Transgenic alteration of ethylene biosynthesis increases grain yield in maize under field drought-stress conditions

Overexpression of zmm28 increases maize grain yield in the field

Increasing maize grain yield has been a major focus of both plant breeding and genetic engineering to meet the global demand for food, feed, and industrial uses. We report that increasing and extending expression of a maize MADS-box transcription factor gene, zmm28, under the control of a moderate-constitutive maize promoter, results in maize plants with increased plant growth, photosynthesis capacity, and nitrogen utilization. Molecular and biochemical characterization of zmm28 transgenic plants demonstrated that their enhanced agronomic traits are associated with elevated plant carbon assimilation, nitrogen utilization, and plant growth. Overall, these positive attributes are associated with a significant increase in grain yield relative to wild-type controls that is consistent across years, environments, and elite germplasm backgrounds.

Maize expressing the sunflower transcription factor HaHB11 has improved productivity in controlled and field conditions

HaHB11 is a sunflower transcription factor from the homeodomain-leucine zipper I family. Transgenic Arabidopsis plants expressing HaHB11 had larger rosettes and improved seed yield. In this work maize plants from hybrid HiII were transformed with 35S:HaHB11, ZmUBI:HaHB11 and ProHaHB11:HaHB11 and then backcrossed to B73 to obtain a more homozygous inbred phenotype. Transgene expression levels were stable at least during three generations. Greenhouse-grown HaHB11 transgenic lines had larger leaf area and delayed senescence than controls, together with increased total biomass (up to 25%) and seed yield (up to 28%). Field trials conducted with T2 and T4 generations indicated that enhanced leaf area (up to 18%), stem diameter (up to 28%) and total biomass (up to 40%) as well as delayed leaf senescence were maintained among transgenic individuals when upscaling from pots in the greenhouse to communal plants in the field. The T4 field-grown transgenic generation had increased light interception and radiation use efficiency as well as seed yield (43-47% for events driven by the 35S promoter). Results suggest that HaHB11 is a promising tool for crop improvement because differential traits observed in the Arabidopsis model plant were preserved in a crop like maize independently of growth conditions and backcross level.

Analysis of differential gene expression in cold-tolerant vs. cold-sensitive varieties of snap bean (Phaseolus vulgaris L.) in response to low temperature stress

BACKGROUND

Snap bean, Phaseolus vulgaris L., as a warm-season vegetable, low temperature stress seriously affect the yield and quality. At present, little is known about the genes and molecular regulation mechanism in cold response in snap bean exposed to low temperature.

OBJECTIVES

Our objectives were to identify the low temperature response genes in snap bean and to examine differences in the gene response between cold-tolerant and cold-sensitive genotypes.

METHODS

We used two highly inbred snap bean lines in this study, the cold-tolerant line '120', and the cold-sensitive line '093'. The plants were grown to the three leaf and one heart stage and exposed to 4 °C low temperature. We used RNA sequencing (RNA-seq) to analyze the differences of gene expression.

RESULTS

988 and 874 cold-responsive genes were identified in 'T120 vs CK120' and 'T093 vs CK093' ('T' stands for low temperature treatment, and 'CK' stands for control at room temperature), respectively. Of these, 555 and 442 genes were unique to cold-stressed lines '120' and '093', respectively compared to the control. Our analysis of these differentially expressed genes indicates that Ca²⁺, ROS, and hormones act as signaling molecules that play important roles in low temperature response in P. vulgaris. Altering the expression of genes in these signaling pathways activates expression of downstream response genes which can interact with other signaling regulatory networks. This may maintained the balance of ROS and hormones, making line '120' more cold-tolerant than line '093'.

CONCLUSION

Our results provide a preliminarily understanding of the molecular basis of low temperature response in snap bean, and also establish a foundation for the future genetic improvement of cold sensitivity in snap bean by incorporating genes for cold tolerance.

Maize: Key agricultural crop in food security and sovereignty in a future with water scarcity

ABSTRACT The objective in this review was to discuss the importance of maize currently and the crucial role it may play in the future for food production in scenarios of water shortage, as well as the importance of conserving its landrace cultivars, which have a considerable portion of the reserve of genetic variability. Maize plants, when exposed to water deficit, may develop physiological, morphological, biochemical and anatomical adaptation mechanisms. With the aid of genetic improvement, characteristics that impart tolerance are fixed in plants through conventional methods. In this context, ‘Tuxpeño Sequia’ cultivars were developed in Mexico, while in Africa, one of the most important strategies was the development of ‘DT’ (Drought-tolerant) cultivars. In the United States, one of the most important processes was the development of PionerAquamax® hybrids, while in Brazil, it was the development of cultivars with the ‘Maya Latente’ gene. Through genetic transformation, the hybrid ‘MON 87460’ was developed. However, it should be mentioned that, for a cultivar to be well accepted by producers, besides having one or more adaptation characteristics, it must have a high grain yield. Biotechnological tools such as the use of molecular markers, genetic transformation, and modeling through bioinformatics, associated with conventional selection, will be fundamental to guarantee the advancement of water deficit tolerance in maize.

Evaluation of the History of Safe Use of the Maize ZMM28 Protein

The ZMM28 protein encoded by the zmm28 gene is endogenous to maize. DP202216 maize was genetically modified to increase and extend expression of the zmm28 gene relative to native zmm28 gene expression, resulting in plants with enhanced grain yield potential. Evaluation of the history of safe use (HOSU) is one component of the safety assessment framework for a newly expressed protein in a GM crop. The deduced amino acid sequence of the introduced ZMM28 protein in DP202216 maize is identical to the ZMM28 protein in nonmodified conventional maize. The ZMM28 protein has also been found in selected varieties of sweet corn kernels, and closely related proteins are found in other commonly consumed food crops. Concentrations of the ZMM28 protein in event DP202216 maize, conventional maize, and sweet corn are reported. This information supports, in part, the evaluation of HOSU, which can be leveraged in the safety assessment of the ZMM28 protein. Additional studies will be considered in the food and feed safety assessment of the DP202216 maize event.

Hormonal regulation of cereal endosperm development with a focus on rice (Oryza sativa)

The endosperm of cereal grain forms the staple diet for most of the world's population, and feeds much of their stock. Grain size and quality are determined largely by events taking place during coenocytic nuclear division, endosperm cellularisation and cell differentiation, and the production of storage molecules. Thus, understanding the complex signalling processes occurring at each of these steps is essential for maintaining and improving our food supply. Here, we critically review evidence for the effects of phytohormones on grain size, as well as hormone homeostasis, signalling and crosstalk. We focus on rice endosperm due to the importance of rice as a food crop and a model grass, as well as its relative neglect in recent reviews; however, data from other cereals are also discussed due to strong evidence for conserved signalling networks operating during grain development. Discussion is restricted to auxin, cytokinin, ethylene, abscisic acid and gibberellin. Our review highlights the need for accurate hormone determinations combined with information on gene expression. We present evidence for separate, localised signalling roles for auxin at different stages of grain development and highlight key research questions for other hormones where much less data are available.

