Source-sink interactions shape herbivore-induced defense responses in Arabidopsis thaliana

Title from PDF of title page (University of Missouri--Columbia, viewed on March 11, 2013).The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file.Dissertation advisor: Dr. Jack C. SchultzIncludes bibliographical references.Vita.Ph. D. University of Missouri--Columbia 2012."December 2012"[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Plant tissues are not attacked uniformly, nor are they uniformly responsive to insect herbivores, in part because they are modular organisms made up of source and sink tissues. This results in complex patterns of defense that may vary spatially and temporally. My dissertation evaluates the importance of source-sink interactions and sink competition in shaping plant defense responses in Arabidopsis thaliana to feeding by two chewing lepidopterans--Pieris rapae (dietary specialist) and Spodoptera exigua (dietary generalist)--and in response to simulated herbivory with the defense elicitor, methyl-jasmonate. I investigated whether the presence of competing sinks naturally occurring during plant development (e.g. reproductive sinks and roots) facilitates or constrains the induction of defense metabolites (glucosinolates and phenolics) by altering whole-plant distribution and metabolic utilization of carbon. The use of short-lived radio-isotopes 11C (t1/2=20.4 min)as 11CO2, and 18F (t1/2= 110 min) as 2-[18F]fluoro-2-deoxy-D-glucose, provides new insight into the roles that plant architecture and competition between sinks play in determining how newly acquired carbon resources are used by plants in their defense against insect herbivory.Includes bibliographical references

Herbivore-induced phenolic accumulation is dependent on the presence of intact source leaves.

<p>The accumulation of total phenolics (A,B), anthocyanins (C,D) and flavonoids (E,F) were measured locally, within herbivore-damaged leaves (A,C,E), and in young, systemic leaves (B,D,F). Bars represent means ±SE. n = 16/treatment group. Results of analysis of variance are summarized in Tables A-C in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123899#pone.0123899.s001" target="_blank">S1 File</a>. Asterisks indicate significant differences between clip-caged control and herbivore-damaged plants for a given leaf type (<i>P</i> < 0.05 Tukey’s <i>post hoc</i> comparisons).</p

Herbivore-induced changes in foliar phenolic accumulation is independent of intact AtSUC2 function.

<p>Herbivore-damaged leaves (A,C,E), and young, systemic leaves (B,D,F) are shown in red. Total phenolics (A,B), anthocyanins (C,D) and flavonoids (E,F) were measured 48h following damage by <i>P</i>. <i>rapae</i> or <i>S</i>. <i>exigua</i> larvae to wild-type (WT) and AtSUC2-silenced plants (<i>suc2-1</i>). Bars represent means ±SE. n = 8/treatment group. Results of analysis of variance are summarized in Tables D-F in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123899#pone.0123899.s001" target="_blank">S1 File</a>. Asterisks indicate significant differences between clip-caged control and herbivore-damaged plants for a given leaf type (<i>P</i> < 0.05 Tukey’s <i>post hoc</i> comparisons).</p

Jasmonate‐dependent depletion of soluble sugars compromises plant resistance to Manduca sexta

Jasmonates regulate plant secondary metabolism and herbivore resistance. How they influence primary metabolites and how this may affect herbivore growth and performance are not well understood. We profiled sugars and starch of jasmonate biosynthesis-deficient and jasmonate-insensitive Nicotiana attenuata plants and manipulated leaf carbohydrates through genetic engineering and in vitro complementation to assess how jasmonate-dependent sugar accumulation affects the growth of Manduca sexta caterpillars. We found that jasmonates reduce the constitutive and herbivore-induced concentration of glucose and fructose in the leaves across different developmental stages. Diurnal, jasmonate-dependent inhibition of invertase activity was identified as a likely mechanism for this phenomenon. Contrary to our expectation, both in planta and in vitro approaches showed that the lower sugar concentrations led to increased M. sexta growth. As a consequence, jasmonate-dependent depletion of sugars rendered N. attenuata plants more susceptible to M. sexta attack. In conclusion, jasmonates are important regulators of leaf carbohydrate accumulation and this determines herbivore growth. Jasmonate-dependent resistance is reduced rather than enhanced through the suppression of glucose and fructose concentrations, which may contribute to the evolution of divergent resistance strategies of plants in nature

The Role of Plant Primary and Secondary Metabolites in Root-Herbivore Behaviour, Nutrition and Physiology

Many insect herbivores feed on belowground plant tissues. In this chapter, we discuss how they have adapted to deal with root primary and secondary metabolites. It is becoming evident that root herbivores can use root volatiles and exudates for host location and foraging. Their complex sensory apparatus suggests a sophisticated recognition and signal transduction system. Furthermore, endogenous metabolites trigger attractive or repellent responses in root feeders, indicating that they may specifically fine-tune food uptake to meet their dietary needs. Little evidence for direct toxic effects of root secondary metabolites has accumulated so far, indicating high prevalence of tolerance mechanisms. Root herbivores furthermore facilitate the entry of soil microbes into the roots, which may influence root nutritional quality. Investigating the role of plant metabolites in an ecologically and physiologically relevant context will be crucial to refine our current models on root-herbivore physiology and behaviour in the future

