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Volume 61, 2023
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Virulence and Ecology of Agrobacteria in the Context of Evolutionary Genomics
Vol. 61 (2023), pp. 1–23More LessAmong plant-associated bacteria, agrobacteria occupy a special place. These bacteria are feared in the field as agricultural pathogens. They cause abnormal growth deformations and significant economic damage to a broad range of plant species. However, these bacteria are revered in the laboratory as models and tools. They are studied to discover and understand basic biological phenomena and used in fundamental plant research and biotechnology. Agrobacterial pathogenicity and capability for transformation are one and the same and rely on functions encoded largely on their oncogenic plasmids. Here, we synthesize a substantial body of elegant work that elucidated agrobacterial virulence mechanisms and described their ecology. We review findings in the context of the natural diversity that has been recently unveiled for agrobacteria and emphasize their genomics and plasmids. We also identify areas of research that can capitalize on recent findings to further transform our understanding of agrobacterial virulence and ecology.
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Ralstonia solanacearum: An Arsenal of Virulence Strategies and Prospects for Resistance
Vol. 61 (2023), pp. 25–47More LessThe group of strains constituting the Ralstonia solanacearum species complex (RSSC) is a prominent model for the study of plant-pathogenic bacteria because of its impact on agriculture, owing to its wide host range, worldwide distribution, and long persistence in the environment. RSSC strains have led to numerous studies aimed at deciphering the molecular bases of virulence, and many biological functions and mechanisms have been described to contribute to host infection and pathogenesis. In this review, we put into perspective recent advances in our understanding of virulence in RSSC strains, both in terms of the inventory of functions that participate in this process and their evolutionary dynamics. We also present the different strategies that have been developed to combat these pathogenic strains through biological control, antimicrobial agents, plant genetics, or microbiota engineering.
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Ill Communication: Host Metabolites as Virulence-Regulating Signals for Plant-Pathogenic Bacteria
Vol. 61 (2023), pp. 49–71More LessPlant bacterial pathogens rely on host-derived signals to coordinate the deployment of virulence factors required for infection. In this review, I describe how diverse plant-pathogenic bacteria detect and respond to plant-derived metabolic signals for the purpose of virulence gene regulation. I highlight examples of how pathogens perceive host metabolites through membrane-localized receptors as well as intracellular response mechanisms. Furthermore, I describe how individual strains may coordinate their virulence using multiple distinct host metabolic signals, and how plant signals may positively or negatively regulate virulence responses. I also describe how plant defenses may interfere with the perception of host metabolites as a means to dampen pathogen virulence. The emerging picture is that recognition of host metabolic signals for the purpose of virulence gene regulation represents an important primary layer of interaction between pathogenic bacteria and host plants that shapes infection outcomes.
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International Trade and Local Effects of Viral and Bacterial Diseases in Ornamental Plants
Vol. 61 (2023), pp. 73–95More LessSince the 1950s, there have been major changes in the scope, value, and organization of the ornamental plant industry. With fewer individual producers and a strong trend toward consolidation and globalization, increasing quantities of diverse plant genera and species are being shipped internationally. Many more ornamentals are propagated vegetatively instead of by seed, further contributing to disease spread. These factors have led to global movement of pathogens to countries where they were not formerly known. The emergence of some previously undescribed pathogens has been facilitated by high-throughput sequencing, but biological studies are often lacking, so their roles in economic diseases are not yet known. Case studies of diseases in selected ornamentals discuss the factors involved in their spread, control measures to reduce their economic impact, and some potential effects on agronomic crops. Advances in diagnostic techniques are discussed, and parallels are drawn to the international movement of human diseases.
