and field experiments will be used as inputs to build models of the underlying physiology of elemental accumulation under elevated [CO2]. Outcomes from this proposal can be translated directly into molecular resources to develop more climate-resilient crops.
Animal Health Component
10%
Research Effort Categories
Basic
90%
Applied
10%
Developmental
0%
Goals / Objectives
The current trend of fossil fuel consumption suggests atmospheric carbon dioxide concentration ([CO2]) will rise to at least 650 ppm by the end of the century. This increase in [CO2] is hypothesized to benefit global food production and help meet the demands of a growing population, as elevated [CO2] increases rates of photosynthesis and water-use efficiency in C3plants through a "fertilization" effect. Experimental and meta-analytic data demonstrate that even though growth is enhanced in plants grown in elevated [CO2], protein content, nitrogen concentrations, and most other nutrients aresignificantly decreased. Despite this large body of literature documenting the experimental effects of elevated [CO2] on biomass and nutrient accumulation, the physiological mechanisms that link the increased biomass response to elevated [CO2] with alterations in mineral nutritional content have not been empirically tested.Several hypotheses that are not mutually exclusive may contribute to lower nutrient content with increased biomass in C3plants grown at elevated [CO2], including1.lower transpiration in leaves,2.down-regulation of photosynthesis,3.altered requirements for minerals due to changes in enzyme/organic complex requirements,4.increased carbohydrate and fiber content that dilutes mineral content in seeds and other ograns,5.inhibition of nitrate assimilation by decreaed photorespiration,6.reduction in mineral absorption by roots and altered root architecture, and7.alteration in expression of mineral transporters. While some of these hypotheses have been tested, these analyses did not address changes in nutrient uptake and structure of roots, nor did they perform anyempirical validationof each hypothesis. It is imperative that these studies be performed to increase our understanding of the mechanisms determining elemental accumulation in seeds and other plant organs under sub-ambient, ambient, and elevated [CO2]. Outcomes from these experiments have the potential to be translated directly into molecular resources and tools to develop more climate-resilient crops.The proposed work will leverage the power of multi-scale modeling and extensive physiological studies to improve fundamental understanding of the mechanisms contributing to lower mineral nutrients in plants in elevated [CO2]. This work will be carried using soybean (Glycine maxL. Merr.) as a model C3plant legume system.This research has three objectives: 1. empirically test the hypothesized physiological and molecular mechanisms of mineral composition response to elevated [CO2]; 2. build mathematical models of the underlying physiology of elemental accumulation to test how elevated [CO2] alters elemental accumulation in plants; and 3. validate outputs of these models in a field setting to determine the most likely mechanism(s) associated with lower mineral nutrients in elevated [CO2] in the field. The multidisciplinary PI team will take advantage of soybean cultivars with contrasting mineral nutrient response to elevated [CO2] and other physiological characteristics to enhance our understanding of the physiological basis of mineral accumulation in plant organs under elevated [CO2].
Project Methods
Aim 1: We will empircally test the potential mechanism(s) associated with decreased nutrient accumulation in plants grown in elevated [CO2]. Plants will be grown in growth chambers in sub-ambient [CO2] (~350 ppm) and elevated [CO2] (650 ppm). Plants will be grown in pots in a 14 h/10 h day/night schedule and 25°C day/21°C night temperature conditions and will be fertilized to ensure nutrient content in the soil is not limiting. A total of six chambers (n = 3) in a randomized complete block design will be used. Plants within each chamber will be rotated every 3 days to minimize chamber effects.Aim 1, Task 1. Test if reduced transpiration or mineral dilution alters nutrient distribution. We will grow four cultivars of soybean in replicated growth chamber experiments in sub-ambient and elevated [CO2]: PI398223, PI567201, Loda and HS93-4118. We will measure rates of transpiration using a mini-lysimeter. We will also sample leaf, root, seed, and xylem sap tissues for ionome analysis at vegetative (V5), full pod (R4), beginning seed (R5), and full seed (R6) stages and seeds at full maturity (R8) as well. We will divide the canopy into three sections (top, middle, and bottom) and analyze the leaf and seed ionome in those three parts. Ionome analysis will include Fe, Zn, and 18 other elements and be analyzed by Co-PI Baxter using an ICP-MS. Photosynthesis and stomatal conductance measurements will be taken using a Portable Photosynthesis System (LI-6800) at midday at the V5-R6 time points. Additionally, carbon isotope discrimination will be measured from the same leaf as gas exchange. We will analyze total nonstructural carbohydrate content (TNC) in leaf and seed tissues at the V5-R6 time points. Finally, total seed weight and aboveground biomass will be determined at maturity. We will also perform tissue sampling for RNA-Seq in order to link gene expression and physiological response to elevated [CO2]. Leaf, seed, and root samples will be collected during vegetative and reproductive stages (V5, R4, R5, and R6). RNA will be isolated from leaf, root, and seed