Eicosapolyenoic Acids as MAMPs

Plants, like animals, recognize certain molecular signatures in microbial cells, triggering a form of immunity that may help the host resist infection by potential pathogens. These molecular signatures are present in microbe-associated molecular patterns, or MAMPs, which include diverse molecules, including proteins, polysaccharides, and lipids.   The eicosapolyenoic acids, arachidonic acid (AA; 20:4;ω-6) and eicosapentaenoic acid (EPA; 20:5 ; ω-3), from Phytophthora species and other oomycetes are MAMPs because they are not present in higher plants, are essential components in microbial cells, and they can induce strong immunity.   The immunity triggered by these and other MAMPs is characterized by a set of responses which may include generation of reactive oxygen species (ROS), programmed host cell death, accumulation of antimicrobial compounds, reinforcement of plant cell walls, and changes in levels and signaling of phytohormones important for orchestrating the response both locally at the site of attack and systemically throughout the plant.  In solanaceous plants such as potato the activity of eicosapolyenoic acids may also be strongly enhanced by β-glucans released by the pathogen during infection.   AA and EPA also are precursors to eicosanoids that mediate inflammatory responses and serve as critical signals for immune and central nervous system functions in mammals.  Eicosapolyenoic acids are compelling molecules for study because they may identify important mechanisms of perception in lipid-based signaling  in induced immunity in plants and conservation of this function across eukaryotic kingdoms.

AA and EPA structures

Model illustrating release of β-glucans and eicosapolyenoic acids from phospholipids and triglyceride (TG)-rich lipid bodies  during early stages of plant–oomycete interactions as suggested by experimental studies.

Thousand Cankers Disease of Walnut

Thousand cankers composite

Thousand cankers disease (TCD) is an emerging disease that is responsible for serious decline and death of ornamental eastern black walnut (Juglans nigra) in Colorado and other western states, and Juglans californica and Juglans hindsii in California.  TCD is now known to occur in the native range of J. nigra,  with reports from Indiana, Maryland, North Carolina, Ohio, Tennessee, Pennsylvania, and Virginia.  Most recently the disease was reported in Italy.  There is great concern over the further spread of TCD throughout the native range of J. nigra, and uncertainty about the potential impact on Juglans species of agricultural and ecological importance in the western U.S. as well.   The disease is caused by a fungal infection following attack by the walnut twig beetle (WTB), Pityophthorus juglandis, which aggregates and transmits the pathogen. The multiple infections result in numerous, coalescing cankers that girdle and kill branches and stems. The pathogen, Geosmithia morbida, was new to science when it was formally described in 2011.  TCD occurs in California and both the pathogen and beetle vector are routinely isolated from symptomatic J. californica, J. hindsii, and Juglans regia (English walnut).  Of great concern is the occurrence of TCD in commercial orchards in both English walnut scions and various rootstocks in the heart of the California walnut industry, which generates over a billion dollars in annual sales.

We are collaborating with Dr. Steve Seybold of the USDA Forest Service  and others to evaluate Juglans species susceptibility to TCD,  WTB host selection, disease and vector distribution in orchards, and the diversity of California isolates of G. morbida in English walnut orchards.  In addition, we are providing science-based education and outreach on TCD through UC IPM and the National Plant Diagnostic Network to inform growers and various stakeholders and decision-makers about this disease.

Fusarium canker disease of stored dormant fruit tree seedlings

 

Figure 1

Symptoms and signs of cold storage canker on almond and apple trees. Almond trees (panels A–H) and apple trees (panels I–K). Most symptoms and signs are localized around the graft union (“GU”, arrowheads A, arrows in H and I). However, some lesions appeared to start around the nodes of the scion (arrows, G). Punctate sporulation often emerges through the lenticels over colonized necrotic tissues (F and I). In advanced stages, sporulation coalesces into beige, brown, or maroon sporodochia-like structures (B–F). Necrosis of the inner bark, cambium, and sapwood primarily occurs on the scion (arrows, F–H,J) but occasionally spreads to the rootstock (arrow, K). From Marek et al Plant Disease (2013).

California fruit and nut tree nurseries lost hundreds of thousands of stone fruit (Prunus spp.) and apple (Malus domestica) trees to a canker disease during the period 1997-2001. The disease was prevalent in dormant tree seedlings maintained in cold storage in refrigerated warehouses, with disease signs and symptoms developing during storage or soon after planting. Diseased trees often had molds growing on the bark, usually near and above the graft union on scions, and necroses of the inner bark, cambium and sapwood, which girdled and killed trees. Infected but non-symptomatic dormant trees planted by growers often developed symptoms and collapsed in the field. We identified the primary causal agents, Fusarium acuminatum and Fusarium avenaceum, along with other opportunistic fungal pathogens such as Cylindrocarpon species that were less frequently encountered but capable of causing similar symptoms. Loss of bark turgidity in almond stem segments due to desiccation stress correlated with significantly increased susceptibility to F. acuminatum. Non-symptomatic nursery trees also were found to harbor F. acuminatum and F. avenaceum.  In 2010-11, the disease reemerged in at least one nursery, resulting in substantial losses of cold-stored seedlings. In addition, the same pathogens were isolated from declining seedlings during establishment of a prune rootstock field trial. In our current research, we are addressing remaining gaps in our understanding of the disease cycle, assessing the occurrence of these pathogens in budwood and seedlings in containerized propagation systems, and evaluating chemical treatments to reduce disease incidence.

 

 

 

 

 

Other Projects and Collaborations