Tracing the evolutionary path to nitrogen-fixing crops

Root endophytic bacterial and fungal communities in a natural hot desert are differentially regulated in dry and wet seasons by stochastic processes and functional traits

Dryland ecosystems experience seasonal cycles of severe drought and moderate precipitation. Desert plants may develop symbiotic relationships with root endophytic microbes to survive under the repeated wet and extremely dry conditions. Although community coalescence has been found in many systems, the colonization by functional microbes and its relationship to seasonal transitions in arid regions are not well understood. Here we examined root endophytic microbial taxa, and their traits in relation to their root colonization, during the dry and wet seasons in a hot desert of the southwestern United States. We used high-throughput DNA sequencing of 16S rRNA and internal transcribed spacer gene profiling of five desert shrubs, and analyzed the seasonal change in endophytic microbial lineages. Goodness of fit to the neutral community model in relationship to microbial traits was evaluated. In summer, Actinobacteria and Bacteroidia increased, although this was not genus-specific. For fungi, Glomeraceae selectively increased in summer. In winter, Gram-negative bacterial genera, including those capable of nitrogen fixation and plant growth promotion, increased. Neutral model analysis revealed a strong stochastic influence on endophytic bacteria but a weak effect for fungi, especially in summer. The taxa with higher frequency than that predicted by neutral model shared environmental adaptability and symbiotic traits, whereas the frequency of pathogenic fungi was at or under the predicted value. These results suggest that community assembly of bacteria and fungi is regulated differently. The bacterial community was affected by stochastic and deterministic processes via bacterial response to drought (response trait), beneficial effect on plants (effect trait), and likely stable mutualistic interactions with plants suggested by the frequency of nodule bacteria. For fungi, mycorrhizal fungi were selected by plants in summer. The regulation of beneficial microbes by plants in both dry and wet seasons suggests the presence of plant-soil positive feedback in this natural desert ecosystem.

Crop adaptation to climate change: An evolutionary perspective

The disciplines of evolutionary biology and plant and animal breeding have been intertwined throughout their development, with responses to artificial selection yielding insights into the action of natural selection and evolutionary biology providing statistical and conceptual guidance for modern breeding. Here we offer an evolutionary perspective on a grand challenge of the 21st century: feeding humanity in the face of climate change. We first highlight promising strategies currently under way to adapt crops to current and future climate change. These include methods to match crop varieties with current and predicted environments and to optimize breeding goals, management practices, and crop microbiomes to enhance yield and sustainable production. We also describe the promise of crop wild relatives and recent technological innovations such as speed breeding, genomic selection, and genome editing for improving environmental resilience of existing crop varieties or for developing new crops. Next, we discuss how methods and theory from evolutionary biology can enhance these existing strategies and suggest novel approaches. We focus initially on methods for reconstructing the evolutionary history of crops and their pests and symbionts, because such historical information provides an overall framework for crop-improvement efforts. We then describe how evolutionary approaches can be used to detect and mitigate the accumulation of deleterious mutations in crop genomes, identify alleles and mutations that underlie adaptation (and maladaptation) to agricultural environments, mitigate evolutionary trade-offs, and improve critical proteins. Continuing feedback between the evolution and crop biology communities will ensure optimal design of strategies for adapting crops to climate change.

Diversity and regulation of symbiotic nitrogen fixation in plants

Plants associate with nitrogen-fixing bacteria to secure nitrogen, which is generally the most limiting nutrient for plant growth. Endosymbiotic nitrogen-fixing associations are widespread among diverse plant lineages, ranging from microalgae to angiosperms, and are primarily one of three types: cyanobacterial, actinorhizal or rhizobial. The large overlap in the signaling pathways and infection components of arbuscular mycorrhizal, actinorhizal and rhizobial symbioses reflects their evolutionary relatedness. These beneficial associations are influenced by environmental factors and other microorganisms in the rhizosphere. In this review, we summarize the diversity of nitrogen-fixing symbioses, key signal transduction pathways and colonization mechanisms relevant to such interactions, and compare and contrast these interactions with arbuscular mycorrhizal associations from an evolutionary standpoint. Additionally, we highlight recent studies on environmental factors regulating nitrogen-fixing symbioses to provide insights into the adaptation of symbiotic plants to complex environments.

