Performing Department
Agronomy & Plant Genetics
Non Technical Summary
U.S. Agriculture is facing an increasingly challenging threat in the form of climate change. In Minnesota, the growing season is expanding, but farmers are exposed to new climatic extremes, particularly increasingly high atmospheric water vapor pressure deficit (VPD or 'atmospheric drought') in the summer and fall seasons, unusually high temperatures in the spring and summer seasons, and increasingly unpredictable frost and freezing stresses in the winter and spring. This 5-year research program aims at addressing these challenges by identifying physiological traits and -when possible- their genetic bases to enable enhanced productivity under these stresses.During the previous 3-year phase of the project, my group focused essentially on the effects of atmospheric drying, enabling the discovery of new daytime and nighttime transpiration (TR) traits that maximize productivity under contrasted VPD regimes in key crops such as barley, maize, soybean and wheat. Moving forward, the research planned for the next 5 years will continue the work done on VPD stress while expanding to address heat and freezing stresses, with the following objectives: 1) Identification of genetic markers controlling various modalities of TR response curves to VPD and their physiological underpinnings, 2) Dissecting the genetic and physiological basis of nocturnal transpiration and its relationship to productivity, 3) Identification of the eco-physiological and genetic drivers of heat stress tolerance in Minnesota-grown oat and 4) Identification of the biophysical basis of winter barley freezing survival under Minnesotan conditions.
Animal Health Component
0%
Research Effort Categories
Basic
50%
Applied
50%
Developmental
0%
Goals / Objectives
1. JustificationAgriculture is facing unprecedented and multiple challenges, which require reducing various environmental footprints while feeding a growing population and enhancing production resilience in the face of climate change. In Minnesota, the growing season is expanding, but farmers are exposed to new climatic extremes, particularly: i) increasingly high atmospheric vapor pressure deficit (VPD) in the summer and fall seasons, ii) unusually high daytime and nighttime temperatures in the spring and summer seasons, and iii) increasingly unpredictable frost and freezing stresses in the winter and spring as a result of global climate change. For states such as in Minnesota, it is expected that such events will dramatically increase in intensity, frequency and unpredictability, thereby potentially offsetting the yield gains that could be expected from longer seasons or atmospheric CO2 fertilization, overall resulting in net yield penalties (IPCC 2018). There is currently an agreement within the scientific community that developing a new generation of locally-adapted, climate-resilient crops is one of the best available options to mitigate the effects of climate change while meeting global production needs.The research proposed seeks to develop new tools and knowledge informed by crop physiology, in order to fast-track breeding and management programs aiming at enhancing productivity under these increasingly ubiquitous abiotic stresses for crops that are critical to Minnesota agriculture (barley, maize, oat, soybean and wheat). More specifically, over the next 5 years, the overarching aim of the proposed research is to identify physiological traits and -when possible- their genetic bases to enable enhanced productivity under 1) increasing atmospheric evaporative demand during the spring and summer seasons (Goal#1), 2) heat stress during the summer (Goal#2) and 3) frost and freezing stress during the winter and early spring (Goal#3). The first 3 years of this project focused essentially on Goal#1, leading to the i) discovery of new traits that maximize productivity under contrasted VPD regimes in key crops such as barley, maize, soybean and wheat and ii) initiation of funded projects addressing Goals#2 and #3 on oat and winter barley, respectively.2. Previous work and present outlookRegarding Goal#1, the research undertaken in 2016-19 enabled the following:The development of a high-throughput phenotyping platform (the GraPh system, Tamang and Sadok 2018). This versatile tool made it possible to screen various diversity panels for their transpiration rate (TR) response curves to increasing VPD. Those response curves reflect different water use strategies that enable tolerance to various VPD regimes.The identification of a substantial variation (2-3 fold) in TR responses to VPD among diversity panels of maize (Tamang and Sadok 2018), barley (Sadok and Tamang 2019) and wheat (Schoppach et al. 2017a, Tamang et al. 2019). One of the key findings was that drought-tolerant cultivars tended to express a water-saving (segmented) TR response to VPD, while genotypes adapted to well-watered environments tended to exhibit a linear response reflecting aggressive water use.The quantification of the potential wheat yield gains that could arise from the deployment of certain modalities of TR responses to VPD (e.g., water-saving or aggressive water use under high VPD) as a function of the local management, soil conditions and historical weather data, using a geospatially-explicit crop modeling approach (Schoppach et al. 2017b, Sadok et al. 2019). This research also