Identifying physiological traits to optimize assimilate partitioning and spike fertility for yield potential in wheat (Triticum aestivum L.) genotypes

Although wheat demand is expected to increase along with a projected growing population, the global rate of wheat yield increase appears to be declining. Genetic progress in yield potential in the past has been achieved mainly through increases in grains m-2 and harvest index with no major changes in above-ground biomass; however, HI has not increased since about 1990 indicating that further increases in yield potential will rely on increases in biomass while optimizing grain DM partitioning. A panel of 26 high biomass CIMMYT elite spring wheat cultivars (CIMMYT Mexico Core Germplasm; CIMCOG) was evaluated for grain yield, above-ground dry-matter (AGDM), DM partitioning and spike fertility in three field experiments (2011, 2012 and 2013) in NW Mexico. An additional field experiment was carried out in 2014 on a subset of four cultivars to examine source-sink related traits by imposing source manipulation treatments (leaf-lamina removal and leaf-sheath shading) on the crop. The main objective was to identify the physiological determinants of grains m-2 and HI in relation to DM partitioning among organs and to prioritise traits for application in breeding to increase HI in high biomass genotypes. Genetic variation in grain yield among genotypes was explained mainly by differences in AGDM. Fruiting efficiency (grains per unit spike DM at anthesis) was affected by high rachis specific weight and appeared to be important in grains m-2 determination. Trends for positive associations were observed between HI and spike partitioning index (Spike DM / AGDM - at anthesis; SPI). Stem DM proportion (mainly structural DM in internodes 2 and 3) was negatively associated with SPI; peduncle length and peduncle DM as a proportion of the stem DM on the other hand were more strongly associated to spike DM per unit area. In 2014, reductions in final grain weight in response to source reduction treatments were relatively low compared to the estimated reductions in light interception. No up-regulation of photosynthesis rate was observed by the flag-leaf or the spike in response to source reductions, suggesting that grain growth was overall sink limited but close to a co-limitation by source and sink. These results indicated scope for designing a plant ideotype to raise HI in high biomass spring wheat cultivars with reductions in stem structural DM (in upper internodes) without major effects on water soluble carbohydrate accumulation (in lower internodes). Finally, results from the source-sink manipulation treatments indicated limited scope for reductions in leaf-sheath and lamina DM partitioning during stem elongation to favour spike growth

Optimizing dry-matter partitioning for increased spike growth, grain number and harvest index in spring wheat

Improving biomass is an important goal for future genetic gains in yield potential in wheat, but it will also be crucial to identify physiological traits to maximize harvest index (HI, proportion of aboveground biomass in grain). Increased grain partitioning will require increased dry-matter (DM) partitioning to the spikes at anthesis as well as enhanced fruiting efficiency (FE, grains per g spike dry matter at anthesis or chaff dry matter at harvest), whilst optimizing the partitioning amongst the non-grain components to maintain post-anthesis photosynthetic capacity and soluble carbohydrate translocation. The objectives of this study were to: i) quantify genetic variation in DM partitioning among plant organs at anthesis (GS65) + 7days and associations with spike growth and FE and ii) identify optimized partitioning traits associated with enhanced HI and grain yield, in CIMMYT elite spring wheat backgrounds. Two field experiments were conducted in 2011-12 and 2012-13 testing 26 CIMMYT spring wheat cultivars in NW Mexico in irrigated conditions in which DM partitioning was assessed in plant organs at anthesis + 7 days, and within-spike (glume, palea, lemma, rachis and awn) partitioning was assessed at harvest. Grain yield, yield components, HI and FE were assessed at harvest. Our results identified new traits for HI (decreased DM partitioning to stem internodes 2 (top down, peduncle-1) and 3, and decreased rachis DM partitioning and rachis specific weight (rachis DM per rachis unit length) and increased lemma DM partitioning), potentially allowing breeders to maximize the exploitation of enhanced carbon assimilation for grain biomass. Further work will focus on understanding the role of soluble carbohydrate re-translocation in these relationships and establishing high-throughput and cost-effective phenotyping methods for these traits for deployment in breeding

Estimating organ contribution to grain filling and potential for source upregulation in wheat cultivars with a contrasting source-sink balance