Developmental and transcriptional responses of maize to drought stress under field conditions

Drought is a common abiotic stress which significantly limits global crop productivity. Maize is an important staple crop and its yield is determined by successful development of the female inflorescence, the ear. We investigated drought stress responses across several developmental stages of the maize B73 inbred line under field conditions. Drought suppressed plant growth, but had little impact on progression through developmental stages. While ear growth was suppressed by drought, the process of spikelet initiation was not significantly affected. Tassel growth was reduced to a lesser extent compared to the observed reduction in ear growth under stress. Parallel RNA-seq profiling of leaves, ears, and tassels at several developmental stages revealed tissue-specific differences in response to drought stress. High temperature fluctuation was an additional environmental factor that also likely influenced gene expression patterns in the field. Drought induced significant transcriptional changes in leaves and ears but only minor changes in the tassel. Additionally, more genes were drought responsive in ears compared to leaves over the course of drought treatment. Genes that control DNA replication, cell cycle, and cell division were significantly down-regulated in stressed ears, which was consistent with inhibition of ear growth under drought. Inflorescence meristem genes were affected by drought to a lesser degree which was consistent with the minimal impact of drought on spikelet initiation. In contrast, genes that are involved in floret and ovule development were sensitive to stress, which is consistent with the detrimental effect of drought on gynoecium development and kernel set.

Leaf Senescence: Systems and Dynamics Aspects

Leaf senescence is an important developmental process involving orderly disassembly of macromolecules for relocating nutrients from leaves to other organs and is critical for plants' fitness. Leaf senescence is the response of an intricate integration of various environmental signals and leaf age information and involves a complex and highly regulated process with the coordinated actions of multiple pathways. Impressive progress has been made in understanding how senescence signals are perceived and processed, how the orderly degeneration process is regulated, how the senescence program interacts with environmental signals, and how senescence regulatory genes contribute to plant productivity and fitness. Employment of systems approaches using omics-based technologies and characterization of key regulators have been fruitful in providing newly emerging regulatory mechanisms. This review mainly discusses recent advances in systems understanding of leaf senescence from a molecular network dynamics perspective. Genetic strategies for improving the productivity and quality of crops are also described.

Amelioration of drought effects in wheat and cucumber by the combined application of super absorbent polymer and potential biofertilizer

Biofertilizer is a good substitute for chemical fertilizer in sustainable agriculture, but its effects are often hindered by drought stress. Super absorbent polymer (SAP), showing good capacity of water absorption and retention, can increase soil moisture. However, limited information is available about the efficiency of biofertilizer amended with SAP. This study was conducted to investigate the effects of synergistic application of SAP and biofertilizers (Paenibacillus beijingensis BJ-18 and Bacillus sp. L-56) on plant growth, including wheat and cucumber. Potted soil was treated with different fertilizer combinations (SAP, BJ-18 biofertilizer, L-56 biofertilizer, BJ-18 + SAP, L-56 + SAP), and pot experiment was carried out to explore its effects on viability of inoculants, seed germination rate, plant physiological and biochemical parameters, and expression pattern of stress-related genes under drought condition. At day 29 after sowing, the highest viability of strain P. beijingensis BJ-18 (264 copies ng^-1 gDNA) was observed in BJ-18 + SAP treatment group of wheat rhizosphere soil, while that of strain Bacillus sp. L-56 (331 copies ng^-1 gDNA) was observed in the L-56 + SAP treatment group of cucumber rhizosphere soil. In addition, both biofertilizers amended with SAP could promote germination rate of seeds (wheat and cucumber), plant growth, soil fertility (urease, sucrose, and dehydrogenase activities). Quantitative real-time PCR analysis showed that biofertilizer + SAP significantly down-regulated the expression levels of genes involved in ROS scavenging (TaCAT, CsCAT, TaAPX, and CsAPX2), ethylene biosynthesis (TaACO2, CsACO1, and CsACS1), stress response (TaDHN3, TaLEA, and CsLEA11), salicylic acid (TaPR1-1a and CsPR1-1a), and transcription activation (TaNAC2D and CsNAC35) in plants under drought stress. These results suggest that SAP addition in biofertilizer is a good tactic for enhancing the efficiency of biofertilizer, which is beneficial for plants in response to drought stress. To the best of our knowledge, this is the first report about the effect of synergistic use of biofertilizer and SAP on plant growth under drought stress.

Ectopic expression of ARGOS8 reveals a role for ethylene in root-lodging resistance in maize

Ethylene plays a critical role in many diverse processes in plant development. Recent studies have demonstrated that overexpression of the maize ARGOS8 gene reduces the plant's response to ethylene by decreasing ethylene signaling and enhances grain yield in transgenic maize plants. The objective of this study was to determine the effects of ethylene on the development of nodal roots, which are primarily responsible for root-lodging resistance in maize. Exogenous application of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) was found to promote the emergence of nodal roots. Transcriptome analysis of nodal tissues revealed that the expression of genes involved in metabolic processes and cell wall biogenesis was upregulated in response to ACC treatment, supporting the notion that ethylene is a positive regulator for the outgrowth of young root primordia. In BSV::ARGOS8 transgenic plants with reduced ethylene sensitivity due to constitutive overexpression of ARGOS8, nodal root emergence was delayed and the promotional effect of ACC on nodal root emergence decreased. Field tests showed that the BSV::ARGOS8 plants had higher root lodging relative to non-transgenic controls. When ARGOS8 expression was controlled by the developmentally regulated promoter FTM1, which conferred ARGOS8 overexpression in adult plants but not in the nodal roots and nodes in juvenile plants, the FTM1::ARGOS8 plants had no significant difference in root lodging compared with the wild type but produced a higher grain yield. These results suggest that ethylene has a role in promoting nodal root emergence and that a delay in nodal root development has a negative effect on root-lodging resistance in maize.

Understanding the Biostimulant Action of Vegetal-Derived Protein Hydrolysates by High-Throughput Plant Phenotyping and Metabolomics: A Case Study on Tomato

Designing and developing new biostimulants is a crucial process which requires an accurate testing of the product effects on the morpho-physiological traits of plants and a deep understanding of the mechanism of action of selected products. Product screening approaches using omics technologies have been found to be more efficient and cost effective in finding new biostimulant substances. A screening protocol based on the use of high-throughput phenotyping platform for screening new vegetal-derived protein hydrolysates (PHs) for biostimulant activity followed by a metabolomic analysis to elucidate the mechanism of the most active PHs has been applied on tomato crop. Eight PHs (A-G, I) derived from enzymatic hydrolysis of seed proteins of Leguminosae and Brassicaceae species were foliarly sprayed twice during the trial. A non-ionic surfactant Triton X-100 at 0.1% was also added to the solutions before spraying. A control treatment foliarly sprayed with distilled water containing 0.1% Triton X-100 was also included. Untreated and PH-treated tomato plants were monitored regularly using high-throughput non-invasive imaging technologies. The phenotyping approach we used is based on automated integrative analysis of photosynthetic performance, growth analysis, and color index analysis. The digital biomass of the plants sprayed with PH was generally increased. In particular, the relative growth rate and the growth performance were significantly improved by PHs A and I, respectively, compared to the untreated control plants. Kinetic chlorophyll fluorescence imaging did not allow to differentiate the photosynthetic performance of treated and untreated plants. Finally, MS-based untargeted metabolomics analysis was performed in order to characterize the functional mechanisms of selected PHs. The treatment modulated the multi-layer regulation process that involved the ethylene precursor and polyamines and affected the ROS-mediated signaling pathways. Although further investigation is needed to strengthen our findings, metabolomic data suggest that treated plants experienced a metabolic reprogramming following the application of the tested biostimulants. Nonetheless, our experimental data highlight the potential for combined use of high-throughput phenotyping and metabolomics to facilitate the screening of new substances with biostimulant properties and to provide a morpho-physiological and metabolomic gateway to the mechanisms underlying PHs action on plants.