revealed that trade-offs (i.e., yield penalties) could arise from deploying genotypes equipped with inappropriate TR response curves to VPD to the wrong regions, offering an actionable framework to advise local breeders to navigate complex G × E interactions underlying yield increase.The discovery of non-negligible rates of nocturnal transpiration (TRN), amounting to up to 35% of daytime TR in barley, maize, soybean and wheat, which was enabled by the GraPh system. This work also revealed a very large phenotypic variability in this trait (up to 2.5-fold) in all examined species, pointing to the possibility to breed in favor or against this trait.The dissection of the functional relevance of variation in TRN. Under well-watered conditions, results indicated that increased TRN may be detrimental to yield, via its effect on increasing respiratory carbon costs (Tamang and Sadok 2018). Under water-deficit conditions, we discovered that TRN is insensitive to decreasing soil water potential, leading to a much higher contribution of TRN to progressive soil drying than previously thought (Claverie et al. 2018). Both studies indicated that these responses are genotype-dependent, opening up new possibilities to breed for increased yields by decreasing TRN. Furthermore, we discovered a previously undocumented role of root auxins in controlling TRN in wheat (Sadok and Schoppach 2019).The discovery of a circadian regulation of TRN during pre-dawn hours in all examined species, which was found to have important implications, particularly under water-limited environments. While limiting overnight TRN was found to be generally beneficial, higher predawn rates of TRN increase resulting from stronger circadian control were hypothesized to be associated with increased drought tolerance, through circadian resonance (Sadok 2016). Our findings confirmed this hypothesis by showing that a stronger circadian control was exhibited by drought-adapted barley and wheat genotypes (Sadok and Tamang 2019; Tamang et al. 2019). This trait was found to correlate with historic precipitation data from locations where barley genotypes were collected and to be under selection by Australian wheat breeders (Tamang et al. 2019; Sadok and Tamang, 2019).The initiation of a high-throughput phenotyping effort to identify the genetic basis of TR responses to VPD, TRN and its circadian control in wheat and soybean. While this effort is on-going, preliminary analyses already identified quantitative trait loci controlling TR response curves to VPD in these species.3. ObjectivesMoving forward, the research objectives for the next 5 years will continue to target Goal#1 but will expand to address Goals#2 and #3, with specific objectives for each of these goals, outlined as follows:Objective 1: Identification of QTLs controlling various modalities of TR response curves to VPD and their physiological underpinningsObjective 2: Dissecting the genetic and physiological basis of nocturnal transpiration and its relationship to productivityObjective 3: Identification of the eco-physiological and genetic drivers of heat stress tolerance in Minnesota-grown oatObjective 4: Identification of the biophysical basis of winter barley freezing survival under Minnesotan conditions
Project Methods
Objective 11.1. Replicating high-throughput phenotyping experiments and identifying QTL controlling TR response to VPD. Using the GraPh platform, our goal is to pursue the high-throughput phenotyping effort we have initiated on wheat and soybean using the protocol we have published (Tamang and Sadok 2018). On wheat, the genetic material will consist of a bi-parental mapping population developed by Jim Anderson (MN99394-1-2 x MN99550-5-2, 145 RILs), and 3 Nested Association Mapping (NAM) families developed by Brian Steffenson (totaling 150 lines), whose parents (PI 220455, PI 519465, PI 430750) exhibited contrasting TR responses to VPD. On soybean, the genetic material will consist in two NAM mapping populations (NAM#34 and #25) consisting of 140 RILs each, which were developed through the SoyNAM project (http://www.soybase.org/SoyNAM/). Phenotypic traits to be used in the QTL mapping will consist in slopes of TR response curves to VPD and VPD breakpoints (i.e., VPD at which there is a change in the slope of TR response to VPD).1.2. Identifying physiological mechanisms underlying favorable QTL. Genotypes having favorable alleles for identified major QTL will be examined for further physiological dissection. Ideally, we will carry out these investigations in near-isogenic lines (NILs) for the detected QTL that will be developed by cooperating wheat and soybean breeders. During those experiments, we will investigate the hypothesis of the involvement of leaf, stem or root hydraulic traits for the favorable responses, using similar approaches as in Sadok and Sinclair (2010), Schoppach et al. (2014) and Schoppach et al. (2016). In addition, based on findings we have recently published, we will investigate the role played by root auxin in modulating the hydrostatic and radial components of root water flow, through its developmental effect on metaxylem patterning and regulatory effect on root aquaporins activity (Sadok and Schoppach 2019). We will also consider other potential mechanisms dependent on the nature of the findings.Objective 22.1. Phenotyping whole-plant TRN among mapping populations and identifying