The mysterious non-arbuscular mycorrhizal status of Brassicaceae species

The Brassicaceae family is unique in not fostering functional symbiosis with arbuscular mycorrhiza (AM). The family is also special in possessing glucosinolates, a class of secondary metabolites predominantly functioning for plant defence. We have reviewed what effect the glucosinolates of this non-symbiotic host have on AM or vice versa. Isothiocyanates, the toxic degradation product of the glucosinolates, particularly the indolic and benzenic glucosinolates, are known to be involved in the inhibition of AM. Interestingly, AM colonization enhances glucosinolate production in two AM-host in the Brassicales family- Moringa oleifera and Tropaeolum spp. PHOSPHATE STARVATION RESPONSE 1 (PHR1), a central transcription factor that controls phosphate starvation response also activates the glucosinolate biosynthesis in AM non-host Arabidopsis thaliana. Recently, the advances in whole-genome sequencing, enabling extensive ecological microbiome studies have helped unravel the Brassicaceae microbiome, identifying new mutualists that compensate for the loss of AM symbiosis, and reporting cues for some influence of glucosinolates on the microbiome structure. We advocate that glucosinolate is an important candidate in determining the mycorrhizal status of Brassicaceae and has played a major role in its symbiosis-defence trade-off. We also identify key open questions in this area that remain to be addressed in the future.

Are legumes different? Origins and consequences of evolving nitrogen fixing symbioses

Nitrogen fixing symbioses between plants and bacteria are ancient and, while not numerous, are formed in diverse lineages of plants ranging from microalgae to angiosperms. One symbiosis stands out as the most widespread one is that between legumes and rhizobia, leading to the formation of nitrogen-fixing nodules. The legume family is one of the largest and most diverse group of plants and legumes have been used by humans since the beginning of agriculture, both as high nitrogen food, as well as pastures and rotation crops. One open question is whether their ability to form a nitrogen-fixing symbiosis has contributed to legumes' success, and whether legumes have any unique characteristics that have made them more diverse and widespread than other groups of plants. This review examines the evolutionary journey that has led to the diversification of legumes, in particular its nitrogen-fixing symbiosis, and asks four questions to investigate which legume traits might have contributed to their success: 1. In what ways do legumes differ from other plant groups that have evolved nitrogen-fixing symbioses? In order to answer this question, the characteristics of the symbioses, and efficiencies of nitrogen fixation are compared between different groups of nitrogen fixing plants. 2. Could certain unique features of legumes be a reason for their success? This section examines the manifestations and possible benefits of a nitrogen-rich 'lifestyle' in legumes. 3. If nitrogen fixation was a reason for such a success, why have some species lost the symbiosis? Formation of symbioses has trade-offs, and while these are less well known for non-legumes, there are known energetic and ecological reasons for loss of symbiotic potential in legumes. 4. What can we learn from the unique traits of legumes for future crop improvements? While exploiting some of the physiological properties of legumes could be used to improve legume breeding, our increasing molecular understanding of the essential regulators of root nodule symbioses raise hope of creating new nitrogen fixing symbioses in other crop species.