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Kitaviruses: A Window to Atypical Plant Viruses Causing Nonsystemic Diseases
Vol. 61 (2023), pp. 97–118More LessKitaviridae is a family of plant-infecting viruses that have multiple positive-sense, single-stranded RNA genomic segments. Kitaviruses are assigned into the genera Cilevirus, Higrevirus, and Blunervirus, mainly on the basis of the diversity of their genomic organization. Cell-to-cell movement of most kitaviruses is provided by the 30K family of proteins or the binary movement block, considered an alternative movement module among plant viruses. Kitaviruses stand out for producing conspicuously unusual locally restricted infections and showing deficient or nonsystemic movement likely resulting from incompatible or suboptimal interactions with their hosts. Transmission of kitaviruses is mediated by mites of many species of the genus Brevipalpus and at least one species of eriophyids. Kitavirus genomes encode numerous orphan open reading frames but RNA-dependent RNA polymerase and the transmembrane helix-containing protein, generically called SP24, typify a close phylogenetic link with arthropod viruses. Kitaviruses infect a large range of host plants and cause diseases of economic concern in crops such as citrus, tomato, passion fruit, tea, and blueberry.
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The Past Is Present: Coevolution of Viruses and Host Resistance Within Geographic Centers of Plant Diversity
Vol. 61 (2023), pp. 119–136More LessUnderstanding the coevolutionary history of plants, pathogens, and disease resistance is vital for plant pathology. Here, I review Francis O. Holmes's work with tobacco mosaic virus (TMV) framed by the foundational work of Nikolai Vavilov on the geographic centers of origin of plants and crop wild relatives (CWRs) and T. Harper Goodspeed's taxonomy of the genus Nicotiana. Holmes developed a hypothesis that the origin of host resistance to viruses was due to coevolution of both at a geographic center. In the 1950s, Holmes proved that genetic resistance to TMV, especially dominant R-genes, was centered in South America for Nicotiana and other solanaceous plants, including Capsicum, potato, and tomato. One seeming exception was eggplant (Solanum melongena). Not until the acceptance of plate tectonics in the 1960s and recent advances in evolutionary taxonomy did it become evident that northeast Africa was the home of eggplant CWRs, far from Holmes's geographic center for TMV–R-gene coevolution. Unbeknownst to most plant pathologists, Holmes's ideas predated those of H.H. Flor, including experimental proof of the gene-for-gene interaction, identification of R-genes, and deployment of dominant host genes to protect crop plants from virus-associated yield losses.
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Tomato Brown Rugose Fruit Virus Pandemic
Vol. 61 (2023), pp. 137–164More LessTomato brown rugose fruit virus (ToBRFV) is an emerging tobamovirus. It was first reported in 2015 in Jordan in greenhouse tomatoes and now threatens tomato and pepper crops around the world. ToBRFV is a stable and highly infectious virus that is easily transmitted by mechanical means and via seeds, which enables it to spread locally and over long distances. The ability of ToBRFV to infect tomato plants harboring the commonly deployed Tm resistance genes, as well as pepper plants harboring the L resistance alleles under certain conditions, limits the ability to prevent damage from the virus. The fruit production and quality of ToBRFV-infected tomato and pepper plants can be drastically affected, thus significantly impacting their market value. Herein, we review the current information and discuss the latest areas of research on this virus, which include its discovery and distribution, epidemiology, detection, and prevention and control measures, that could help mitigate the ToBRFV disease pandemic.
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Genome-Enabled Insights into Downy Mildew Biology and Evolution
Vol. 61 (2023), pp. 165–183More LessOomycetes that cause downy mildew diseases are highly specialized, obligately biotrophic phytopathogens that can have major impacts on agriculture and natural ecosystems. Deciphering the genome sequence of these organisms provides foundational tools to study and deploy control strategies against downy mildew pathogens (DMPs). The recent telomere-to-telomere genome assembly of the DMP Peronospora effusa revealed high levels of synteny with distantly related DMPs, higher than expected repeat content, and previously undescribed architectures. This provides a road map for generating similar high-quality genome assemblies for other oomycetes. This review discusses biological insights made using this and other assemblies, including ancestral chromosome architecture, modes of sexual and asexual variation, the occurrence of heterokaryosis, candidate gene identification, functional validation, and population dynamics. We also discuss future avenues of research likely to be fruitful in studies of DMPs and highlight resources necessary for advancing our understanding and ability to forecast and control disease outbreaks.
