Recent Presentations

Guild-specific herbivory alters physiology and the induction of plant defenses in Nicotiana attenuata

Paul D. Nabity, Jorge A. Zavala, Ian T. Baldwin, Evan H. DeLucia

Insect herbivory elicits guild-specific responses in gene expression in tobacco depending upon the insect and its constituent feeding guild (e.g., defoliators vs. phloem or xylem feeders), yet the known physiological responses of these plants to herbivory are not well investigated. In this study we tested guild-specific affects on photosynthesis and the induction of plant defense by examining the responses of N. attenuata to phloem and leaf mass consumption. If plants respond to insect herbivory depending upon the feeding mechanism, then the induction of plant defenses should be correlated to the type of physiological response and the mechanism of injury. We characterized photosynthesis using an open-system infrared gas analyzer and a novel chlorophyll fluorescence imaging system. Defoliation by a native specialist lepidopteran, Manduca sexta, and a generalist lepidopteran, Helicoverpa zea, did not alter photosynthesis; there were no short or long term decreases in assimilation or photosystem efficiency. However, phloem feeding by the tobacco-adapted aphid, Myzus persicae, decreased assimilation and reduced quantum yield revealing an insect-induced spatial pattern in chlorophyll fluorescence. Physical damage has been shown to induce the jasmonic acid (JA) signaling cascade but did not alter assimilation in this study. Artificial stimulation of the defensive pathway using methyl jasmonate, a JA precursor, reduced assimilation over time. Phloem consumption did not induce the signaling cascade indicating context dependent physiological induction of the JA signaling cascade. This study provides preliminary evidence for a correlation between the manner in which physiological responses and plant defenses are elicited and known gene expression.

(For ESA 2007)

Divergence in plant gene expression because of elevated atmospheric O3 and CO2 and their affect on host- pathogen interactions

Damla D. Bilgin, Steven J. Clough, Evan H. DeLucia

Plants have numerous defence mechanisms many of which are induced by the pathogen attack or various environmental stresses. Although the ultimate response may be different, abiotic and biotic stress-induced signaling pathways share many common genes and nodes. Changes in the concentrations of atmospheric gases played a significant role in the evolution of organisms and their interactions with each other. Increasing levels of ozone (O3) and carbon dioxide (CO2) in the atmosphere causes changes in the gene expression profile of plants and their response to pathogens and other stresses.

We used soybean plants, an economically important crop, to study the effects of global atmospheric change on gene expression and plant-pathogen interaction. Soybean plants grown in the field under elevated O3 or CO2 were tested against virus infection. Both treatments increased the resistance of susceptible plants to Soybean Mosaic Virus (SMV). Gene expression analysis with Affymetrix arrays showed that the nonspecific resistance response induced by elevated O3 is different than CO2 induced response.  Elevated O3 treatment induced isoflavone biosynthesis and pathogenesis- related genes PR1, PR5 and PR10. Elevated CO2 changed the cell number and size of foliar tissue and differentially regulated cell cycle and plant development related genes. The similar and different effects of elevated O3 and CO2 on plant gene expression and plant –pathogen interactions will be discussed. Various reasons of nonspecific resistance induced by the two major components of global atmospheric change will be presented.

(For ASPB 2007)

Canopy photosynthesis drives diel patterns of fine root respiration in a loblolly pine (Pinus taeda) forest exposed to elevated CO2 and nitrogen deposition

