Heat Stress in Legume Seed Setting: Effects, Causes, and Future Prospects

Grain legumes provide a rich resource of plant nutrition to human diets and are vital for food security and sustainable cropping. Heat stress during flowering has a detrimental effect on legume seed yield, mainly due to irreversible loss of seed number. To start with, we provide an overview of the developmental and physiological basis of controlling seed setting in response to heat stress. It is shown that every single process of seed setting including male and female gametophyte development, fertilization, and early seed/fruit development is sensitive to heat stress, in particular male reproductive development in legume crops is especially susceptible. A series of physiochemical processes including heat shock proteins, antioxidants, metabolites, and hormones centered with sugar starvation are proposed to play a key role in regulating legume seed setting in response to heat stress. The exploration of the molecular mechanisms underlying reproductive heat tolerance is in its infancy. Medicago truncatula, with a small diploid genome, and well-established transformation system and molecular platforms, has become a valuable model for testing gene function that can be applied to advance the physiological and molecular understanding of legume reproductive heat tolerance.The work is supported by National Key R & D Program of China (No. 2017YFD0301307 and 2017YFD0300204-3) and Anhui Agricultural University startup funds for YS, and National Natural Science Foundation of China (No. 31760579) and Hainan University startup funds (No. kyqd1663) for YL

Analysis on Heat Characteristics for Summer Maize Cropping in a Semi-Arid Region

Heat stress during flowering is a critical limitation for summer maize production. However, the incidence of heat varies with years and locations, and it poses a great risk to successful maize reproduction and kernel setting. Therefore, it is essential to provide a sound quantification of heat occurrence in relation to maize growth and development. Here, we analyzed the characteristics of heat occurrence based on climate data for over 60 years on Huaibei Plain, China. The effective accumulated temperature showed a slight interannual variation. The average maximum temperature (Tmax) during flowering was 32◦C–33◦C, which was approximately 2◦C higher than that over the whole growing season. The probability (P) for the daily Tmax > 33◦C during flowering was closer to 50% and this maximum temperature ranged between 33◦C and 37◦C. The five levels from normal to extreme heat for Tmax were defined. Across the six studied sites, the mild level heat stress accounted for most of incidents (P, 25–50%), followed by moderate (P, 13–25%) and severe (P, 0.5–13%), and the minimum for extreme heat stress (P, 0.5%). Four phases bracketing flowering during maize development were given, i.e., 1 week prior to anthesis, 1 week during anthesis, 1 week for anthesis-silking, and 1week post silking. There was a greater probability for heat stress incidents from anthesis to silking compared to the other developmental stages. Additionally, maize grain yield slightly increased with the increase in Tmax to 33◦C, but it declined as Tmax surpassed 33◦C. In conclusion, the pattern and characteristics of heat stress were quantified bracketing maize flowering. These findings assist to advise summer maize cropping strategies on the semi-arid and semi-humid Huaibei Plain, China or similar climate and cropping regions

Assessment of Heat Risk of Winter Wheat Cropping Based on Long-Term Meteorological Data

The frequency of heat events is likely to increase due to global climate change, posing an increasing risk to wheat production. To optimize crop management strategies for coping with future climates, it is crucial to quantify the high-temperature occurrence during cropping seasons. Here, sixty-six years (1955~2020) of meteorological data during wheat reproductive growth were collected from six meteorological stations in the Huaibei Plain of Anhui Province. These data were analyzed to quantify the pattern and characteristics of post-anthesis heat stress for wheat crops. Five levels of annual mean daily maximum temperature (Tmax) were defined, from normal to extreme temperatures. Six crop developmental phases of winter wheat, i.e., phase i to phase vi, were divided from flowering to maturity. The data suggest an annual mean temperature of 17~24 °C from flowering to maturity, with an annual effective cumulative temperature ranging from 725 °C d to 956 °C d. The mean temperature and effective cumulative temperature increased as crop growth progressed, along with more frequent heat events during phase ii (8~14 days after anthesis) and phase iii (15~21 days after anthesis). We also found that the frequency of extremely high temperatures (≥33 °C) from 1990 to 2020 was significantly greater than that from 1957 to 1990. Interestingly, it was found that the intensity of post-anthesis night temperatures also increased with crop growth, i.e., from phase i to phase vi. Wheat grain yield increased with increasing effective accumulative temperature and Tmax, but it started to decline when thresholds of effective accumulative temperature and Tmax were reached. Overall, these findings could provide guidelines for winter wheat cropping in the Huaibei Plain, China, or similar climate and cropping regions.This study was funded by the National Key Research and Development Program of China (2017YFD0300204-3)