QTL controlling TRN and its circadian control. We will leverage the GraPh system to phenotype TRN time courses over the nocturnal period for all the mapping populations that will be examined for TR responses to VPD, as described in Tamang and Sadok (2018). The phenotypic traits that will be used in the QTL mapping will consist of average TRN and pre-dawn slopes of TRN increases, which are indicative of circadian control of TRN.2.2. Identifying physiological mechanisms underlying favorable QTL. Ideally, we will follow the same approach described for TR response curves to VPD to develop NILs. Regarding the physiological dissection, we will evaluate several hypotheses underlying the functional relevance of TRN (i.e., why stomates are not closing). Such hypotheses include i) the 'leaky stomata hypothesis', or the inability of plants to close their stomata during the night (Barbour et al. 2005), ii) the 'anticipative hypothesis' (Sadok 2016; Resco de Dios et al. 2016), which reflects the need for plants to increase pre-dawn TRN to maximize early morning gas exchange, or (iii) the 'CO2 flushing hypothesis' (Marks and Lechowicz 2007; Fricke 2019), which reflect the need for plants to release respiratory CO2 during the night. Other hypotheses that would emerge in the meantime will be considered.Objective 33.1. Identification of the eco-physiological basis of heat stress tolerance in oat. In collaboration with the oat breeder cooperating in the project, we have assembled preliminary evidence suggesting that the decline in oat productivity in Minnesota is associated with historic increases in temperature during anthesis. A core panel of 30 oat genotypes selected from a diversity panel consisting of 200 advanced lines will be screened in the field for heat stress tolerance for at least 2 years. At anthesis, heat tents will be deployed in the field for approximately one week during which various reproductive and vegetative traits (floret fertility, pollen viability, leaf gas exchange and leaf anatomy) will be examined. At the end of the experiment, grain yield (1000 kernel weight) and seed quality (protein content, plumpness, test weight) will be examined. Data analysis of this experiment will be performed to identify among the investigated traits those that are the best predictors of increased seed yield and maintenance of seed quality under heat stress. Ad hoc physiological experiments will be set up on contrasted genotypes to examine potential mechanisms underlying heat stress tolerance as detailed in Prasad et al. (2017) and Tricker et al. (2018). For instance, if gas exchange variables are found to be strongly correlated with differential yield responses to heat stress treatments in the field, and dependently on the sign of this relationship, ad hoc experiments will be designed to identify the mechanistic basis of this link. In this specific case, multiple experiments could be implemented, on a limited number of genotypes and under more controlled conditions, for instance targeting the quantification of the relative influence of daytime vs. nighttime temperature increases or soil temperature on photosynthesis, transpiration or respiratory carbon loss. If reproductive traits are found to be strongly related to yield performance under heat stress, as is highly expected, follow-up experiments will be conducted to identify the physiological basis of pollen or floret tolerance to heat stress based on Jagadish et al. (2008), Prasad et al. (2015), Prasad et al. (2017) and Impa et al. (2019).3.2. Identification of the genetic basis of heat stress tolerance in oat. Dependently on the traits that will be identified in the previous step, we will develop an approach for high throughput screening of heat stress tolerance traits for the entire panel. The goal of such screening effort will be to identify QTL controlling heat stress tolerance in oat under MN conditions.Objective 4In collaboration with the winter barley breeding team, we have assembled pilot data indicating that freezing survival may involve differential resistance to freezing damage enabled by vasculature-related traits. Specifically, the findings seem to indicate that smaller xylem vasculature is associated with enhanced resistance to freezing damage, consistently with hydraulic vulnerability theory and recent findings on the evolution of freezing stress tolerance in angiosperms (Zanne et al. 2014; Olson et al. 2018). To further examine this question, we will be working on a set of 8 highly diverse winter barley cultivars assembled from different countries (Bulgaria, Canada, France, Japan, Russia, Turkmenistan, Poland, USA) and 3 check cultivars from the winter barley breeding program of the University of Minnesota. These genotypes exhibited highly contrasted levels of freezing stress tolerance, based on a 3-year experiment over 7 locations. Experiments will be set up under various environmental settings, including a newly acquired programmable freezing chamber which will be used to impose freezing treatments that mimic scenarios encountered in Minnesota. Specifically, we will conduct experiments to test whether differential freezing tolerance is associated with enhanced physical resistance of xylem vasculature and crown meristems to freezing-triggered cell collapse, based on a suite of anatomical and functional methods developed by Breman et al. (2009) and Livingston et al. (2018).