Developing Transgenic Agronomic Traits for Crops: Targets, Methods, and Challenges

The last two decades have witnessed a surge of investment by the agricultural biotechnology industry in the development of transgenic agronomic traits. These are traits that improve yield performance by modifying endogenous physiological processes such as energy capture, nutrient utilization, and stress tolerance. In this chapter we provide a foundation for understanding these fundamental processes and then outline approaches that have been taken to use this knowledge for yield improvement. We characterize the current status of product development pipelines in the industry and illustrate the trait discovery process with three important examples-bacterial cold-shock proteins, alanine aminotransferase, and auxin-regulated genes. The challenges with developing and commercializing an agronomic trait product are discussed.

Improving photosynthesis, plant productivity and abiotic stress tolerance - current trends and future perspectives

With unfavourable climate changes and an increasing global population, there is a great need for more productive and stress-tolerant crops. As traditional methods of crop improvement have probably reached their limits, a further increase in the productivity of crops is expected to be possible using genetic engineering. The number of potential genes and metabolic pathways, which when genetically modified could result in improved photosynthesis and biomass production, is multiple. Photosynthesis, as the only source of carbon required for the growth and development of plants, attracts much attention is this respect, especially the question concerning how to improve CO₂ fixation and limit photorespiration. The most promising direction for increasing CO₂ assimilation is implementating carbon concentrating mechanisms found in cyanobacteria and algae into crop plants, while hitherto performed experiments on improving the CO₂ fixation versus oxygenation reaction catalyzed by Rubisco are less encouraging. On the other hand, introducing the C₄ pathway into C₃ plants is a very difficult challenge. Among other points of interest for increased biomass production is engineering of metabolic regulation, certain proteins, nucleic acids or phytohormones. In this respect, enhanced sucrose synthesis, assimilate translocation to sink organs and starch synthesis is crucial, as is genetic engineering of the phytohormone metabolism. As abiotic stress tolerance is one of the key factors determining crop productivity, extensive studies are being undertaken to develop transgenic plants characterized by elevated stress resistance. This can be accomplished due to elevated synthesis of antioxidants, osmoprotectants and protective proteins. Among other promising targets for the genetic engineering of plants with elevated stress resistance are transcription factors that play a key role in abiotic stress responses of plants. In this review, most of the approaches to improving the productivity of plants that are potentially promising and have already been undertaken are described. In addition to this, the limitations faced, potential challenges and possibilities regarding future research are discussed.

Phytohormones as targets for improving plant productivity and stress tolerance

In this review, we summarize the results of experiments that lead to altered levels of phytohormones in transgenic plants to improve plant productivity. The available data indicate that manipulating the level of phytohormones might also be a promising way to enhance the environmental stress tolerance of crop plants. In the regulation of the level of phytohormones, both biosynthesis and their catabolism pathways can be targeted for engineering purposes. Moreover, the signaling pathways of phytohormones should explored in this respect. In genetic modifications, conditional promoters must be developed to avoid undesired effects on growth. In order to find a practical application, the effects of genetic modifications should be further verified under field conditions and over a longer time scale.

Where are the drought tolerant crops? An assessment of more than two decades of plant biotechnology effort in crop improvement

Since the dawn of modern biotechnology public and private enterprise have pursued the development of a new breed of drought tolerant crop products. After more than 20 years of research and investment only a few such products have reached the market. This is due to several technical and market constraints. The technical challenges include the difficulty in defining tractable single-gene trait development strategies, the logistics of moving traits from initial to commercial genetic backgrounds, and the disconnect between conditions in farmer's fields and controlled environments. Market constraints include the significant difficulty, and associated costs, in obtaining access to markets around the world. Advances in the biology of plant water management, including response to water deficit reveal new opportunities to improve crop response to water deficit and new genome-based tools promise to usher in the next era of crop improvement. As biotechnology looks to improve crop productivity under drought conditions, the environmental and food security advantages will influence public perception and shift the debate toward benefits rather than risks.

Abiotic Stress Signaling in Wheat - An Inclusive Overview of Hormonal Interactions During Abiotic Stress Responses in Wheat

Rapid global warming directly impacts agricultural productivity and poses a major challenge to the present-day agriculture. Recent climate change models predict severe losses in crop production worldwide due to the changing environment, and in wheat, this can be as large as 42 Mt/°C rise in temperature. Although wheat occupies the largest total harvested area (38.8%) among the cereals including rice and maize, its total productivity remains the lowest. The major production losses in wheat are caused more by abiotic stresses such as drought, salinity, and high temperature than by biotic insults. Thus, understanding the effects of these stresses becomes indispensable for wheat improvement programs which have depended mainly on the genetic variations present in the wheat genome through conventional breeding. Notably, recent biotechnological breakthroughs in the understanding of gene functions and access to whole genome sequences have opened new avenues for crop improvement. Despite the availability of such resources in wheat, progress is still limited to the understanding of the stress signaling mechanisms using model plants such as Arabidopsis, rice and Brachypodium and not directly using wheat as the model organism. This review presents an inclusive overview of the phenotypic and physiological changes in wheat due to various abiotic stresses followed by the current state of knowledge on the identified mechanisms of perception and signal transduction in wheat. Specifically, this review provides an in-depth analysis of different hormonal interactions and signaling observed during abiotic stress signaling in wheat.

Drought affects abortion of reproductive organs by exacerbating developmentally driven processes via expansive growth and hydraulics

Abortion of reproductive organs is a major limiting factor of yield under water deficit, but is also a trait selected for by evolutionary processes. The youngest reproductive organs must be prone to abortion so older organs can finish their development in case of limited resources. Water deficit increases natural abortion via two developmentally driven processes, namely a signal from the first fertilized ovaries and a simultaneous arrest of the expansive growth of all ovaries at a precise stage. In maize (Zea mays) subjected to water deficits typically encountered in dryland agriculture, these developmental mechanisms account for 90% of drought-associated abortion and are irreversible 3 d after silk emergence. Consistently, transcripts and enzyme activities suggest that the molecular events associated with abortion affect expansive growth in silks whereas ovaries maintain a favourable carbon status. Abortion due to carbon starvation is only observed for severe drought scenarios occurring after silking. Both kinetic and genetic evidence indicates that vegetative and reproductive structures share a partly common hydraulic control of expansive growth. Hence, the control of expansive growth of reproductive structures probably has a prominent effect on abortion for mild water deficits occurring at flowering time, while carbon starvation dominates in severe post-flowering drought scenarios.