Characteristics of historical precipitation for winter wheat cropping in the semi-arid and semi-humid area

Winter wheat (Triticum aestivum L.) is one of major crops in the area along Huai river, China where it is a semi-arid and semi-humid region with sufficient precipitation for an entire season, but with uneven distribution within various growth stages. The instability of precipitation is an important factor in limiting wheat production potential under climate change. Therefore, it is essential to characterise the precipitation associated with different crop developmental stages. Based on climate data from 1999 to 2020 in six representative meteorological stations, we characterised the historical precipitation relating to seven key growth stages in winter wheat. There is no clear trend of interannual variation of precipitation for wheat season, with an average of precipitation of 414.4 ± 121.2 mm. In terms of the distribution of precipitation grade within a season, light rain was dominant. Continuous rain occurred frequently during the pre-winter seedling and overwintering stages. The critical period of water demand, such as jointing and booting, has less precipitation. The fluctuation range of precipitation in sowing, heading-filling and maturation stages is large, which means that there is flood and drought at times. In conclusion, these findings provide a foundation for instructing winter wheat cropping in confronting with waterlogging and drought risk due to uneven precipitation in ‘Yanhuai’ region, China

Smoking Cessation With 20 Hz Repetitive Transcranial Magnetic Stimulation (rTMS) Applied to Two Brain Regions: A Pilot Study

Chronic smoking impairs brain functions in the prefrontal cortex and the projecting meso-cortical limbic system. The purpose of this pilot study is to examine whether modulating the frontal brain activity using high-frequency repetitive transcranial magnetic stimulation (rTMS) can improve smoking cessation and to explore the changing pattern of the brain activity after treatment. Fourteen treatment-seeking smokers were offered a program involving 10 days of rTMS treatment with a follow-up for another 25 days. A frequency of 20 Hz rTMS was sequentially applied on the left dorso-lateral prefrontal cortex (DLPFC) and the superior medial frontal cortex (SMFC). The carbon monoxide (CO) level, withdrawal, craving scales, and neuroimaging data were collected. Ten smokers completed the entire treatment program, and 90% of them did not smoke during the 25-day follow-up time. A significant smoking craving reduction and resting brain activity reduction measured by the cerebral blood flow (CBF) and brain entropy (BEN) were observed after 10 days of 20 Hz rTMS treatments compared to the baseline. Although limited by sample size, these pilot findings definitely showed a high potential of multiple-target high-frequency rTMS in smoking cessation and the utility of fMRI for objectively assessing the treatment effects

Genetic regulation of embryo development and formation of seed storage products in the legume model Medicago truncatula

Research Doctorate - Doctor of Philosphy (PhD)Legumes are increasingly recognised as playing a critical role in addressing the food crisis and meeting growing demands for bioenergy in a framework of sustainable cropping. M. truncatula is a widely used model to study legume biology particularly for legume-specific developmental questions such as nodulation. Accordingly, rich genetic sequencing information and extensive databases with bioinformatic analyses are made available by the legume community. This facilitates the use of M. truncatula as a model to study legume seed biology. A global picture of proteomics and transcriptomics of M. truncatula seed filling has been explored. However, there is still limited information available for M. truncatula as a legume model for studying seed biology and seed filling. This study in the thesis characterised M. truncatula embryo development from embryo sac to protein and oil body formation using histological, biochemical and molecular methods. A multicellular hypophysis, suspensor development and a clear procambial connection between shoot and root apical meristems are featured. TEM images clearly show a specific oil body arrangement aligning the protein bodies and plasma membrane. Gene expression during early seed development and the accumulation of storage protein and oil was profiled, with a focus on transcription factors. Embryogenesis establishes the embryonic pattern that acts as the centre for growth after germination and develops the cotyledons that are a repository for protein bodies and oil bodies in legumes. Early embryogenesis with a series of complex development events is completed in 6-8 days and is tightly regulated by a network of transcription factors. Early embryogenesis is characterised by three distinctive expression patterns, with MtSERF1 and MtWOX9 expression associated with embryo growth, connecting the early and late embryo development. Medicago orbicularis was chosen to compare the accumulation of seed storage products with M. truncatula because of its lower seed protein and oil. Further biochemical and histology studies clearly showed a contrasting seed nutritional spectrum in terms of protein, oil, starch and cell wall between M. truncatula and M. orbicularis, which provides a pair of close species to investigate genetic mechanisms of seed storage accumulation. The major regulators of seed maturation and seed filling are LEC1/L1l, LEC2, FUS3 and ABI3 in Arabidopsis while in Medicago only L1L and ABI3 are shown to be possible equivalent regulators. The expression of selected pathway genes further confirmed the transcriptional regulation. Reduced oil content in M. orbicularis is also associated with increased seed coat mucilage and cotyledon cell wall material, and gene expression of associated biosynthetic pathways. The storage compounds can potentially be influenced by modifying carbon flow (e.g. the mucilage biosynthesis pathway), or modifying regulatory genes (e.g. L1L). Apart from the biosynthesis of protein and oil, the process of packing protein and oil is also important for the final storage in the cotyledons. A highly efficient plant transformation system is essential for gene functional analysis and creating transgenic lines. Accordingly, an improved transformation procedure was developed using a new hormonal combination of ABA+GA (an unusual synergism) imposed from the beginning of tissue culture. In essence, the study in this thesis completed a detailed examination of morphological, cellular and molecular aspects of embryogenesis in M. truncatula, providing a legume perspective to complement studies in Arabidopsis embryogenesis. By comparing the cellular, biochemical and molecular basis of seed storage between M. truncatula and M. orbicularis, these species were found to be a promising pair for investigating mechanisms of regulating partitioning of legume seed storage compounds. The improvement of transformation efficiency further facilitates the genetic modification of specific genes of interest in M. truncatula in this context. Therefore, this thesis lays a foundation for further studies in legume seed biology and the regulation of seed filling and storage partitioning

