Date of Award

Fall 2023

Document Type

Open Access Dissertation

Department

Biological Sciences

First Advisor

Johannes Stratmann

Abstract

As sessile organisms, plants rely on extracellular signals and chemical responses to defend against stressors. Extracellular stress signals inducing defense responses originate both from biotic or abiotic threats, and from products of tissue damage or hormones. In defense against herbivores, essential responses include rapid generation of jasmonic acid (JA) at the wound site, followed by the global expression of genes that impede digestion and growth. Early defense gene expression amplifies JA production, while later JA-responsive gene expression causes the accumulation of defense products. MAP kinase (MAPK) pathways play a central role in mediating the perception of self, non-self, and hormonal molecules that trigger defense. The MAPKs MPK6 and MPK3 are a major signaling hub for many stress signals in plants and are essential to JA production in many plant species. In Arabidopsis however, JA accumulation is not dependent on MPK6/3 activity, so the role of this critical MAPK in defense against insects is not clear. The MPK6/3 pathway is activated by wounding, and its activity can be negatively regulated by phosphatases. MAPK inactivating phosphatases (MIPPs) confer an additional layer of signal specificity by modulating the intensity and duration of a kinase signal in specific contexts. In this study, we investigate the role of MPK6/3 in defense against insects via MIPP mutation in Arabidopsis. The MIPPs MKP1, DsPTP1, PP2C5, and AP2C1 have only been implicated in infection, drought or osmotic signaling. These pathways are known to modulate the success of the JA-responsive pathway against insects. We hypothesized that since drought, infection, and herbivore defense MAPK pathways share significant crosstalk, these MIPPs can impact responses to herbivores. We found that mutation of either individual MIPPs, MPK6, or MPK3 has an impact on MPK6/3 kinetics, and the accumulation of the defense hormones jasmonic acid, abscisic acid, and salicylic acid. MPK6/3 misregulation via MIPP or MAPK knockout resulted in weight changes in Trichoplusia ni and Spodoptera exigua larvae, and in S. exigua day of pupation compared to wild type (Col-0). We concluded that MPK6 and MPK3, and individual MIPPs, contribute moderately and uniquely to defense against insects.

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© 2024, Claire Teresa Hann

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