Nitrogen fixation in maize: breeding opportunities

Maize (Zea mays L.) is a highly versatile crop with huge demand of nitrogen (N) for its growth and development. N is the most essential macronutrient for crop production. Despite being the highest abundant element in the atmosphere (~ 78%), it is scarcely available for plant growth. To fulfil the N demand, commercial agriculture is largely dependent on synthetic fertilizers. Excessive dependence on inorganic fertilizers has created extensive ecological as well as economic problems worldwide. Hence, for a sustainable solution to nitrogenous fertilizer use, development of biological nitrogen fixation (BNF) in cereals will be the best alternative. BNF is a well-known mechanism in legumes where diazotrophs convert atmospheric nitrogen (N≡N) to plant-available form, ammonium (NH₄⁺). From many decades, researchers have dreamt to develop a similar symbiotic partnership as in legumes to the cereal crops. A large number of endophytic diazotrophs have been found associated with maize. Elucidation of the genetic and molecular aspects of their interaction will open up new avenues to introgress BNF in maize breeding. With the advanced understanding of N-fixation process, researchers are at a juncture of breeding and engineering this symbiotic relationships in cereals. Different breeding, genetic engineering, omics, gene editing, and synthetic biology approaches will be discussed in this review to make BNF a reality in cereals. It will help to provide a road map to develop/improve the BNF in maize to an advance step for the sustainable production system to achieve the food and nutritional security.

How rhizobia adapt to the nodule environment

Rhizobia are a phylogenetically diverse group of soil bacteria that engage in mutualistic interactions with legume plants. Although specifics of the symbioses differ between strains and plants, all symbioses ultimately result in the formation of specialized root nodule organs which host the nitrogen-fixing microsymbionts called bacteroids. Inside nodules, bacteroids encounter unique conditions that necessitate global reprogramming of physiological processes and rerouting of their metabolism. Decades of research have addressed these questions using genetics, omics approaches, and more recently computational modelling. Here we discuss the common adaptations of rhizobia to the nodule environment that define the core principles of bacteroid functioning. All bacteroids are growth-arrested and perform energy-intensive nitrogen fixation fueled by plant-provided C₄-dicarboxylates at nanomolar oxygen levels. At the same time, bacteroids are subject to host control and sanctioning that ultimately determine their fitness and have fundamental importance for the evolution of a stable mutualistic relationship.

Molecular Biology in the Improvement of Biological Nitrogen Fixation by Rhizobia and Extending the Scope to Cereals

The contribution of biological nitrogen fixation to the total N requirement of food and feed crops diminished in importance with the advent of synthetic N fertilizers, which fueled the “green revolution”. Despite being environmentally unfriendly, the synthetic versions gained prominence primarily due to their low cost, and the fact that most important staple crops never evolved symbiotic associations with bacteria. In the recent past, advances in our knowledge of symbiosis and nitrogen fixation and the development and application of recombinant DNA technology have created opportunities that could help increase the share of symbiotically-driven nitrogen in global consumption. With the availability of molecular biology tools, rapid improvements in symbiotic characteristics of rhizobial strains became possible. Further, the technology allowed probing the possibility of establishing a symbiotic dialogue between rhizobia and cereals. Because the evolutionary process did not forge a symbiotic relationship with the latter, the potential of molecular manipulations has been tested to incorporate a functional mechanism of nitrogen reduction independent of microbes. In this review, we discuss various strategies applied to improve rhizobial strains for higher nitrogen fixation efficiency, more competitiveness and enhanced fitness under unfavorable environments. The challenges and progress made towards nitrogen self-sufficiency of cereals are also reviewed. An approach to integrate the genetically modified elite rhizobia strains in crop production systems is highlighted.

Unique and common traits in mycorrhizal symbioses

Mycorrhizas are among the most important biological interkingdom interactions, as they involve ~340,000 land plants and ~50,000 taxa of soil fungi. In these mutually beneficial interactions, fungi receive photosynthesis-derived carbon and provide the host plant with mineral nutrients such as phosphorus and nitrogen in exchange. More than 150 years of research on mycorrhizas has raised awareness of their biology, biodiversity and ecological impact. In this Review, we focus on recent phylogenomic, molecular and cell biology studies to present the current state of knowledge of the origin of mycorrhizal fungi and the evolutionary history of their relationship with land plants. As mycorrhizas feature a variety of phenotypes, depending on partner taxonomy, physiology and cellular interactions, we explore similarities and differences between mycorrhizal types. During evolution, mycorrhizal fungi have refined their biotrophic capabilities to take advantage of their hosts as food sources and protective niches, while plants have developed multiple strategies to accommodate diverse fungal symbionts. Intimate associations with pervasive ecological success have originated at the crossroads between these two evolutionary pathways. Our understanding of the biological processes underlying these symbioses, where fungi act as biofertilizers and bioprotectors, provides the tools to design biotechnological applications addressing environmental and agricultural challenges.