© 2020 by the authors. Grain filling may be limited by the joint source and sink capacity in modern wheat cultivars, indicating a need to research the co-limitation of yield by both photosynthesis and the number and potential size of grains. The extent to which the post-anthesis source may be limiting final grain size can be estimated by partial degraining of spikes, while defoliation and shading treatments can be useful to estimate if any excess photosynthetic capacity exists. In the current study, degraining was applied to a set of 26 elite spring wheat cultivars from the International Maize and Wheat Improvement Center (CIMMYT)'s core germplasm (CIMCOG) panel, while lamina defoliation and shading through stem-and-leaf-sheath covering treatments were applied to a subset of the same cultivars. Responses to source treatments in grain weight, pre-anthesis reserve contribution to grain weight, dry-matter translocation efficiency, and flag-leaf and spike photosynthetic rate were measured and compared to an unmanipulated control treatment. Grain weight responses to degraining among cultivars ranged from no response to increases of 28%, suggesting a range of responses from sink limitation, to probable source and sink co-limitation of grain growth. Grain weight's response to degraining increased linearly with the years of cultivar release from 1966 to 2009, indicating that the current highest yield potential CIMMYT spring wheats have a co-limitation of grain growth by source and sink. This may have been due to an increase in grain sink strength with years of cultivar release with no commensurate increase in post-anthesis source capacity. The relatively low decreases in grain weight with defoliation compared to decreases in light interception by defoliation indicated that sink limitation was still likely predominating in the cultivars with co-limitation. The stem-and-leaf-sheath covering treatment decreased grain weight by nearly 10%, indicating that stem-and-leafsheath photosynthesis plays a key role in grain growth during grain filling. In addition, pre-anthesis reserve contribution to grain weight was increased by ca. 50% in response to lamina defoliation. Our results showed that increasing the post-anthesis source capacity, through increases in stem-and-leaf-sheath photosynthetic rate during grain filling and pre-anthesis reserve contribution to grain weight, is an important objective in enhancing yield potential in wheat through maintaining a source-sink balance

Yielding to the image: how phenotyping reproductive growth can assist crop improvement and production

Reproductive organs are the main reason we grow and harvest most plant species as crops, yet they receive less attention from phenotyping due to their complexity and inaccessibility for analysis. This review highlights recent progress towards the quantitative high-throughput phenotyping of reproductive development, focusing on three impactful areas that are pivotal for plant breeding and crop production. First, we look at phenotyping phenology, summarizing the indirect and direct approaches that are available. This is essential for analysis of genotype by environment, and to enable effective management interpretation and agronomy and physiological interventions. Second, we look at pollen development and production, in addition to anther characteristics, these are critical points of vulnerability for yield loss when stress occurs before and during flowering, and are of particular interest for hybrid technology development. Third, we elaborate on phenotyping yield components, indirectly or directly during the season, with a numerical or growth related approach and post-harvest processing. Finally, we summarise the opportunities and challenges ahead for phenotyping reproductive growth and their feasibility and impact, with emphasis on plant breeding applications and targeted yield increases

Exploring genetic diversity for grain partitioning traits to enhance yield in a high biomass spring wheat panel

© 2020 Breeding to raise yield potential through enhancing photosynthesis will have limited impact unless harvest index (HI: proportion of above-ground biomass as grain yield) is maintained or ideally increased. Boosting grain dry matter (DM) partitioning will require increased allocation of assimilates to sink organs to enhance spike growth. A high biomass spring wheat panel of 150 genotypes encompassing elite, landrace-derived and synthetic-derived lines was grown under yield potential conditions in two seasons in NW Mexico. Results showed that the incorporation of landrace-derived and synthetic-derived backgrounds into elite lines resulted in higher expression of above-ground biomass (AGDM), leaf lamina and stem DM partitioning at anthesis. However, no grain yield advantage was observed over elite lines, due to lower grain number per unit area (GN) and decreased harvest index (HI). Positive linear associations were found among spike fertility-related traits - fruiting efficiency (grains per unit of spike DM at anthesis; FE), GN and HI - which were, in turn, related positively with grain yield (GY). Stem-internode 3 length and internode 3 DM partioning were negatively associated with spike partitioning index (SPI: ratio of spike DM to total above-ground DM at anthesis) and GN, suggesting an enhanced competition for assimilates between the spike and stem internode 3 during stem elongation. Within-spike DM partitioning analysis (glume, lemma, palea, rachis, awn) showed decreased partitioning to awns was associated with increased FE and thousand grain weight (TGW). While the use of exotic material can enhance biomass, special attention needs to be paid in the selection for novel DM partitioning traits that raise HI and GN coming from the elite genepool. The selection for grain partitioning traits in wheat breeding combined with sources expressing high biomass can potentially allow breeders to increase grain carbon assimilation that will deliver higher yields

Associations between endogenous spike cytokinins and grain-number traits in spring wheat genotypes

Genetic variation in grain number has been positively associated with levels of cytokinins in inflorescences in cereals, although studies quantifying endogenous levels in the field are currently lacking. The present study, using a spring wheat association mapping panel (HiBAP II) of 150 lines, quantified associations between spike hormone levels and grain number and associated traits. The HiBAP II panel was grown in the field in NW Mexico under irrigated conditions for one year and a subset of ten genotypes in the glasshouse under well-watered conditions for three years. The spike levels of four cytokinins (trans-zeatin riboside, trans-zeatin, isopentenyladenosine, and isopentenyladenine) were measured by using ultra-high-performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry. In the glasshouse experiments, spike hormone levels were measured at booting and anthesis, and in the field experiment at anthesis. In the glasshouse experiments, cytokinin levels were also measured in the basal, central, and apical spikelets separately in addition to at the whole spike level. The spike cytokinin levels did not differ significantly between the basal, central and apical sections of the spike. or show a spike position × genotype interaction. In the glasshouse experiments, significant genetic variation was detected for the expression of the four cytokinins in spikes at booting. At booting, spike trans-zeatin concentration ranged amongst genotypes from 4.5 to 16.0 ng g−1 FW and was positively correlated with grain number per main shoot (r = 0.77, P < 0.05). In the field at anthesis, the spike levels of each of trans-zeatin, trans-zeatin riboside and isopentenyl adenosine were positively correlated with grains per m2 (r = 0.17–0.19, P < 0.05). Our results indicated that selection for high spike cytokinin levels in wheat germplasm offers scope to raise grain number and yield potential in wheat