John Drake, Gabriel Katul, and Evan DeLucia

Recent research suggests that canopy photosynthesis drives soil respiration on short timescales (hours to days) in some temperate forests, presumably by affecting root respiration. However, measurements of fine root respiration (Rr) have lacked the sampling intensity and temporal precision to detect diel patterns, if present. We used a novel automated sampling system with temperature controlled cuvettes to make >7,000 measurements of Rr in a loblolly pine (Pinus taeda) forest exposed to elevated carbon dioxide and nitrogen fertilization. We detected diel patterns of Rr that track soil temperature as well as daily net ecosystem exchange (NEE) as measured by an on-site eddy-covariance tower. The direct response of Rr to temperature explained most of the diel variation in Rr on days with low NEE, but not on days with high NEE. The magnitude of the diel variation in Rr was correlated with the current and previous days’ NEE (r2=0.83, p<0.001 and r2=0.57, p<0.01, respectively), with no significant correlation with NEE from preceding days (p>0.1). Glucose additions to excised roots in solution consistently stimulated Rr measured in oxygen electrodes, suggesting that Rr was substrate limited and that the observed diel patterns in Rr could be explained by sugar supply from recent canopy photosynthesis. These results suggest a surprisingly close linkage between canopy and soil carbon processes, with implications for predicting Rr from measurements of NEE or photosynthesis. The combination of elevated [CO2] and N fertilization reduced basal Rr by 61% (p=0.07) indicating that Rr may be decreased by global change.

(For ESA 2007)

Elevated CO2 and Plant-Insect Interactions: Connecting Gene Expression to Ecological Responses

Evan H. DeLucia, Clare Casteel, Bridget F. O’Neill, Jorge Zavala, Arthur R. Zangerl, May R. Berenbaum

The compensatory feeding hypothesis posits that by increasing the carbon:nitrogen ratio (C:N), growth under elevated CO2 will increase herbivory by folivorous insects. Exposure of field-grown soybean to elevated levels of CO2 anticipated by 2050 increased the concentration of soluble sugars and starch in leaves and also increased foliar herbivory by Japanese beetles, western corn rootworm, and soybean aphids. Consuming foliage grown under elevated CO2 increased longevity and fitness of Japanese beetles, but elevated leaf carbohydrates or C:N could not explain these changes. Analysis of the soybean transcriptome with microarrays revealed that under field conditions key genes coding for the production of jasmonic acid (JA), an important plant defense hormone, were down-regulated under elevated CO2. Lower constitutive levels and a dampened induction of JA following beetle herbivory reduced expression of the gene coding for cysteine protease inhibitor (CystPI), a defense employed by soybean against insects. Consistent with the reduction in soybean CystPI, Japanese beetles consuming foliage grown under elevated CO2 had greater gut protease activity; more efficient digestion of leaves grown under elevated CO2 may have contributed to increased fitness of Japanese beetles. By ignoring variation in plant defenses, the compensatory feeding hypothesis may incorrectly invoke C:N ratios as the explanation for enhanced feeding in the context of elevated CO2. At least for the soybean agro-ecosystem, ever increasing levels of CO2 in the atmosphere may increase herbivory by dampening hormonal signaling and chemical defense.

(For ESA 2007)

Biotic damage to leaves universally down-regulates the transcription of photosynthesis genes

Evan H. DeLucia, Damla Bilgin, J.A. Zavala, J. Zhu, S.J. Clough, D.R. Ort

In addition to producing a myriad of constitutive and induced defenses, a reduction in photosynthesis in remaining tissues may represent an indirect “cost” of herbivory and other forms of biotic damage. Using a comparative genomic approach we tested the hypothesis that biotic assault to leaf tissue causes a universal and balanced down-regulation of genes coding for photosynthetic proteins. Microarray data were obtained from public web sites or contributed by authors. The metadata included the transcription response to foliar damage inflicted by fungi, bacteria and virus, and by two insect feeding guilds, aphids and chewing insects.  Fold-change data were subject to hierarchical clustering. With the exception of a putative chloroplast ferredoxin reductase, genes involved in light harvesting and photosynthetic electron transport were down-regulated in concert following damage. In carbon metabolism, the majority of the genes coding for aldolase, ketolase and isomerase reactions were up-regulated, while those coding for photosynthetic carbon reduction cycle proteins were down-regulated. The response to physical wounding, powdery mildew, and aphid was mild; viral, bacterial and other fungal infections and chewing damage by caterpillars caused a relatively strong reduction in the expression of photosynthetic genes. The universal nature of the response suggests that the coordinated down regulation of photosynthesis following biotic assault may be adaptive, perhaps by redirecting resources to defense or by reducing the capacity for supra-optimal generation of reactive oxygen species.

(For ASPB 2007)