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Phytophthora capsici: Recent Progress on Fundamental Biology and Disease Management 100 Years After Its Description
Vol. 61 (2023), pp. 185–208More LessPhytophthora capsici is a destructive oomycete pathogen of vegetable, ornamental, and tropical crops. First described by L.H. Leonian in 1922 as a pathogen of pepper in New Mexico, USA, P. capsici is now widespread in temperate and tropical countries alike. Phytophthora capsici is notorious for its capability to evade disease management strategies. High genetic diversity allows P. capsici populations to overcome fungicides and host resistance, the formation of oospores results in long-term persistence in soils, zoospore differentiation in the presence of water increases epidemic potential, and a broad host range maximizes economic losses and limits the effectiveness of crop rotation. The severity of disease caused by P. capsici and management challenges have led to numerous research efforts in the past 100 years. Here, we discuss recent findings regarding the biology, genetic diversity, disease management, fungicide resistance, host resistance, genomics, and effector biology of P. capsici.
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Integrated Nematode Management in a World in Transition: Constraints, Policy, Processes, and Technologies for the Future
Vol. 61 (2023), pp. 209–230More LessPlant-parasitic nematodes are one of the most insidious pests limiting agricultural production, parasitizing mostly belowground and occasionally aboveground plant parts. They are an important and underestimated component of the estimated 30% yield loss inflicted on crops globally by biotic constraints. Nematode damage is intensified by interactions with biotic and abiotic factors constraints: soilborne pathogens, soil fertility degradation, reduced soil biodiversity, climate variability, and policies influencing the development of improved management options. This review focuses on the following topics: (a) biotic and abiotic constraints, (b) modification of production systems, (c) agricultural policies, (d) the microbiome, (e) genetic solutions, and (f) remote sensing. Improving integrated nematode management (INM) across all scales of agricultural production and along the Global North–Global South divide, where inequalities influence access to technology, is discussed. The importance of the integration of technological development in INM is critical to improving food security and human well-being in the future.
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The Reemergence of Phycopathology: When Algal Biology Meets Ecology and Biosecurity
Vol. 61 (2023), pp. 231–255More LessViruses, bacteria, and eukaryotic symbionts interact with algae in a variety of ways to cause disease complexes, often shaping marine and freshwater ecosystems. The advent of phyconomy (a.k.a. seaweed agronomy) represents a need for a greater understanding of algal disease interactions, where underestimated cryptic diversity and lack of phycopathological basis are prospective constraints for algal domestication. Here, we highlight the limited yet increasing knowledge of algal pathogen biodiversity and the ecological interaction with their algal hosts. Finally, we discuss how ecology and cultivation experience contribute to and reinforce aquaculture practice, with the potential to reshape biosecurity policies of seaweed cultivation worldwide.
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Engineering the Crop Microbiota Through Host Genetics
Vol. 61 (2023), pp. 257–277More LessThe microbiota populating the plant–soil continuum defines an untapped resource for sustainable crop production. The host plant is a driver for the taxonomic composition and function of these microbial communities. In this review, we illustrate how the host genetic determinants of the microbiota have been shaped by plant domestication and crop diversification. We discuss how the heritable component of microbiota recruitment may represent, at least partially, a selection for microbial functions underpinning the growth, development, and health of their host plants and how the magnitude of this heritability is influenced by the environment. We illustrate how host–microbiota interactions can be treated as an external quantitative trait and review recent studies associating crop genetics with microbiota-based quantitative traits. We also explore the results of reductionist approaches, including synthetic microbial communities, to establish causal relationships between microbiota and plant phenotypes. Lastly, we propose strategies to integrate microbiota manipulation into crop selection programs. Although a detailed understanding of when and how heritability for microbiota composition can be deployed for breeding purposes is still lacking, we argue that advances in crop genomics are likely to accelerate wider applications of plant–microbiota interactions in agriculture.
