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Research Project: PHYSIOLOGICAL AND MOLECULAR SIGNALING IN VIROID AND BACTERIAL DISEASE

Location: Molecular Plant Pathology Laboratory

2016 Annual Report


Objectives
Objective 1: Identify changes in host gene expression and small RNA-mediated regulation associated with viroid and bacterial infection and disease development as potential targets for disease management. Objective 2: Identify key metabolites that are involved in the early stages of pathogenesis and may have global effects on disease resistance through either their bioactive nature or effect on redox-status. Sub-objective 2.A. Identify and quantify secondary metabolites induced upon infection of tomato with either P. syringae or potato spindle tuber viroid (PSTVd). Sub-objective 2.B. Determine bioactivity of secondary metabolites induced upon infection of tomato with either P. syringae or PSTVd. Sub-objective 2.C. Determine the effect on redox status of secondary metabolites induced upon infection of tomato with either P. syringae or PSTVd. Objective 3: Identify the molecular signals and pathways used by viroids to move through the cytoplasm, enter the nucleus or chloroplast of the host cell, and begin replication. Sub-objective 3.A. Determine the role of host protein 4/1 (and other proteins interacting with 4/1) in the intra- and intercellular movement of PSTVd. Sub-objective 3.B. Use sequence motifs derived from Eggplant latent viroid (ELVd) to redirect mRNAs encoding enzymes involved in terpenoid biosynthesis into the chloroplast.


Approach
This project seeks to elucidate specific signaling mechanisms that are involved in plant disease resistance. Plants respond to biotic stress by the integration of responses located in two different cellular compartments, the symplast and the apoplast. Improvement of existing resistance to plant disease requires a more comprehensive understanding of key apoplastic and symplastic responses to pathogen invasion and how they are integrated. In the apoplast, we will examine the complex interplay between secondary metabolites and redox signaling that controls early events in bacterial and viroid pathogenesis as well as other responses triggered by long distance signaling. In the symplast, we will examine the molecular interactions between viroids and their hosts to determine how these small RNA molecules replicate and move between organelles within a single cell or over long distances between cells. The long-term objective of this multidisciplinary project is to understand the role of plant-pathogen signaling in disease resistance in sufficient detail that novel strategies can be developed to render plants resistant/immune to pathogen infection. In the intermediate term, we will test the ability of certain viroid-derived targeting signals to redirect mRNAs to the chloroplast, thereby adding novel biosynthetic capabilities to chloroplast metabolism with the need for chloroplast genome transformation.


Progress Report
In regard to disease resistance, the plant apoplast, which surrounds the plant cell, is analogous to a moat protecting a castle within. The tactics used by both the host and pathogen on this battleground, outside the protected cell, can be complex, having evolved over time, and can create very toxic conditions and molecules. Only pathogens that have evolved adequate counter measures to slip through the apoplast will survive and cause disease. In regard to disease control, the leaf apoplast is highly vulnerable to manipulation either by genetic engineering or direct application of materials to leaves, via stomata or additions to ground water, and delivered by xylem. By knowing how pathogen and host tactics operate in the apoplast we may be able to enhance specific attributes and better protect the cell within from pathogens and other stressors. We have continued to examine secondary metabolites that are induced in the apoplast and examine their bioactivity, such as apocyanin which is being tested widely for anti-cancer therapy in humans, (see accomplishments). In addition we have been shown that symplastic metabolites from the cell vacuole (within the cell) will leak out into the apoplast several hours after infection with successful pathogenic bacteria. Subsequently, these bacteria will multiply and spread further suggesting that nutrients are also leaking out; the details will be examined further. In addition, we have been examining the role and mechanisms of ‘redox status’ and how this is maintained in the complex mixture of phenolics found in the apoplast.


Accomplishments
1. A new technique to examine how and when bacterial pathogens overcome resistance in plants. Bacterial plant diseases cause major damage to crops each year and the cost of controlling them adds greatly to production costs and often involves antibiotics which are a public concern; therefore, ARS researchers at Beltsville, Maryland, developed a technique to examine how bacterial pathogens overcome resistance to cause disease in plants. The plant leaf apoplast, which is the cell wall region just outside the plant cell itself, is the first line of defense against most aerial pathogens. The new method monitors changes in apoplast components and changes in the physical integrity of the apoplast/cytoplasmic barrier, through which successful bacteria are able to gain nutrients. This method will be used to select and test strategies to prevent leakage of this apoplast barrier by pathogens and the leaf apoplast offers a good arena for improving control strategies either by genetic engineering or direct application of materials to leaves or roots. This information will benefit plant scientists and breeders who are devising new strategies to improve disease resistance while decreasing the use of chemical pesticides and antibiotics.


None.


Review Publications
Baker, C.J., Mock, N.M., Smith, J.M., Averyanov, A.A. 2015. The dynamics of apoplast phenolics in tobacco leaves following inoculation with bacteria. Frontiers in Plant Science. 6:649.
Averyanov, A.A., Pasechnik, T.D., Lapikova, V.P., Romanova, T.S., Baker, C.J. 2015. Systemic reduction of rice blast by inhibitors of antioxidant enzymes. Russian Journal of Plant Physiology. 62:628-633.