Ethylene mediates dichromate-induced inhibition of primary root growth by altering AUX1 expression and auxin accumulation in Arabidopsis thaliana

The hexavalent form of chromium [Cr(VI)] causes a major reduction in yield and quality of crops worldwide. The root is the first plant organ that interacts with Cr(VI) toxicity, which inhibits primary root elongation, but the underlying mechanisms of this inhibition remain elusive. In this study, we investigate the possibility that Cr(VI) reduces primary root growth of Arabidopsis by modulating the cell cycle-related genes and that ethylene signalling contributes to this process. We show that Cr(VI)-mediated inhibition of primary root elongation was alleviated by the ethylene perception and biosynthesis antagonists silver and cobalt, respectively. Furthermore, the ethylene signalling defective mutants (ein2-1 and etr1-3) were insensitive, whereas the overproducer mutant (eto1-1) was hypersensitive to Cr(VI). We also report that high levels of Cr(VI) significantly induce the distribution and accumulation of auxin in the primary root tips, but this increase was significantly suppressed in seedlings exposed to silver or cobalt. In addition, genetic and physiological investigations show that AUXIN-RESISTANT1 (AUX1) participates in Cr(VI)-induced inhibition of primary root growth. Taken together, our results indicate that ethylene mediates Cr(VI)-induced inhibition of primary root elongation by increasing auxin accumulation and polar transport by stimulating the expression of AUX1.

Strategies and tools to improve crop productivity by targeting photosynthesis

Crop productivity needs to substantially increase to meet global food and feed demand for a rapidly growing world population. Agricultural technology developers are pursuing a variety of approaches based on both traditional technologies such as genetic improvement, pest control and mechanization as well as new technologies such as genomics, gene manipulation and environmental modelling to develop crops that are capable of meeting growing demand. Photosynthesis is a key biochemical process that, many suggest, is not yet optimized for industrial agriculture or the modern global environment. We are interested in identifying control points in maize photoassimilation that are amenable to gene manipulation to improve overall productivity. Our approach encompasses: developing and using novel gene discovery techniques, translating our discoveries into traits and evaluating each trait in a stepwise manner that reflects a modern production environment. Our aim is to provide step change advancement in overall crop productivity and deliver this new technology into the hands of growers.This article is part of the themed issue 'Enhancing photosynthesis in crop plants: targets for improvement'.

The Pivotal Role of Ethylene in Plant Growth

Being continuously exposed to variable environmental conditions, plants produce phytohormones to react quickly and specifically to these changes. The phytohormone ethylene is produced in response to multiple stresses. While the role of ethylene in defense responses to pathogens is widely recognized, recent studies in arabidopsis and crop species highlight an emerging key role for ethylene in the regulation of organ growth and yield under abiotic stress. Molecular connections between ethylene and growth-regulatory pathways have been uncovered, and altering the expression of ethylene response factors (ERFs) provides a new strategy for targeted ethylene-response engineering. Crops with optimized ethylene responses show improved growth in the field, opening new windows for future crop improvement. This review focuses on how ethylene regulates shoot growth, with an emphasis on leaves.

Heat and Drought Stresses in Crops and Approaches for Their Mitigation

Drought and heat are major abiotic stresses that reduce crop productivity and weaken global food security, especially given the current and growing impacts of climate change and increases in the occurrence and severity of both stress factors. Plants have developed dynamic responses at the morphological, physiological and biochemical levels allowing them to escape and/or adapt to unfavorable environmental conditions. Nevertheless, even the mildest heat and drought stress negatively affects crop yield. Further, several independent studies have shown that increased temperature and drought can reduce crop yields by as much as 50%. Response to stress is complex and involves several factors including signaling, transcription factors, hormones, and secondary metabolites. The reproductive phase of development, leading to the grain production is shown to be more sensitive to heat stress in several crops. Advances coming from biotechnology including progress in genomics and information technology may mitigate the detrimental effects of heat and drought through the use of agronomic management practices and the development of crop varieties with increased productivity under stress. This review presents recent progress in key areas relevant to plant drought and heat tolerance. Furthermore, an overview and implications of physiological, biochemical and genetic aspects in the context of heat and drought are presented. Potential strategies to improve crop productivity are discussed.

Heat and Drought Stresses in Crops and Approaches for Their Mitigation

Drought and heat are major abiotic stresses that reduce crop productivity and weaken global food security, especially given the current and growing impacts of climate change and increases in the occurrence and severity of both stress factors. Plants have developed dynamic responses at the morphological, physiological and biochemical levels allowing them to escape and/or adapt to unfavorable environmental conditions. Nevertheless, even the mildest heat and drought stress negatively affects crop yield. Further, several independent studies have shown that increased temperature and drought can reduce crop yields by as much as 50%. Response to stress is complex and involves several factors including signaling, transcription factors, hormones, and secondary metabolites. The reproductive phase of development, leading to the grain production is shown to be more sensitive to heat stress in several crops. Advances coming from biotechnology including progress in genomics and information technology may mitigate the detrimental effects of heat and drought through the use of agronomic management practices and the development of crop varieties with increased productivity under stress. This review presents recent progress in key areas relevant to plant drought and heat tolerance. Furthermore, an overview and implications of physiological, biochemical and genetic aspects in the context of heat and drought are presented. Potential strategies to improve crop productivity are discussed.

Are GM Crops for Yield and Resilience Possible?

Crop yield improvements need to accelerate to avoid future food insecurity. Outside Europe, genetically modified (GM) crops for herbicide- and insect-resistance have been transformative in agriculture; other traits have also come to market. However, GM of yield potential and stress resilience has yet to impact on food security. Genes have been identified for yield such as grain number, size, leaf growth, resource allocation, and signaling for drought tolerance, but there is only one commercialized drought-tolerant GM variety. For GM and genome editing to impact on yield and resilience there is a need to understand yield-determining processes in a cell and developmental context combined with evaluation in the grower environment. We highlight a sugar signaling mechanism as a paradigm for this approach.

A Brief History of Promoter Development for Use in Transgenic Maize Applications

Promoters regulate gene expression, and are essential biotechnology tools. Since its introduction in the mid-1990s, biotechnology has greatly enhanced maize productivity primarily through the development of insect control and herbicide tolerance traits. Additional biotechnology applications include improving seed nutrient composition, industrial protein production, therapeutic production, disease resistance, abiotic stress resistance, and yield enhancement. Biotechnology has also greatly expanded basic research into important mechanisms that govern plant growth and reproduction. Many novel promoters have been developed to facilitate this work, but only a few are widely used. Transgene optimization includes a variety of strategies some of which effect promoter structure. Recent reviews examine the state of the art with respect to transgene design for biotechnology applications. This chapter examines the use of transgene technology in maize, focusing on the way promoters are selected and used. The impact of new developments in genomic technology on promoter structure is also discussed.