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Induced Resistance in Fruit and Vegetables: A Host Physiological Response Limiting Postharvest Disease Development
Vol. 61 (2023), pp. 279–300More LessHarvested fruit and vegetables are perishable, subject to desiccation, show increased respiration during ripening, and are colonized by postharvest fungal pathogens. Induced resistance is a strategy to control diseases by eliciting biochemical processes in fruits and vegetables. This is accomplished by modulating the progress of ripening and senescence, which maintains the produce in a state of heightened resistance to decay-causing fungi. Utilization of induced resistance to protect produce has been improved by scientific tools that better characterize physiological changes in plants. Induced resistance slows the decline of innate immunity after harvest and increases the production of defensive responses that directly inhibit plant pathogens. This increase in defense response in fruits and vegetables contributes to higher amounts of phenols and antioxidant compounds, improving both the quality and appearance of the produce. This review summarizes mechanisms and treatments that induce resistance in harvested fruits and vegetables to suppress fungal colonization. Moreover, it highlights the importance of host maturity and stage of ripening as limiting conditions for the improved expression of induced-resistance processes.
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Functional Peptides for Plant Disease Control
Vol. 61 (2023), pp. 301–324More LessPlant disease control requires novel approaches to mitigate the spread of and losses caused by current, emerging, and re-emerging diseases and to adapt plant protection to global climate change and the restrictions on the use of conventional pesticides. Currently, disease management relies mainly on biopesticides, which are required for the sustainable use of plant-protection products. Functional peptides are candidate biopesticides because they originate from living organisms or are synthetic analogs and provide novel mechanisms of action against plant pathogens. Hundreds of compounds exist that cover an extensive range of activities against viruses, bacteria and phytoplasmas, fungi and oomycetes, and nematodes. Natural sources, chemical synthesis, and biotechnological platforms may provide peptides at large scale for the industry and growers. The main challenges for their use in plant disease protection are (a) the requirement of stability in the plant environment and counteracting resistance in pathogen populations, (b) the need to develop suitable formulations to increase their shelf life and methods of application, (c) the selection of compounds with acceptable toxicological profiles, and (d) the high cost of production for agricultural purposes. In the near future, it is expected that several functional peptides will be commercially available for plant disease control, but more effort is needed to validate their efficacy at the field level and fulfill the requirements of the regulatory framework.
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Traffic Control: Subversion of Plant Membrane Trafficking by Pathogens
Vol. 61 (2023), pp. 325–350More LessMembrane trafficking pathways play a prominent role in plant immunity. The endomembrane transport system coordinates membrane-bound cellular organelles to ensure that immunological components are utilized effectively during pathogen resistance. Adapted pathogens and pests have evolved to interfere with aspects of membrane transport systems to subvert plant immunity. To do this, they secrete virulence factors known as effectors, many of which converge on host membrane trafficking routes. The emerging paradigm is that effectors redundantly target every step of membrane trafficking from vesicle budding to trafficking and membrane fusion. In this review, we focus on the mechanisms adopted by plant pathogens to reprogram host plant vesicle trafficking, providing examples of effector-targeted transport pathways and highlighting key questions for the field to answer moving forward.
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The Plant Ubiquitin–Proteasome System as a Target for Microbial Manipulation
Vol. 61 (2023), pp. 351–375More LessThe plant immune system perceives pathogens to trigger defense responses. In turn, pathogens secrete effector molecules to subvert these defense responses. The initiation and maintenance of defense responses involve not only de novo synthesis of regulatory proteins and enzymes but also their regulated degradation. The latter is achieved through protein degradation pathways such as the ubiquitin–proteasome system (UPS). The UPS regulates all stages of immunity, from the perception of the pathogen to the execution of the response, and, therefore, constitutes an ideal candidate for microbial manipulation of the host. Pathogen effector molecules interfere with the plant UPS through several mechanisms. This includes hijacking general UPS functions or perturbing its ability to degrade specific targets. In this review, we describe how the UPS regulates different immunity-related processes and how pathogens subvert this to promote disease.