Climate change impacts on crop breeding: Targeting interacting biotic and abiotic stresses for wheat improvement

Wheat (Triticum aestivum L.) as a staple crop is closely interwoven into the development of modern society. Its influence on culture and economic development is global. Recent instability in wheat markets has demonstrated its importance in guaranteeing food security across national borders. Climate change threatens food security as it interacts with a multitude of factors impacting wheat production. The challenge needs to be addressed with a multidisciplinary perspective delivered across research, private, and government sectors. Many experimental studies have identified the major biotic and abiotic stresses impacting wheat production, but fewer have addressed the combinations of stresses that occur simultaneously or sequentially during the wheat growth cycle. Here, we argue that biotic and abiotic stress interactions, and the genetics and genomics underlying them, have been insufficiently addressed by the crop science community. We propose this as a reason for the limited transfer of practical and feasible climate adaptation knowledge from research projects into routine farming practice. To address this gap, we propose that novel methodology integration can align large volumes of data available from crop breeding programs with increasingly cheaper omics tools to predict wheat performance under different climate change scenarios. Underlying this is our proposal that breeders design and deliver future wheat ideotypes based on new or enhanced understanding of the genetic and physiological processes that are triggered when wheat is subjected to combinations of stresses. By defining this to a trait and/or genetic level, new insights can be made for yield improvement under future climate conditions

Uncovering candidate genes involved in photosynthetic capacity using unexplored genetic variation in Spring Wheat

To feed an ever-increasing population we must leverage advances in genomics and phenotyping to harness the variation in wheat breeding populations for traits like photosynthetic capacity which remains unoptimized. Here we survey a diverse set of wheat germplasm containing elite, introgression and synthetic derivative lines uncovering previously uncharacterized variation. We demonstrate how strategic integration of exotic material alleviates the D genome genetic bottleneck in wheat, increasing SNP rate by 62% largely due to Ae. tauschii synthetic wheat donors. Across the panel, 67% of the Ae. tauschii donor genome is represented as introgressions in elite backgrounds. We show how observed genetic variation together with hyperspectral reflectance data can be used to identify candidate genes for traits relating to photosynthetic capacity using association analysis. This demonstrates the value of genomic methods in uncovering hidden variation in wheat and how that variation can assist breeding efforts and increase our understanding of complex traits

Heteroatom-tagged proteomics of lung cancer and chronic obstructive pulmonary disease human serum reveal alterations in selenoproteins

Heteroatom-tagged proteomics allows the absolute quantification of selenoproteins using the heteroatom as a "tag" into a selective and sensitive atomic detector instead of a molecular one. Using this analytical method, about 90% of total selenium in human serum/plasma can be measured as selenoproteins and total selenometabolites and thus, the status of selenium can be determined. Herein, we determined the absolute concentration of selenoproteins in human serum patients with lung cancer (LC) and chronic obstructive pulmonary disease (COPD), a competing cause of morbidity and mortality in smokers as well as an independent risk factor for LC. We conducted an observational study of 154 human serum samples obtained from LC and COPD patients with varying severity of disease, including COPD patients who developed LC during follow-up and healthy controls (HC). Using heteroatom-tagged proteomics, we determined extracellular glutathione peroxidase (eGPx), selenoprotein P (SELENOP), and selenoalbumin (SeAlb). Associations between selenoproteins were also studied as potential biomarkers of disease. The concentration of eGPx was significantly higher in the all-inclusive COPD cohort compared to HC, COPD patients with LC, or those with mild obstructive lung disease, while SELENOP concentration was significantly decreased in LC patients compared to HC and COPD. We found an inverse correlation between SELENOP and SeAlb in HC, but also in LC patients, and especially in patients with COPD and LC. Moreover, we found that eGPx and selenometabolite concentrations were positively associated with LC human serum. Selenoprotein concentrations were altered in COPD and LC when compared to healthy controls suggesting a potential role of the selenoproteome in the diagnosis and/or treatment of these tobacco-related diseases.Funding: This work has been supported by the project “Heteroatom-tagged proteomics and metabolomics to study lung cancer. Influence of gut microbiota” (Ref.: PY20_00366). Project of Excellence. Regional Ministry of Economy, Knowledge, Business and University, Andalusia, Spain. The authors also thank the grants Ref. 651/2018 and 115/2020 from the Spanish Society of Pneumology and Surgery (SEPAR) and 08/2018 from the Association of Pneumology and Thoracic Surgery (Neumosur) that supported samples recruitment at the hospitals and biobank registration. The authors also thank Instituto de Salud Carlos III (AES16/01783) and unrestricted funding from Menarini Group and AstraZeneca“. Funding for open access charge: Universidad de Huelva / CBUA. Acknowledgements: We thank all the patients who have volunteered and donated their biomaterials for the study