Connecting Biochemical Photosynthesis Models with Crop Models to Support Crop Improvement

The next advance in field crop productivity will likely need to come from improving crop use efficiency of resources (e.g. light, water and nitrogen), aspects of which are closely linked with overall crop photosynthetic efficiency. Progress in genetic manipulation of photosynthesis is confounded by uncertainties of consequences at crop level because of difficulties connecting across scales. Crop growth and development simulation models that integrate across biological levels of organization and use a gene-to-phenotype modelling approach may present a way forward. There has been a long history of development of crop models capable of simulating dynamics of crop physiological attributes. Many crop models incorporate canopy photosynthesis (source) as a key driver for crop growth, while others derive crop growth from the balance between source- and sink-limitations. Modelling leaf photosynthesis has progressed from empirical modelling via light response curves to a more mechanistic basis, having clearer links to the underlying biochemical processes of photosynthesis. Cross-scale modelling that connects models at the biochemical and crop levels and utilises developments in upscaling leaf-level models to canopy models has the potential to bridge the gap between photosynthetic manipulation at the biochemical level and its consequences on crop productivity. Here we review approaches to this emerging cross-scale modelling framework and reinforce the need for connections across levels of modelling. Further, we propose strategies for connecting biochemical models of photosynthesis into the cross-scale modelling framework to support crop improvement through photosynthetic manipulation

Regulation of Carbon Partitioning in the Seed of the Model Legume Medicago truncatula and Medicago orbicularis: A Comparative Approach

The proportion of starch, protein and oil in legume seeds is species dependent. The model legume, Medicago truncatula, has predominantly oil and protein stores. To investigate the regulation of seed oil production we compared M. truncatula with M. orbicularis, which has less oil and protein. The types of protein and fatty acids are similar between the two species. Electron microscopy indicated that the size and distribution of the oil bodies in M. orbicularis, is consistent with reduced oil production. M. orbicularis has more extruded endosperm mucilage compared to M. truncatula. The cotyledons have a greater cell wall content, visualized as thicker cell walls. The reduced oil content in M. orbicularis is associated with increased expression of the MtGLABRA2-like (MtGL2) transcription factor, linked to an inverse relationship between mucilage and oil content in Arabidopsis. The expression of the pectin biosynthesis GALACTURONOSYLTRANSFERASE (GAUT) genes, is also increased in M. orbicularis. These increases in extruded mucilage and cell wall storage components in M. orbicularis are accompanied by reduced expression of transcriptional regulators of oil biosynthesis, MtLEAFY COTYLEDON1-LIKE (MtL1L), MtABSCISIC ACID-INSENSITIVE3 (MtABI3), and MtWRINKLED-like (MtWRI), in M. orbicularis. The reduced oil in M. orbicularis, is consistent with increased synthesis of cell wall polysaccharides and decreased expression of master transcription factors regulating oil biosynthesis and embryo maturation. Comparative investigations between these two Medicago species is a useful system to investigate the regulation of oil content and carbon partitioning in legumes