Diverse Roles of Ethylene in Regulating Agronomic Traits in Rice

Gaseous hormone ethylene has diverse effects in various plant processes. These processes include seed germination, plant growth, senescence, fruit ripening, biotic and abiotic stresses responses, and many other aspects. The biosynthesis and signaling of ethylene have been extensively studied in model Arabidopsis in the past two decades. However, knowledge about the ethylene signaling mechanism in crops and roles of ethylene in regulation of crop agronomic traits are still limited. Our recent findings demonstrate that rice possesses both conserved and diverged mechanism for ethylene signaling compared with Arabidopsis. Here, we mainly focused on the recent advances in ethylene regulation of important agronomic traits. Of special emphasis is its impact on rice growth, flowering, grain filling, and grain size control. Similarly, the influence of ethylene on other relevant crops will be compared. Additionally, interactions of ethylene with other hormones will also be discussed in terms of crop growth and development. Increasing insights into the roles and mechanisms of ethylene in regulating agronomic traits will contribute to improvement of crop production through precise manipulation of ethylene actions in crops.

High-biomass C4 grasses-Filling the yield gap

A significant increase in agricultural productivity will be required by 2050 to meet the needs of an expanding and rapidly developing world population, without allocating more land and water resources to agriculture, and despite slowing rates of grain yield improvement. This review examines the proposition that high-biomass C₄ grasses could help fill the yield gap. High-biomass C₄ grasses exhibit high yield due to C₄ photosynthesis, long growth duration, and efficient capture and utilization of light, water, and nutrients. These C₄ grasses exhibit high levels of drought tolerance during their long vegetative growth phase ideal for crops grown in water-limited regions of agricultural production. The stems of some high-biomass C₄ grasses can accumulate high levels of non-structural carbohydrates that could be engineered to enhance biomass yield and utility as feedstocks for animals and biofuels production. The regulatory pathway that delays flowering of high-biomass C₄ grasses in long days has been elucidated enabling production and deployment of hybrids. Crop and landscape-scale modeling predict that utilization of high-biomass C₄ grass crops on land and in regions where water resources limit grain crop yield could increase agricultural productivity.

Phenotypic and genome-wide association analysis of spike ethylene in diverse wheat genotypes under heat stress

The gaseous phytohormone ethylene plays an important role in spike development in wheat (Triticum aestivum). However, the genotypic variation and the genomic regions governing spike ethylene (SET) production in wheat under long-term heat stress remain unexplored. We investigated genotypic variation in the production of SET and its relationship with spike dry weight (SDW) in 130 diverse wheat elite lines and landraces under heat-stressed field conditions. We employed an Illumina iSelect 90K single nucleotide polymorphism (SNP) genotyping array to identify the genetic loci for SET and SDW through a genome-wide association study (GWAS) in a subset of the Wheat Association Mapping Initiative (WAMI) panel. The SET and SDW exhibited appreciable genotypic variation among wheat genotypes at the anthesis stage. There was a strong negative correlation between SET and SDW. The GWAS uncovered five and 32 significant SNPs for SET, and 22 and 142 significant SNPs for SDW, in glasshouse and field conditions, respectively. Some of these SNPs closely localized to the SNPs for plant height, suggesting close associations between plant height and spike-related traits. The phenotypic and genetic elucidation of SET and its relationship with SDW supports future efforts toward gene discovery and breeding wheat cultivars with reduced ethylene effects on yield under heat stress.

Improved drought tolerance in wheat plants overexpressing a synthetic bacterial cold shock protein gene SeCspA

Cold shock proteins (CSPs) enhance acclimatization of bacteria to adverse environmental circumstances. The Escherichia coli CSP genes CspA and CspB were modified to plant-preferred codon sequences and named as SeCspA and SeCspB. Overexpression of exogenous SeCspA and SeCspB in transgenic Arabidopsis lines increased germination rates, survival rates, and increased primary root length compared to control plants under drought and salt stress. Investigation of several stress-related parameters in SeCspA and SeCspB transgenic wheat lines indicated that these lines possessed stress tolerance characteristics, including lower malondialdehyde (MDA) content, lower water loss rates, lower relative Na⁺ content, and higher chlorophyll content and proline content than the control wheat plants under drought and salt stresses. RNA-seq and qRT-PCR expression analysis showed that overexpression of SeCsp could enhance the expression of stress-responsive genes. The field experiments showed that the SeCspA transgenic wheat lines had great increases in the 1000-grain weight and grain yield compared to the control genotype under drought stress conditions. Significant differences in the stress indices revealed that the SeCspA transgenic wheat lines possessed significant and stable improvements in drought tolerance over the control plants. No such improvement was observed for the SeCspB transgenic lines under field conditions. Our results indicated that SeCspA conferred drought tolerance and improved physiological traits in wheat plants.

Time of day determines Arabidopsis transcriptome and growth dynamics under mild drought

Drought stress is a major problem for agriculture worldwide, causing significant yield losses. Plants have developed highly flexible mechanisms to deal with drought, including organ- and developmental stage-specific responses. In young leaves, growth is repressed as an active mechanism to save water and energy, increasing the chances of survival but decreasing yield. Despite its importance, the molecular basis for this growth inhibition is largely unknown. Here, we present a novel approach to explore early molecular mechanisms controlling Arabidopsis leaf growth inhibition following mild drought. We found that growth and transcriptome responses to drought are highly dynamic. Growth was only repressed by drought during the day, and our evidence suggests that this may be due to gating by the circadian clock. Similarly, time of day strongly affected the extent, specificity, and in certain cases even direction of drought-induced changes in gene expression. These findings underscore the importance of taking into account diurnal patterns to understand stress responses, as only a small core of drought-responsive genes are affected by drought at all times of the day. Finally, we leveraged our high-resolution data to demonstrate that phenotypic and transcriptome responses can be matched to identify putative novel regulators of growth under mild drought.

ARGOS8 variants generated by CRISPR-Cas9 improve maize grain yield under field drought stress conditions

Maize ARGOS8 is a negative regulator of ethylene responses. A previous study has shown that transgenic plants constitutively overexpressing ARGOS8 have reduced ethylene sensitivity and improved grain yield under drought stress conditions. To explore the targeted use of ARGOS8 native expression variation in drought-tolerant breeding, a diverse set of over 400 maize inbreds was examined for ARGOS8 mRNA expression, but the expression levels in all lines were less than that created in the original ARGOS8 transgenic events. We then employed a CRISPR-Cas-enabled advanced breeding technology to generate novel variants of ARGOS8. The native maize GOS2 promoter, which confers a moderate level of constitutive expression, was inserted into the 5'-untranslated region of the native ARGOS8 gene or was used to replace the native promoter of ARGOS8. Precise genomic DNA modification at the ARGOS8 locus was verified by PCR and sequencing. The ARGOS8 variants had elevated levels of ARGOS8 transcripts relative to the native allele and these transcripts were detectable in all the tissues tested, which was the expected results using the GOS2 promoter. A field study showed that compared to the WT, the ARGOS8 variants increased grain yield by five bushels per acre under flowering stress conditions and had no yield loss under well-watered conditions. These results demonstrate the utility of the CRISPR-Cas9 system in generating novel allelic variation for breeding drought-tolerant crops.