Sequencing and Analyzing the Transcriptomes of a Thousand Species Across the Tree of Life for Green Plants

The 1,000 Plants (1KP) initiative was the first large-scale effort to collect next-generation sequencing (NGS) data across a phylogenetically representative sampling of species for a major clade of life, in this case theViridiplantae, or green plants. As an international multidisciplinary consortium, we focused on plant evolution and its practical implications. Among the major outcomes were the inference of a reference species tree for green plants by phylotranscriptomic analysis of low-copy genes, a survey of paleopolyploidy (whole-genome duplications) across the Viridiplantae, the inferred evolutionary histories for many gene families and biological processes, the discovery of novel light-sensitive proteins for optogenetic studies in mammalian neuroscience, and elucidation of the genetic network for a complex trait (C₄ photosynthesis). Altogether, 1KP demonstrated how value can be extracted from a phylodiverse sequencing data set, providing a template for future projects that aim to generate even more data, including complete de novo genomes, across the tree of life.

An ancestral signalling pathway is conserved in intracellular symbioses-forming plant lineages

Plants are the foundation of terrestrial ecosystems, and their colonization of land was probably facilitated by mutualistic associations with arbuscular mycorrhizal fungi. Following this founding event, plant diversification has led to the emergence of a tremendous diversity of mutualistic symbioses with microorganisms, ranging from extracellular associations to the most intimate intracellular associations, where fungal or bacterial symbionts are hosted inside plant cells. Here, through analysis of 271 transcriptomes and 116 plant genomes spanning the entire land-plant diversity, we demonstrate that a common symbiosis signalling pathway co-evolved with intracellular endosymbioses, from the ancestral arbuscular mycorrhiza to the more recent ericoid and orchid mycorrhizae in angiosperms and ericoid-like associations of bryophytes. By contrast, species forming exclusively extracellular symbioses, such as ectomycorrhizae, and those forming associations with cyanobacteria, have lost this signalling pathway. This work unifies intracellular symbioses, revealing conservation in their evolution across 450 million yr of plant diversification.

Molecular Basis of Root Nodule Symbiosis between Bradyrhizobium and 'Crack-Entry' Legume Groundnut (Arachis hypogaea L.)

Nitrogen is one of the essential plant nutrients and a major factor limiting crop productivity. To meet the requirements of sustainable agriculture, there is a need to maximize biological nitrogen fixation in different crop species. Legumes are able to establish root nodule symbiosis (RNS) with nitrogen-fixing soil bacteria which are collectively called rhizobia. This mutualistic association is highly specific, and each rhizobia species/strain interacts with only a specific group of legumes, and vice versa. Nodulation involves multiple phases of interactions ranging from initial bacterial attachment and infection establishment to late nodule development, characterized by a complex molecular signalling between plants and rhizobia. Characteristically, legumes like groundnut display a bacterial invasion strategy popularly known as "crack-entry'' mechanism, which is reported approximately in 25% of all legumes. This article accommodates critical discussions on the bacterial infection mode, dynamics of nodulation, components of symbiotic signalling pathway, and also the effects of abiotic stresses and phytohormone homeostasis related to the root nodule symbiosis of groundnut and Bradyrhizobium. These parameters can help to understand how groundnut RNS is programmed to recognize and establish symbiotic relationships with rhizobia, adjusting gene expression in response to various regulations. This review further attempts to emphasize the current understanding of advancements regarding RNS research in the groundnut and speculates on prospective improvement possibilities in addition to ways for expanding it to other crops towards achieving sustainable agriculture and overcoming environmental challenges.

