Like animals, plants continually interact with a plethora of other organisms in their environment, including potentially pathogenic microbes and herbivorous insects that can cause disease or damage. To minimize the detrimental impact of invading organisms, plants must mount a rapid and robust immune response. My research interests center on plant peptide signals, Peps, which regulate broad spectrum plant defense responses against pathogens and herbivores, and their application to manipulate plant resistance to biotic attack.
Plant recognition of invading organisms occurs through perception of foreign molecules associated with attacking microbes and herbivores, known as elicitors, MAMPs (microbe-associated molecular patterns) or HAMPs (herbivore-associated molecular patterns). Perception of these molecules by pattern recognition receptors (PRRs) leads to subsequent launching of complex and multilayered defenses. These defenses, also known as basal or innate immune responses, have damaging effects on a wide array of pathogens and pests and involve accumulation of antimicrobial proteins and toxic metabolites. In addition to perceiving molecules from foreign organisms to detect biotic attack, plants also produce and recognize endogenous elicitors to assist in recognition and amplification of the immune response.
Arabidopsis Plant Elicitor Peptide1 (AtPep1) was the first endogenous peptide signal found to regulate plant anti-microbial defenses. AtPep1 is derived from a larger precursor encoded by the Arabidopsis gene designated AtPROPEP1, which is expressed in response to treatment with microbe-derived elicitors. AtPROPEP1 is a member of an eight gene family in Arabidopsis thaliana, each of which encode peptides that interact with the Plant Elicitor Peptide Receptors (PEPRs) to regulate plant immune responses. Treatment of Arabidopsis plants with AtPep1 induces production of defensive phytohormones and expression of pathogen defense genes. Transgenic over-expression of AtPROPEP1 causes constitutive expression of defense genes and enhances resistance to root degradation by the oomycete Pythium irregulare. Similarly, pretreatment of Arabidopsis plants with AtPeps prior to infection with Pseudomonas syringae Pv. tomato DC3000 restricts bacterial growth and decreases symptoms of disease.
AtPeps are active only in Arabidopsis and closely related Brassicaceous species, but gene orthologs have been identified in more than 50 plant species. These Pep orthologs also regulate innate immunity. Treatment of maize plants with the ortholog ZmPep1 prior to infection with fungal pathogens caused accumulation of defense transcripts and metabolites and reduced symptoms and cell death caused by both Cochliobolus heterostrophus (Southern leaf blight) and Colletotrichum heterostrophus (Anthracnose stalk rot).
Peps also mediate anti-herbivore defenses. Upon attack by herbivores, plants emit a complex blend of chemicals composed predominately of terpenes and six carbon (C6) green leafy volatiles (GLVs). These volatiles deployed by the plant recruit other organisms from the community to defend it from damage. Beneficial insects such as wasp species are attracted to the volatile blend and either eat or lay eggs in the attacking herbivorous pests. A maize Plant Elicitor Peptide, ZmPep3, is a potent signal promoting emission of herbivore associated volatiles and treatment of maize leaves with ZmPep3 results in accumulation of transcripts encoding biosynthetic enzymes for volatiles and other defense metabolites. These ZmPep3-induced responses are effective as direct and indirect defenses against herbivores, limiting growth of Spodoptera exigua larvae and attracting naïve Cotesia marginiventris, a parasitoid of Lepidopteran larvae. Similar regulation of anti-herbivore defense by Peps occurs in numerous and diverse plant species.
Because PROPEP genes and signaling by Peps are ubiquitous throughout angiosperm plants, knowledge gleaned from Pep research in maize and Arabidopsis can be extrapolated and applied to other plants. Peps act as regulatory nodes for defenses against pathogens and herbivores, and are valuable candidate tools for genetic improvement of crop resistance. Study of Peps have generated a number of compelling questions for future research. How is signal specificity achieved? What signaling components are involved? How can this endogenous regulation of the plant immune system be improved to protect plants?
As individual Peps appear to have differing functions, we’re studying the overlap of signaling and defense strategies mediated by individual Peps and by exogenous elicitors from other organisms. We hope to decode Pep-induced response specificity and to identify molecular components that can be successfully used to enhance resistance against a variety of attackers, including biotrophic and nectrotrophic pathogens as well as specialist and generalist herbivores. Current lab efforts are focused on 1. Study of Pep-PEPR ligand-receptor interactions and determination of structural requirements for signaling activity. 2. Characterization of downstream Pep signaling components identified through genetic screens and profiling of rapid changes in transcriptome and phosphoproteome of maize and Arabidopsis. 3. Development of Pep-based strategies for enhancement of plant immunity against pathogens and pests.
Dr. Huffaker received her Ph.D. from the Institute of Biological Chemistry at Washington State University. She continued at the Institute of Biological Chemistry for her postdoctoral training in plant defense signaling. Dr. Huffaker joined the section of Cell and Developmental Biology in 2014.