Ethylene Mediates Alkaline-Induced Rice Growth Inhibition by Negatively Regulating Plasma Membrane H+-ATPase Activity in Roots

pH is an important factor regulating plant growth. Here, we found that rice was better adapted to low pH than alkaline conditions, as its growth was severely inhibited at high pH, with shorter root length and an extreme biomass reduction. Under alkaline stress, the expression of genes for ethylene biosynthesis enzymes in rice roots was strongly induced by high pH and exogenous ethylene precursor ACC and ethylene overproduction in etol1-1 mutant aggravated the alkaline stress-mediated inhibition of rice growth, especially for the root elongation with decreased cell length in root apical regions. Conversely, the ethylene perception antagonist silver (Ag⁺) and ein2-1 mutants could partly alleviate the alkaline-induced root elongation inhibition. The H⁺-ATPase activity was extremely inhibited by alkaline stress and exogenous ACC. However, the H⁺-ATPase-mediated rhizosphere acidification was enhanced by exogenous Ag⁺, while H⁺ efflux on the root surface was extremely inhibited by exogenous ACC, suggesting that ethylene negatively regulated H⁺-ATPase activity under high-pH stress. Our results demonstrate that H⁺-ATPase is involved in ethylene-mediated inhibition of rice growth under alkaline stress.

Moderate Drought Stress Induces Increased Foliar Dimethylsulphoniopropionate (DMSP) Concentration and Isoprene Emission in Two Contrasting Ecotypes of Arundo donax

The function of dimethylsulphoniopropionate (DMSP) in plants is unclear. It has been proposed as an antioxidant, osmolyte and overflow for excess energy under stress conditions. The formation of DMSP is part of the methionine (MET) pathway that is involved in plant stress responses. We used a new analytical approach to accurately quantify the changes in DMSP concentration that occurred in two ecotypes of the biomass crop Arundo donax subject to moderate drought stress under field conditions. The ecotypes of A. donax were from a hot semi-arid habitat in Morocco and a warm-humid environment in Central Italy. The Moroccan ecotype showed more pronounced reductions in photosynthesis, stomatal conductance and photochemical electron transport than the Italian ecotype. An increase in isoprene emission occurred in both ecotypes alongside enhanced foliar concentrations of DMSP, indicative of a protective function of these two metabolites in the amelioration of the deleterious effects of excess energy and oxidative stress. This is consistent with the modification of carbon within the methyl-erythritol and MET pathways responsible for increased synthesis of isoprene and DMSP under moderate drought. The results of this study indicate that DMSP is an important adaptive component of the stress response regulated via the MET pathway in A. donax. DMSP is likely a multifunctional molecule playing a number of roles in the response of A. donax to reduced water availability.

Implications of climate change predictions for UK cropping and prospects for possible mitigation: a review of challenges and potential responses

The UK, like the rest of the world, is confronting the impacts of climate change. Further changes are expected and they will have a profound effect on agriculture. Future crop production will take place against increasing CO₂ levels and temperatures, decreasing water availability, and increasing frequency of extreme weather events. This review contributes to research on agricultural practices for climate change, but with a more regional perspective. The present study explores climate change impacts on UK agriculture, particularly food crop production, and how to mitigate and build resilience to climate change by adopting and/or changing soil management practices, including fertilisation and tillage systems, new crop adoption and variety choice. Some mitigation can be adopted in the shorter term, such as changes in crop type and reduction in fertiliser use, but in other cases the options will need greater investment and longer adaptation period. This is the case for new crop variety development and deployment, and possible changes to soil cultivations. Uncertainty of future weather conditions, particularly extreme weather, also affect decision-making for adoption of practices by farmers to ensure more stable and sustainable production. Even when there is real potential for climate change mitigation, it can sometimes be more difficult to accomplish with certainty on-farm. Better future climate projections and long-term investments will be required to create more resilient agricultural systems in the UK in the face of climate change challenges. © 2016 Society of Chemical Industry.

Gravimetric phenotyping of whole plant transpiration responses to atmospheric vapour pressure deficit identifies genotypic variation in water use efficiency

There is increasing interest in rapidly identifying genotypes with improved water use efficiency, exemplified by the development of whole plant phenotyping platforms that automatically measure plant growth and water use. Transpirational responses to atmospheric vapour pressure deficit (VPD) and whole plant water use efficiency (WUE, defined as the accumulation of above ground biomass per unit of water used) were measured in 100 maize (Zea mays L.) genotypes. Using a glasshouse based phenotyping platform with naturally varying VPD (1.5-3.8kPa), a 2-fold variation in WUE was identified in well-watered plants. Regression analysis of transpiration versus VPD under these conditions, and subsequent whole plant gas exchange at imposed VPDs (0.8-3.4kPa) showed identical responses in specific genotypes. Genotype response of transpiration versus VPD fell into two categories: 1) a linear increase in transpiration rate with VPD with low (high WUE) or high (low WUE) transpiration rate at all VPDs, 2) a non-linear response with a pronounced change point at low VPD (high WUE) or high VPD (low WUE). In the latter group, high WUE genotypes required a significantly lower VPD before transpiration was restricted, and had a significantly lower rate of transpiration in response to VPD after this point, when compared to low WUE genotypes. Change point values were significantly positively correlated with stomatal sensitivity to VPD. A change point in stomatal response to VPD may explain why some genotypes show contradictory WUE rankings according to whether they are measured under glasshouse or field conditions. Furthermore, this novel use of a high throughput phenotyping platform successfully reproduced the gas exchange responses of individuals measured in whole plant chambers, accelerating the identification of plants with high WUE.

Responses to combined abiotic and biotic stress in tomato are governed by stress intensity and resistance mechanism

Stress conditions in agricultural ecosystems can occur at variable intensities. Different resistance mechanisms against abiotic stress and pathogens are deployed by plants. Thus, it is important to examine plant responses to stress combinations under different scenarios. Here, we evaluated the effect of different levels of salt stress ranging from mild to severe (50, 100, and 150mM NaCl) on powdery mildew resistance and overall performance of tomato introgression lines with contrasting levels of partial resistance, as well as near-isogenic lines (NILs) carrying the resistance gene Ol-1 (associated with a slow hypersensitivity response; HR), ol-2 (an mlo mutant associated with papilla formation), and Ol-4 (an R gene associated with a fast HR). Powdery mildew resistance was affected by salt stress in a genotype- and stress intensity-dependent manner. In susceptible and partial resistant lines, increased susceptibility was observed under mild salt stress (50mM) which was accompanied by accelerated cell death-like senescence. In contrast, severe salt stress (150mM) reduced disease symptoms. Na(+) and Cl(-) accumulation in the leaves was linearly related to the decreased pathogen symptoms under severe stress. In contrast, complete resistance mediated by ol-2 and Ol-4 was unaffected under all treatment combinations, and was associated with a decreased growth penalty. Increased susceptibility and senescence under combined stress in NIL-Ol-1 was associated with the induction of ethylene and jasmonic acid pathway genes and the cell wall invertase gene LIN6. These results highlight the significance of stress severity and resistance type on the plant's performance under the combination of abiotic and biotic stress.