Developmental Responses to Water and Salinity in Root Systems

Roots provide the primary mechanism that plants use to absorb water and nutrients from their environment. These functions are dependent on developmental mechanisms that direct root growth and branching into regions of soil where these resources are relatively abundant. Water is the most limiting factor for plant growth, and its availability is determined by the weather, soil structure, and salinity. In this review, we define the developmental pathways that regulate the direction of growth and branching pattern of the root system, which together determine the expanse of soil from which a plant can access water. The ability of plants to regulate development in response to the spatial distribution of water is a focus of many recent studies and provides a model for understanding how biological systems utilize positional cues to affect signaling and morphogenesis. A better understanding of these processes will inform approaches to improve crop water use efficiency to more sustainably feed a growing population.

Virus-induced gene silencing: empowering genetics in non-model organisms

Virus-induced gene silencing (VIGS) is an RNA interference-based technology used to transiently knock down target gene expression by utilizing modified plant viral genomes. VIGS can be adapted to many angiosperm species that cover large phylogenetic distances, allowing the analysis of gene functions in species that are not amenable to stable genetic transformation. With a vast amount of sequence information already available and even more likely to become available in the future, VIGS provides a means to analyze the functions of candidate genes identified in large genomic or transcriptomic screens. Here, we provide a comprehensive overview of target species and VIGS vector systems, assess recent key publications in the field, and explain how plant viruses are modified to serve as VIGS vectors. As many reports on the VIGS technique are being published, we also propose minimal reporting guidelines for carrying out these experiments, with the aim of increasing comparability between experiments. Finally, we propose methods for the statistical evaluation of phenotypic results obtained with VIGS-treated plants, as analysis is challenging due to the predominantly transient nature of the silencing effect.

A phylogenetic framework of the legume genus Aeschynomene for comparative genetic analysis of the Nod-dependent and Nod-independent symbioses

BACKGROUND

Among semi-aquatic species of the legume genus Aeschynomene, some have the property of being nodulated by photosynthetic Bradyrhizobium lacking the nodABC genes necessary for the synthesis of Nod factors. Knowledge of the specificities underlying this Nod-independent symbiosis has been gained from the model legume Aeschynomene evenia but our understanding remains limited due to the lack of comparative genetics with related taxa using a Nod factor-dependent process. To fill this gap, we combined different approaches to perform a thorough comparative analysis in the genus Aeschynomene.

RESULTS

This study significantly broadened previous taxon sampling, including in allied genera, in order to construct a comprehensive phylogeny. In the phylogenetic tree, five main lineages were delineated, including a novel lineage, the Nod-independent clade and another one containing a polytomy that comprised several Aeschynomene groups and all the allied genera. This phylogeny was matched with data on chromosome number, genome size and low-copy nuclear gene sequences to reveal the diploid species and a polytomy containing mostly polyploid taxa. For these taxa, a single allopolyploid origin was inferred and the putative parental lineages were identified. Finally, nodulation tests with different Bradyrhizobium strains revealed new nodulation behaviours and the diploid species outside of the Nod-independent clade were compared for their experimental tractability and genetic diversity.

CONCLUSIONS

The extended knowledge of the genetics and biology of the different lineages sheds new light of the evolutionary history of the genus Aeschynomene and they provide a solid framework to exploit efficiently the diversity encountered in Aeschynomene legumes. Notably, our backbone tree contains all the species that are diploid and it clarifies the genetic relationships between the Nod-independent clade and the Nod-dependent lineages. This study enabled the identification of A. americana and A. patula as the most suitable species to undertake a comparative genetic study of the Nod-independent and Nod-dependent symbioses.