Auxin is rapidly induced by herbivory attack and regulates systemic, jasmonate-dependent defenses

Plant responses to herbivore attack are regulated by phytohormonal networks. To date, the role of the auxin indole-3-acetic acid (IAA) in this context is not well understood. We quantified and manipulated the spatiotemporal patterns of IAA accumulation in herbivore-attacked Nicotiana attenuata plants to unravel its role in the regulation of plant secondary metabolism. We found that IAA is strongly, rapidly and specifically induced by herbivore attack. IAA is elicited by herbivore oral secretions and fatty acid conjugate elicitors and is accompanied by a rapid transcriptional increase of auxin biosynthetic YUCCA-like genes. IAA accumulation starts 30-60 seconds after local induction and peaks within 5 minutes after induction, thereby preceding the jasmonate (JA) burst. IAA accumulation does not require JA signaling and spreads rapidly from the wound site to systemic tissues. Complementation and transport inhibition experiments reveal that IAA is required for the herbivore-specific, jasmonate-dependent accumulation of anthocyanins and phenolamides in the stems. In contrast, IAA does not affect the accumulation of nicotine or 7-hydroxygeranyllinalool diterpene glycosides in the same tissue. Taken together, our results uncover IAA as a rapid and specific signal that regulates a subset of systemic, jasmonate-dependent secondary metabolites in herbivore-attacked plants

Diurnal cycling of rhizosphere bacterial communities is associated with shifts in carbon metabolism

Abstract Background The circadian clock regulates plant metabolic functions and is an important component in plant health and productivity. Rhizosphere bacteria play critical roles in plant growth, health, and development and are shaped primarily by soil communities. Using Illumina next-generation sequencing and high-resolution mass spectrometry, we characterized bacterial communities of wild-type (Col-0) Arabidopsis thaliana and an acyclic line (OX34) ectopically expressing the circadian clock-associated cca1 transcription factor, relative to a soil control, to determine how cycling dynamics affected the microbial community. Microbial communities associated with Brachypodium distachyon (BD21) were also evaluated. Results Significantly different bacterial community structures (P = 0.031) were observed in the rhizosphere of wild-type plants between light and dark cycle samples. Furthermore, 13% of the community showed cycling, with abundances of several families, including Burkholderiaceae, Rhodospirillaceae, Planctomycetaceae, and Gaiellaceae, exhibiting fluctuation in abundances relative to the light cycle. However, limited-to-no cycling was observed in the acyclic CCAox34 line or in soil controls. Significant cycling was also observed, to a lesser extent, in Brachypodium. Functional gene inference revealed that genes involved in carbohydrate metabolism were likely more abundant in near-dawn, dark samples. Additionally, the composition of organic matter in the rhizosphere showed a significant variation between dark and light cycles. Conclusions The results of this study suggest that the rhizosphere bacterial community is regulated, to some extent, by the circadian clock and is likely influenced by, and exerts influences, on plant metabolism and productivity. The timing of bacterial cycling in relation to that of Arabidopsis further suggests that diurnal dynamics influence plant-microbe carbon metabolism and exchange. Equally important, our results suggest that previous studies done without relevance to time of day may need to be reevaluated with regard to the impact of diurnal cycles on the rhizosphere microbial community

A Nicotiana attenuata cell wall invertase inhibitor (NaCWII) reduces growth and increases secondary metabolite biosynthesis in herbivore-attacked plants

Plant invertases are sucrolytic enzymes that are essential for the regulation of carbohydrate metabolism and source–sink relationships. While their activity has been well documented during abiotic and biotic stresses, the role of proteinaceous invertase inhibitors in regulating these changes is unknown. Here, we identify a putative Nicotiana attenuata cell wall invertase inhibitor (NaCWII) which is strongly up-regulated in a jasmonate (JA)-dependent manner following simulated attack by the specialist herbivore Manduca sexta. To understand the role of NaCWII in planta, we silenced its expression by RNA interference and measured changes in primary and secondary metabolism and plant growth following simulated herbivory. NaCWII-silenced plants displayed a stronger depletion of carbohydrates and a reduced capacity to increase secondary metabolite pools relative to their empty vector control counterparts. This coincided with the attenuation of herbivore-induced CWI inhibition and growth suppression characteristic of wild-type plants. Together our findings suggest that NaCWII may act as a regulatory switch located downstream of JA accumulation which fine-tunes the plant's balance between growth and defense metabolism under herbivore attack. Although carbohydrates are not typically viewed as key factors in plant growth and defense, our study shows that interfering with their catabolism strongly influences plant responses to herbivory