Foliar Abscisic Acid-To-Ethylene Accumulation and Response Regulate Shoot Growth Sensitivity to Mild Drought in Wheat

Although, plant hormones play an important role in adjusting growth in response to environmental perturbation, the relative contributions of abscisic acid (ABA) and ethylene remain elusive. Using six spring wheat genotypes differing for stress tolerance, we show that young seedlings of the drought-tolerant (DT) group maintained or increased shoot dry weight (SDW) while the drought-susceptible (DS) group decreased SDW in response to mild drought. Both the DT and DS groups increased endogenous ABA and ethylene concentrations under mild drought compared to control. The DT and DS groups exhibited different SDW response trends, whereby the DS group decreased while the DT group increased SDW, to increased concentrations of ABA and ethylene under mild drought, although both groups decreased ABA/ethylene ratio under mild drought albeit at different levels. We concluded that SDW of the DT and DS groups might be distinctly regulated by specific ABA:ethylene ratio. Further, a foliar-spray of low concentrations (0.1 μM) of ABA increased shoot relative growth rate (RGR) in the DS group while ACC (1-aminocyclopropane-1-carboxylic acid, ethylene precursor) spray increased RGR in both groups compared to control. Furthermore, the DT group accumulated a significantly higher galactose while a significantly lower maltose in the shoot compared to the DS group. Taken all together, these results suggest an impact of ABA, ethylene, and ABA:ethylene ratio on SDW of wheat seedlings that may partly underlie a genotypic variability of different shoot growth sensitivities to drought among crop species under field conditions. We propose that phenotyping based on hormone accumulation, response and hormonal ratio would be a viable, rapid, and an early-stage selection tool aiding genotype selection for stress tolerance.

Future-proof crops: challenges and strategies for climate resilience improvement

Breeding for stress-resilient crops strongly depends on technological and biological advancements that have provided a wealth of information on genetic variants and their contribution to stress tolerance. In the context of the upcoming challenges for agriculture due to climate change, such as prolonged and/or increased stress intensities, CO2 increase and stress combinations, hierarchizing this information is key to accelerating crop improvement towards sustained or even increased productivity. We propose traits with high scalability to yield and crop performance that can be targeted for improvement and provide examples of recent discoveries with potential applicability in breeding. Critical to success is the integrated analysis of the phenotypes of genetic variants across different environmental variables using modelling approaches and high-throughput phenotyping.

Expression of trehalose-6-phosphate phosphatase in maize ears improves yield in well-watered and drought conditions

Maize, the highest-yielding cereal crop worldwide, is particularly susceptible to drought during its 2- to 3-week flowering period. Many genetic engineering strategies for drought tolerance impinge on plant development, reduce maximum yield potential or do not translate from laboratory conditions to the field. We overexpressed a gene encoding a rice trehalose-6-phosphate phosphatase (TPP) in developing maize ears using a floral promoter. This reduced the concentration of trehalose-6-phosphate (T6P), a sugar signal that regulates growth and development, and increased the concentration of sucrose in ear spikelets. Overexpression of TPP increased both kernel set and harvest index. Field data at several sites and over multiple seasons showed that the engineered trait improved yields from 9% to 49% under non-drought or mild drought conditions, and from 31% to 123% under more severe drought conditions, relative to yields from nontransgenic controls.

Plant Growth Beyond Limits

Growth processes, governed by complex genetic networks in a coordinated manner, are determining factors for numerous crop traits. Many components of these networks, described in Arabidopsis and to a lesser extent in crops, enhance organ growth when perturbed. However, translating our understanding of plant growth into crop improvement has been very limited. We argue here that this lack of success is due to the fact that modifying the expression of single genes in a complex growth regulatory network might be buffered by other components of the network. We discuss the observation that simultaneous perturbations of multiple genes have more pronounced effects, and present novel perspectives to use knowledge of growth regulatory networks to enhance crop yield in a targeted manner.

Implications of ethylene biosynthesis and signaling in soybean drought stress tolerance

BACKGROUND

Ethylene is a phytohormone known for inducing a triple response in seedlings, leaf abscission and other responses to various stresses. Several studies in model plants have evaluated the importance of this hormone in crosstalk signaling with different metabolic pathways, in addition to responses to biotic stresses. However, the mechanism of action in plants of agricultural interest, such as soybean, and its participation in abiotic stresses remain unclear.

RESULTS

The studies presented in this work allowed for the identification of 176 soybean genes described elsewhere for ethylene biosynthesis (108 genes) and signal transduction (68 genes). A model to predict these routes in soybean was proposed, and it had great representability compared to those described for Arabidopsis thaliana and Oryza sativa. Furthermore, analysis of putative gene promoters from soybean gene orthologs permitted the identification of 29 families of cis-acting elements. These elements are essential for ethylene-mediated regulation and its possible crosstalk with other signaling pathways mediated by other plant hormones. From genes that are differentially expressed in the transcriptome database, we analyzed the relative expression of some selected genes in resistant and tolerant soybean plants subjected to water deficit. The differential expression of a set of five soybean ethylene-related genes (MAT, ACS, ACO, ETR and CTR) was validated with RT-qPCR experiments, which confirmed variations in the expression of these soybean target genes, as identified in the transcriptome database. In particular, two families of ethylene biosynthesis genes (ACS and ACO) were upregulated under these experimental conditions, whereas CTR (involved in ethylene signal transduction) was downregulated. In the same samples, high levels of ethylene production were detected and were directly correlated with the free fraction levels of ethylene's precursor. Thus, the combination of these data indicated the involvement of ethylene biosynthesis and signaling in soybean responses to water stress.

CONCLUSIONS

The in silico analysis, combined with the quantification of ethylene production (and its precursor) and RT-qPCR experiments, allowed for a better understanding of the importance of ethylene at a molecular level in this crop as well as its role in the response to abiotic stresses. In summary, all of the data presented here suggested that soybean responses to water stress could be regulated by a crosstalk network among different signaling pathways, which might involve various phytohormones, such as auxins, ABA and jasmonic acid. The integration of in silico and physiological data could also contribute to the application of biotechnological strategies to the development of improved cultivars with regard to different stresses, such as the isolation of stress-specific plant promoters.