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The Global Forest Health Crisis: A Public-Good Social Dilemma in Need of International Collective Action
Vol. 61 (2023), pp. 377–401More LessSociety is confronted by interconnected threats to ecological sustainability. Among these is the devastation of forests by destructive non-native pathogens and insects introduced through global trade, leading to the loss of critical ecosystem services and a global forest health crisis. We argue that the forest health crisis is a public-good social dilemma and propose a response framework that incorporates principles of collective action. This framework enables scientists to better engage policymakers and empowers the public to advocate for proactive biosecurity and forest health management. Collective action in forest health features broadly inclusive stakeholder engagement to build trust and set goals; accountability for destructive pest introductions; pooled support for weakest-link partners; and inclusion of intrinsic and nonmarket values of forest ecosystems in risk assessment. We provide short-term and longer-term measures that incorporate the above principles to shift the societal and ecological forest health paradigm to a more resilient state.
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More Than the Sum of Its Parts: Unlocking the Power of Network Structure for Understanding Organization and Function in Microbiomes
Vol. 61 (2023), pp. 403–423More LessPlant and soil microbiomes are integral to the health and productivity of plants and ecosystems, yet researchers struggle to identify microbiome characteristics important for providing beneficial outcomes. Network analysis offers a shift in analytical framework beyond “who is present” to the organization or patterns of coexistence between microbes within the microbiome. Because microbial phenotypes are often significantly impacted by coexisting populations, patterns of coexistence within microbiomes are likely to be especially important in predicting functional outcomes. Here, we provide an overview of the how and why of network analysis in microbiome research, highlighting the ways in which network analyses have provided novel insights into microbiome organization and functional capacities, the diverse network roles of different microbial populations, and the eco-evolutionary dynamics of plant and soil microbiomes.
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Previous Volumes
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Volume 61 (2023)
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Volume 60 (2022)
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Volume 59 (2021)
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Volume 58 (2020)
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Volume 57 (2019)
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Volume 56 (2018)
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Volume 55 (2017)
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Volume 54 (2016)
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Volume 53 (2015)
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Volume 52 (2014)
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Volume 51 (2013)
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Volume 50 (2012)
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Volume 49 (2011)
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Volume 48 (2010)
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Volume 47 (2009)
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Volume 46 (2008)
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Volume 45 (2007)
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Volume 44 (2006)
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Volume 43 (2005)
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Volume 42 (2004)
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Volume 41 (2003)
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Volume 40 (2002)
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Volume 39 (2001)
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Volume 38 (2000)
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Volume 37 (1999)
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Volume 36 (1998)
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Volume 35 (1997)
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Volume 34 (1996)
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Volume 33 (1995)
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Volume 32 (1994)
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Volume 31 (1993)
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Volume 30 (1992)
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Volume 29 (1991)
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Volume 28 (1990)
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Volume 27 (1989)
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Volume 26 (1988)
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Volume 25 (1987)
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Volume 24 (1986)
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Volume 23 (1985)
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Volume 22 (1984)
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Volume 21 (1983)
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Volume 20 (1982)
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Volume 19 (1981)
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Volume 18 (1980)
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Volume 17 (1979)
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Volume 16 (1978)
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Volume 15 (1977)
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Volume 14 (1976)
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Volume 13 (1975)
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Volume 12 (1974)
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Volume 11 (1973)
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Volume 10 (1972)
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Volume 9 (1971)
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Volume 8 (1970)
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Volume 7 (1969)
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Volume 6 (1968)
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Volume 5 (1967)
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Volume 4 (1966)
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Volume 3 (1965)
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Volume 2 (1964)
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Volume 1 (1963)
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Volume 0 (1932)