tissue collected. DNA libraries will be synthesized from RNA samples sequenced using the Illumina HiSeq 4000 sequencer. Reads will be filtered for quality and mapped to the Wm82.a2.v1 soybean reference genome assembly to determination of transcript abundance values. Differential gene expression analysis will be done using DESeq2.Aim 1, Task 2. Test if down-regulation of photosynthesis or inhibition of nitrate assimilation by decreased photorespiration limits nutrient uptake in elevated [CO2]. For this task will use two genotypes of soybean: a non-nodulating line of soybean without a symbiotic relationship with rhizobia (Williams82-NN) and the corresponding nodulated line (Williams82). We will grow all genotypes in replicated growth chamberexperiments (n = 3) in sub-ambient and elevated [CO2]. Two plants of each cultivar will be assigned to one of three different nutrient treatments corresponding with different nitrogen sources (soil collected at the SoyFACE facility in Illinois (Treatiment 1), Long Ashton solution substituting for the N source with 1 mM NH4Cl (Treatment 2) and 1 mM KNO3 (Treatment 3). We will assess photosynthesis and stomatal conductance and transpiration using the same methods and same time points (V5-R6) as in Aim 1, Task 1. Additionally, A-Ci curves will test whether the different cultivars grown under different nitrogenregimes acclimate to elevated CO2. Leaf and seed tissues will also be sampled for ionomic and TNC analysis using the same methodology as in Aim 1, Task 1. We will only sample at one position in the canopy for ionomic analysis depending on the results in Aim 1, Task 1. Total nitrogencontent and free nitrate will be calculated from the aboveground biomass, and we will use the natural abundance method utilizing 15N isotopic analysis.Aim 1, Task 3. Test if altered requirements for minerals or changes in root architecture/root mineral transport are linked with decreased mineral content in elevated [CO2]. Clark, Flyer, Williams, and Spencer will be grown in replicated GC experiments (n = 3) in sub-ambient and elevated [CO2]. We will perform A-Ci curve analysis as in Aim 1, Task 2. The root structure, ionome, and transcriptome will also be sampled. Root tissue will also be analyzed using a bench top scanner and software (WinRHIZO). Aboveground biomass and total seed weight will also be collected.Aim 2: For the modeling work, we will focus on the critical nutrients, Fe and Zn, although the approach can be easily scaled for other nutrients. We will use a series of systems models with increasing complexity to evaluate the physiological data generated iin Aim 1 for their ability to explain the variation in Fe/Zn and other mineral accumulation in seeds. We will use approximate Bayesian computation to infer parameter values for the underlying processes and identify areas of low sensitivity (more information needed) or high sensitivity (high model confidence/ identifiability). By using genotypes that contrast in their physiological response to elevated [CO2] , we will be able to explore the parameter space of the models more fully. The models will be fit to each genotype and [CO2] separately, and then the parameters and fitting statistics will be compared across genotypes to evaluate how well each model is performing. The models consist of tissue compartments that describe elemental storage dynamics as a function of plant growth. Since little is known about this system, we propose to start with the simplest possible model based on models of root-shoot interactions and resource allocation and uptake. The model has two compartments that increase in size (root, shoot), and two sub-compartments that increase in number and size as the plant grows (trifoliate node sub-compartments, which include seed and leaf). This model will utilize all of the data collected during Aim 1. From the linked genotype and phenotypic analyses, we can begin to distinguish among hypotheses 1-7 by comparing with the predicted model parameters from this aim.Aim 3: We will test the most likely mechanism(s) associated with lower mineral nutrients in elevated [CO2] in the field using the SoyFACE facility in Illinois. We will be grow replicated plots of soybean at ambient [CO2] (~410 ppm) and elevated [CO2] (650 ppm). We will grow all eight cultivars plus Williams82-NN with contrasting phenotypes in the field and assess transpiration rate, down-regulation of photosynthesis, and plant mineral content to determine a link between transpiration rate and/or mineral dilution on seed mineral nutrient concentration and assess the possibility of down-regulation on photosynthesis and altered requirements for minerals on seed mineral concentration in elevated [CO2]. To assess down-regulation of photosynthesis, A-Ci curves will be performed as described in Aim 1, Task 2. We will also sample leaves, seeds, xylem sap, and roots for ionome analysis. We will analyze TNC in leaf and seed tissues as in Aim 1, Task 1. We will also perform tissue sampling for RNA-Seq in order to link gene expression and physiological response to elevated [CO2]. In addition to ionome and RNA sampling in the roots, we will also use a "shovelomic" approach to determine if changes in root architecture occur in plants grown under elevated [CO2]. We will calculate the amount of assimilated nitrogenby analyzing total nitrogencontent and free nitrate, as well as fixed nitrogen from aboveground biomass as in Aim 1, Task 2. Yield will be recorded at R8. Total aboveground biomass will be weighed, and seeds will be threshed with a belt thresher and then weighed. These data will be used to calculate yield and harvest index.