Integrating Crop Growth Models with Whole Genome Prediction through Approximate Bayesian Computation

Genomic selection, enabled by whole genome prediction (WGP) methods, is revolutionizing plant breeding. Existing WGP methods have been shown to deliver accurate predictions in the most common settings, such as prediction of across environment performance for traits with additive gene effects. However, prediction of traits with non-additive gene effects and prediction of genotype by environment interaction (G×E), continues to be challenging. Previous attempts to increase prediction accuracy for these particularly difficult tasks employed prediction methods that are purely statistical in nature. Augmenting the statistical methods with biological knowledge has been largely overlooked thus far. Crop growth models (CGMs) attempt to represent the impact of functional relationships between plant physiology and the environment in the formation of yield and similar output traits of interest. Thus, they can explain the impact of G×E and certain types of non-additive gene effects on the expressed phenotype. Approximate Bayesian computation (ABC), a novel and powerful computational procedure, allows the incorporation of CGMs directly into the estimation of whole genome marker effects in WGP. Here we provide a proof of concept study for this novel approach and demonstrate its use with synthetic data sets. We show that this novel approach can be considerably more accurate than the benchmark WGP method GBLUP in predicting performance in environments represented in the estimation set as well as in previously unobserved environments for traits determined by non-additive gene effects. We conclude that this proof of concept demonstrates that using ABC for incorporating biological knowledge in the form of CGMs into WGP is a very promising and novel approach to improving prediction accuracy for some of the most challenging scenarios in plant breeding and applied genetics.

The ARGOS gene family functions in a negative feedback loop to desensitize plants to ethylene

BACKGROUND

Ethylene plays critical roles in plant growth and development, including the regulation of cell expansion, senescence, and the response to biotic and abiotic stresses. Elements of the initial signal transduction pathway have been determined, but we are still defining regulatory mechanisms by which the sensitivity of plants to ethylene is modulated.

RESULTS

We report here that members of the ARGOS gene family of Arabidopsis, previously implicated in the regulation of plant growth and biomass, function as negative feedback regulators of ethylene signaling. Expression of all four members of the ARGOS family is induced by ethylene, but this induction is blocked in ethylene-insensitive mutants. The dose dependence for ethylene induction varies among the ARGOS family members, suggesting that they could modulate responses across a range of ethylene concentrations. GFP-fusions of ARGOS and ARL localize to the endoplasmic reticulum, the same subcellular location as the ethylene receptors and other initial components of the ethylene signaling pathway. Seedlings with increased expression of ARGOS family members exhibit reduced ethylene sensitivity based on physiological and molecular responses.

CONCLUSIONS

These results support a model in which the ARGOS gene family functions as part of a negative feedback circuit to desensitize the plant to ethylene, thereby expanding the range of ethylene concentrations to which the plant can respond. These results also indicate that the effects of the ARGOS gene family on plant growth and biomass are mediated through effects on ethylene signal transduction.

Planning for food security in a changing climate

The Intergovernmental Panel on Climate Change and other international agencies have concluded that global crop production is at risk due to climate change, population growth, and changing food preferences. Society expects that the agricultural sciences will innovate solutions to these problems and provide food security for the foreseeable future. My thesis is that an integrated research plan merging agronomic and genetic approaches has the greatest probability of success. I present a template for a research plan based on the lessons we have learned from the Green Revolution and from the development of genetically engineered crops that may guide us to meet this expectation. The plan starts with a vision of how the crop management system could change, and I give a few examples of innovations that are very much in their infancy but have significant potential. The opportunities need to be conceptualized on a regional basis for each crop to provide a target for change. The plan gives an overview of how the tools of plant biotechnology can be used to create the genetic diversity needed to implement the envisioned changes in the crop management system, using the development of drought tolerance in maize (Zea mays L.) as an example that has led recently to the commercial release of new hybrids in the USA. The plan requires an interdisciplinary approach that integrates and coordinates research on plant biotechnology, genetics, physiology, breeding, agronomy, and cropping systems to be successful.

Ethylene signaling in rice and Arabidopsis: conserved and diverged aspects

Ethylene as a gas phytohormone plays significant roles in the whole life cycle of plants, ranging from growth and development to stress responses. A linear ethylene signaling pathway has been established in the dicotyledonous model plant Arabidopsis. However, the ethylene signaling mechanism in monocotyledonous plants such as rice is largely unclear. In this review, we compare the ethylene response phenotypes of dark-grown seedlings of Arabidopsis, rice, and other monocotyledonous plants (maize, wheat, sorghum, and Brachypodium distachyon) and pinpoint that rice has a distinct phenotype of root inhibition but coleoptile promotion in etiolated seedlings upon ethylene treatment. We further summarize the homologous genes of Arabidopsis ethylene signaling components in these monocotyledonous plants and discuss recent progress. Although conserved in most aspects, ethylene signaling in rice has evolved new features compared with that in Arabidopsis. These analyses provide novel insights into the understanding of ethylene signaling in the dicotyledonous Arabidopsis and monocotyledonous plants, particularly rice. Further characterization of rice ethylene-responsive mutants and their corresponding genes will help us better understand the whole picture of ethylene signaling mechanisms in plants.

Next biotech plants: new traits, crops, developers and technologies for addressing global challenges

Most of the genetically modified (GM) plants currently commercialized encompass a handful of crop species (soybean, corn, cotton and canola) with agronomic characters (traits) directed against some biotic stresses (pest resistance, herbicide tolerance or both) and created by multinational companies. The same crops with agronomic traits already on the market today will continue to be commercialized, but there will be also a wider range of species with combined traits. The timeframe anticipated for market release of the next biotech plants will not only depend on science progress in research and development (R&D) in laboratories and fields, but also primarily on how demanding regulatory requirements are in countries where marketing approvals are pending. Regulatory constraints, including environmental and health impact assessments, have increased significantly in the past decades, delaying approvals and increasing their costs. This has sometimes discouraged public research entities and small and medium size plant breeding companies from using biotechnology and given preference to other technologies, not as stringently regulated. Nevertheless, R&D programs are flourishing in developing countries, boosted by the necessity to meet the global challenges that are food security of a booming world population while mitigating climate change impacts. Biotechnology is an instrument at the service of these imperatives and a wide variety of plants are currently tested for their high yield despite biotic and abiotic stresses. Many plants with higher water or nitrogen use efficiency, tolerant to cold, salinity or water submergence are being developed. Food security is not only a question of quantity but also of quality of agricultural and food products, to be available and accessible for the ones who need it the most. Many biotech plants (especially staple food) are therefore being developed with nutritional traits, such as biofortification in vitamins and metals. The main international seed companies continue to be the largest investors in plant biotechnology R&D, and often collaborate in the developing world with public institutions, private entities and philanthropic organizations. These partnerships are particularly present in Africa. In developed countries, plant biotechnology is also used for non-food purposes, such as the pharmaceutical, biofuel, starch, paper and textile industries. For example, plants are modified to specifically produce molecules with therapeutic uses, or with an improved biomass conversion efficiency, or producing larger volumes of feedstocks for biofuels. Various plant breeding technologies are now used in the entire spectrum of plant biotechnology: transgenesis producing proteins or RNAi. Cisgenesis (transgenes isolated from a crossable donor plant) and intragenesis (transgenes originate from the same species or a crossable species), null segregants are also used. To date, the next generation precision gene editing tools are developed in basic research. They include: clustered regularly interspaced short palindromic repeats (CRISPR), oligonucleotide-directed mutagenesis (ODM), transcription activator-like effects nucleases (TALENs) and zinc-finger nuclease (ZFN).