Nitrogen Fixation in Cereals

Cereals such as maize, rice, wheat and sorghum are the most important crops for human nutrition. Like other plants, cereals associate with diverse bacteria (including nitrogen-fixing bacteria called diazotrophs) and fungi. As large amounts of chemical fertilizers are used in cereals, it has always been desirable to promote biological nitrogen fixation in such crops. The quest for nitrogen fixation in cereals started long ago with the isolation of nitrogen-fixing bacteria from different plants. The sources of diazotrophs in cereals may be seeds, soils, and even irrigation water and diazotrophs have been found on roots or as endophytes. Recently, culture-independent molecular approaches have revealed that some rhizobia are found in cereal plants and that bacterial nitrogenase genes are expressed in plants. Since the levels of nitrogen-fixation attained with nitrogen-fixing bacteria in cereals are not high enough to support the plant’s needs and never as good as those obtained with chemical fertilizers or with rhizobium in symbiosis with legumes, it has been the aim of different studies to increase nitrogen-fixation in cereals. In many cases, these efforts have not been successful. However, new diazotroph mutants with enhanced capabilities to excrete ammonium are being successfully used to promote plant growth as commensal bacteria. In addition, there are ambitious projects supported by different funding agencies that are trying to genetically modify maize and other cereals to enhance diazotroph colonization or to fix nitrogen or to form nodules with nitrogen-fixing symbiotic rhizobia.

Symbiotic nitrogen fixation by rhizobia-the roots of a success story

By evolving the dual capacity of intracellular survival and symbiotic nitrogen fixation in legumes, rhizobia have achieved an ecological and evolutionary success that has reshaped our biosphere. Despite complex challenges, including a dual lifestyle of intracellular infection separated by a free-living phase in soil, rhizobial symbiosis has spread horizontally to hundreds of bacterial species and geographically throughout the globe. This symbiosis has also persisted and been reshaped through millions of years of history. Here, we summarize recent advances in our understanding of the molecular mechanisms, ecological settings, and evolutionary pathways that are collectively responsible for this symbiotic success story. We offer predictions of how this symbiosis can evolve under new influences and for the benefit of a burgeoning human population.

Crops, Nitrogen, Water: Are Legumes Friend, Foe, or Misunderstood Ally?

Biological nitrogen fixation (BNF) by crop legumes reduces demand for industrial nitrogen fixation (INF). Nonetheless, rates of BNF in agriculture remain low, with strong negative feedback to BNF from reactive soil nitrogen (N) and drought. We show that breeding for yield has resulted in strong relationships between photosynthesis and leaf N in non-leguminous crops, whereas grain legumes show strong relations between leaf N and water use efficiency (WUE). We contrast these understandings with other studies that draw attention to the water costs of grain legume crops, and their potential for polluting the biosphere with N. We propose that breeding grain legumes for reduced stomatal conductance can increase WUE without compromising production or BNF. Legume crops remain a better bet than relying on INF.

The Role of Soil Microorganisms in Plant Mineral Nutrition-Current Knowledge and Future Directions

In their natural environment, plants are part of a rich ecosystem including numerous and diverse microorganisms in the soil. It has been long recognized that some of these microbes, such as mycorrhizal fungi or nitrogen fixing symbiotic bacteria, play important roles in plant performance by improving mineral nutrition. However, the full range of microbes associated with plants and their potential to replace synthetic agricultural inputs has only recently started to be uncovered. In the last few years, a great progress has been made in the knowledge on composition of rhizospheric microbiomes and their dynamics. There is clear evidence that plants shape microbiome structures, most probably by root exudates, and also that bacteria have developed various adaptations to thrive in the rhizospheric niche. The mechanisms of these interactions and the processes driving the alterations in microbiomes are, however, largely unknown. In this review, we focus on the interaction of plants and root associated bacteria enhancing plant mineral nutrition, summarizing the current knowledge in several research fields that can converge to improve our understanding of the molecular mechanisms underpinning this phenomenon.

The calmodulin fused kinase novel gene family is the major system in plants converting Ca2+ signals to protein phosphorylation responses

Eukaryotes utilize Ca²⁺ as a universal second messenger to convert and multiply environmental and developmental signals to downstream protein phosphorylation responses. However, the phylogenetic relationships of the genes that convert Ca²⁺ signal (CS) to protein phosphorylation responses (PPRs) remain highly controversial, and their origin and evolutionary trajectory are unclear, which greatly hinders functional studies. Here we examined the deep phylogeny of eukaryotic CS converter gene families and identified a phylogenetically and structurally distinctive monophyly in Archaeplastida. This monophyly can be divided into four subfamilies, and each can be traced to ancestral members that contain a kinase domain and a calmodulin-like domain. This strongly indicates that the ancestor of this monophyly originated by a de novo fusion of a kinase gene and a calmodulin gene. This gene family, with a proposed new name, Calmodulin Fused Kinase (CFK), had expanded and diverged significantly both in sizes and in structures for efficient and accurate Ca²⁺ signalling, and was shown to play pivotal roles in all the six major plant adaptation events in evolution. Our findings elucidated the common origin of all CS-PPR converter genes except CBL-CIPK converter genes, and revealed that CFKs act as the main CS conversion system in plants.

Starting points in plant-bacteria nitrogen-fixing symbioses: intercellular invasion of the roots

Agricultural practices contribute to climate change by releasing greenhouse gases such as nitrous oxide that are mainly derived from nitrogen fertilizers. Therefore, understanding biological nitrogen fixation in farming systems is beneficial to agriculture and environmental preservation. In this context, a better grasp of nitrogen-fixing systems and nitrogen-fixing bacteria-plant associations will contribute to the optimization of these biological processes. Legumes and actinorhizal plants can engage in a symbiotic interaction with nitrogen-fixing rhizobia or actinomycetes, resulting in the formation of specialized root nodules. The legume-rhizobia interaction is mediated by a complex molecular signal exchange, where recognition of different bacterial determinants activates the nodulation program in the plant. To invade plants roots, bacteria follow different routes, which are determined by the host plant. Entrance via root hairs is probably the best understood. Alternatively, entry via intercellular invasion has been observed in many legumes. Although there are common features shared by intercellular infection mechanisms, differences are observed in the site of root invasion and bacterial spread on the cortex reaching and infecting a susceptible cell to form a nodule. This review focuses on intercellular bacterial invasion of roots observed in the Fabaceae and considers, within an evolutionary context, the different variants, distribution and molecular determinants involved. Intercellular invasion of actinorhizal plants and Parasponia is also discussed.

Rooting for cassava: insights into photosynthesis and associated physiology as a route to improve yield potential

Contents 50 I. 50 II. 52 III. 54 IV. 55 V. 57 VI. 57 VII. 59 60 References 61 SUMMARY: As a consequence of an increase in world population, food demand is expected to grow by up to 110% in the next 30-35 yr. The population of sub-Saharan Africa is projected to increase by > 120%. In this region, cassava (Manihot esculenta) is the second most important source of calories and contributes c. 30% of the daily calorie requirements per person. Despite its importance, the average yield of cassava in Africa has not increased significantly since 1961. An evaluation of modern cultivars of cassava showed that the interception efficiency (ɛ_i ) of photosynthetically active radiation (PAR) and the efficiency of conversion of that intercepted PAR (ɛ_c ) are major opportunities for genetic improvement of the yield potential. This review examines what is known of the physiological processes underlying productivity in cassava and seeks to provide some strategies and directions toward yield improvement through genetic alterations to physiology to increase ɛ_i and ɛ_c . Possible physiological limitations, as well as environmental constraints, are discussed.

Towards the minimal nitrogen-fixing symbiotic genome

diCenzo and coworkers have reverse engineered a rhizobium into a non-nitrogen fixer, creating a genomic platform for gain-of-function genetics studies, which should aid to identify the minimal nitrogen fixing symbiotic genome.